Not sure why, but this just seems more intuitive/correct. Maybe because
LFSR_TAG_NAME is always the first tag in a file's attr set:
LFSR_TAG_NAMELIMIT 0x0039 v--- ---- --11 1--1
LFSR_TAG_FILELIMIT 0x003a v--- ---- --11 1-1-
Seeing as several parts of the codebase still use the previous order,
it seems reasonable to switch back to that.
No code changes.
This carves out two more bits in cksum tags to store the "phase" of the
rbyd block (maybe the name is too fancy, this is just the lowest 2 bits
of the block address):
LFSR_TAG_CKSUM 0x300p v-11 ---- ---- -pqq
^ ^
| '-- phase bits
'---- perturb bit
The intention here is to catch mrootanchors that are "out-of-phase",
i.e. they've been shifted by a small number of blocks.
This can happen if we find the wrong mrootanchor (after, say, a magic
scan), and risks filesystem corruption:
formatted
.-----------------'-----------------.
mounted
.-----------------'-----------------.
.--------+--------+--------+--------+ ...
|(erased)| mroot |
| | anchor | ...
| | |
'--------+--------+--------+--------+ ...
Including the lower 2 bits of the block address in cksum tags avoids
this, for up to a 3 block shift (the maximum number of redund
mrootanchors).
---
Note that cksum tags really are the only place we could put these bits.
Anywhere else and they would interfere with the canonical cksum, which
would break error correction. By definition these need to be different
per block.
We include these phase bits in every cksum tag (because it's easier),
but these don't really say much about mdirs that are not the
mrootanchor. Non-anchor mdirs can have arbitrary block addresses,
therefore arbitrary phase bits.
You _might_ be able to do something interesting if you sort the rbyd
addresses and use the index as the phase bits, but that would add quite
a bit of code for questionable benefit...
You could argue this adds noise to our cksums, but:
1. 2 bits seems like a really small amount of noise
2. our cksums are just crc32cs
3. the phase bits humorously never change when you rewrite a block
---
As with any feature this adds code, but only a small amount. I think
it's worth the extra protection:
code stack ctx
before: 35792 2368 636
after: 35824 (+0.1%) 2368 (+0.0%) 636 (+0.0%)
Also added test_mount_incompat_out_of_phase to test this.
The dbg scripts _don't_ error (block mismatch seems likely when
debugging), but dbgrbyd.py at least adds phase mismatch notes in
-l/--log mode.
I was toying with making this look more like the mtree API in lfs.c (so
no lookupleaf/namelookupleaf, only lookup/namelookup), but dropped the
idea:
- It would be tedious
- The Mtree class's lookupleaf/namelookupleaf are also helpful for
returning inner btree nodes when printing debug info
- Not embedding mids in the Mdir class would complicate things
It's ok for these classes to not match littlefs's internal API
_exactly_. The goal is easy access for debug info, not to port the
filesystem to Python.
At least dropped Mtree.lookupnext, because that function really makes no
sense.
Now that we don't have to worry about name tag conflicts as much, we
can add name tags for things that aren't files.
This adds LFSR_TAG_BNAME for branch names, and LFSR_TAG_MNAME for mtree
names. Note that the upper 4 bits of the subtype match LFSR_TAG_BRANCH
and LFSR_TAG_MDIR respectively:
LFSR_TAG_BNAME 0x0200 v--- --1- ---- ----
LFSR_TAG_MNAME 0x0220 v--- --1- --1- ----
LFSR_TAG_BRANCH 0x030r v--- --11 ---- --rr
LFSR_TAG_MDIR 0x0324 v--- --11 --1- -1rr
The encoding is somewhat arbitrary, but I figured reserving ~31 types
for files is probably going to be plenty for littlefs. POSIX seems to
do just fine with only ~7 all these years, and I think custom attributes
will be more enticing for "niche" file types (symlinks, compressed
files, etc), given the easy backwards compatibility.
---
In addition to the debugging benefits, the new name tags let us stop
btree lookups on the first non-bname/branch tag. Previously we always
had to fetch the first struct tag as well to check if it was a branch.
In theory this saves one rbyd lookup, but in practice it's a bit muddy.
The problem is that there's two ways to use named btrees:
1. As buckets: mtree -> mdir -> mid
2. As a table: ddtree -> ddid
The only named btree we _currently_ have is the mtree. And the mtree
operates in bucket mode, with each mdir acting more-or-less as an
extension to the btree. So we end up needing to do the second tag lookup
anyways, and all we've done is complicated up the code.
But we will _eventually_ need the table mode for the ddtree, where we
care if the ddname is an exact match.
And returning the first tag is arguably the more "correct" internal API,
vs arbitrarily the first struct tag.
But then again this change is pretty pricey...
code stack ctx
before: 35732 2440 640
after: 35888 (+0.4%) 2480 (+1.6%) 640 (+0.0%)
---
It's worth noting the new BNAME/MNAME tags don't _require_ the btree
lookup changes (which is why we can get away with not touching the dbg
scripts). The previous algorithm of always checking for branch tags
still works.
Maybe there's an argument for conditionally using the previous API when
compiling without the ddtree, but that sounds horrendously messy...
Mainly to make room for some future planned stuff:
- Moved the mroot's redund bits from LFSR_TAG_GEOMETRY to
LFSR_TAG_MAGIC:
LFSR_TAG_MAGIC 0x003r v--- ---- --11 --rr
This has the benefit of living in a fixed location (off=0x5), which
may make mounting/debugging easier. It also makes LFSR_TAG_GEOMETRY
less of a special case (LFSR_TAG_MAGIC is already a _very_ special
case).
Unfortunately, this does get in the way of our previous magic=0x3
encoding. To compensate (and to avoid conflicts with LFSR_TAG_NULL),
I've added the 0x3_ prefix. This has the funny side-effect of
rendering redunds 0-3 as ascii 0-3 (0x30-0x33), which is a complete
accident but may actually be useful when debugging.
Currently all config tags fit in the 0x3_ prefix, which is nice for
debugging but not a hard requirement.
- Flipped LFSR_TAG_FILELIMIT/NAMELIMIT:
LFSR_TAG_FILELIMIT 0x0039 v--- ---- --11 1--1
LFSR_TAG_NAMELIMIT 0x003a v--- ---- --11 1-1-
The file limit is a _bit_ more fundamental. It's effectively the
required integer size for the filesystem.
These may also be followed by LFSR_TAG_ATTRLIMIT based on how future
attr revisits go.
- Rearranged struct tags so that LFSR_TAG_BRANCH = 0x300:
LFSR_TAG_BRANCH 0x030r v--- --11 ---- --rr
LFSR_TAG_DATA 0x0304 v--- --11 ---- -1--
LFSR_TAG_BLOCK 0x0308 v--- --11 ---- 1err
LFSR_TAG_DDKEY* 0x0310 v--- --11 ---1 ----
LFSR_TAG_DID 0x0314 v--- --11 ---1 -1--
LFSR_TAG_BSHRUB 0x0318 v--- --11 ---1 1---
LFSR_TAG_BTREE 0x031c v--- --11 ---1 11rr
LFSR_TAG_MROOT 0x032r v--- --11 --1- --rr
LFSR_TAG_MDIR 0x0324 v--- --11 --1- -1rr
LFSR_TAG_MTREE 0x032c v--- --11 --1- 11rr
*Planned
LFSR_TAG_BRANCH is a very special tag when it comes to bshrub/btree
traversal, so I think it deserves the subtype=0 slot.
This also just makes everything fit together better, and makes room
for the future planned ddkey tag.
Code changes minimal:
code stack ctx
before: 35728 2440 640
after: 35732 (+0.0%) 2440 (+0.0%) 640 (+0.0%)
Now that LFS_TYPE_STICKYNOTE is a real type users can interact with, it
makes sense to group it with REG/DIR. This also has the side-effect of
making these contiguous.
---
LFSR_TAG_BOOKMARKs, however, are still hidden from the user. This
unfortunately means there will be a bit of a jump if we ever add
LFS_TYPE_SYMLINK in the future, but I'm starting to wonder if that's the
best way to approach symlinks in littlefs...
If instead LFS_TYPE_SYMLINKS were implied via custom attribute, you
could avoid the headache that comes with adding a new tag encoding, and
allow perfect compatibility with non-symlink drivers. Win win.
This seems like a better approach for _all_ of the theoretical future
types (compressed files, device files, etc), and avoids the risk of
oversaturating the type space.
---
This had a surprising impact on code for just a minor encoding tweak. I
guess the contiguousness pushed the compiler to use tables/ranges for
more things? Or maybe 3 vs 5 is just an easier constant to encode?
code stack ctx
before: 35952 2440 640
after: 35928 (-0.1%) 2440 (+0.0%) 640 (+0.0%)
This was caused by including the shrub bit in the tag comparison in
Rbyd.lookup.
Fixed by adding an extra key mask (0xfff). Note this is already how
lfsr_rbyd_lookup works in lfs.c.
- Fixed Mtree.lookupleaf accepting mbid=0, which caused dbglfs.py to
double print all files with mbid=-1
- Fixed grm mids not being mapped to mbid=-1 and related orphan false
positives
This was a surprising side-effect the script rework: Realizing the
internal btree/rbyd lookup APIs were awkwardly inconsistent and could be
improved with a couple tweaks:
- Adopted lookupleaf name for functions that return leaf rbyds/mdirs.
There's an argument this should be called lookupnextleaf, since it
returns the next bid, unlike lookup, but I'm going to ignore that
argument because:
1. A non-next lookupleaf doesn't really make sense for trees where
you don't have to fetch the leaf (the mtree)
2. It would be a bit too verbose
- Adopted commitleaf name for functions that accept leaf rbyds.
This makes the lfsr_bshrub_commit -> lfsr_btree_commit__ mess a bit
more readable.
- Strictly limited lookup and lookupnext to return rattrs, even in
complex trees like the mtree.
Most use cases will probably stick to the lookupleaf variants, but at
least the behavior will be consistent.
- Strictly limited lookup to expect a known bid/rid.
This only really matters for lfsr_btree/bshrub_lookup, which as a
quirk of their implementation _can_ lookup both bid + rattr at the
same time. But I don't think we'll need this functionality, and
limited the behavior may allow for future optimizations.
Note there is no lfsr_file_lookup. File btrees currently only ever
have a single leaf rattr, so this API doesn't really make sense.
Internal API changes:
- lfsr_btree_lookupnext_ -> lfsr_btree_lookupleaf
- lfsr_btree_lookupnext -> lfsr_btree_lookupnext
- lfsr_btree_lookup -> lfsr_btree_lookup
- added lfsr_btree_namelookupleaf
- lfsr_btree_namelookup -> lfsr_btree_namelookup
- lfsr_btree_commit__ -> lfsr_btree_commit_
- lfsr_btree_commit_ -> lfsr_btree_commitleaf
- lfsr_btree_commit -> lfsr_btree_commit
- added lfsr_bshrub_lookupleaf
- lfsr_bshrub_lookupnext -> lfsr_bshrub_lookupnext
- lfsr_bshrub_lookup -> lfsr_bshrub_lookup
- lfsr_bshrub_commit_ -> lfsr_bshrub_commitleaf
- lfsr_bshrub_commit -> lfsr_bshrub_commit
- lfsr_mtree_lookup -> lfsr_mtree_lookupleaf
- added lfsr_mtree_lookupnext
- added lfsr_mtree_lookup
- added lfsr_mtree_namelookupleaf
- lfsr_mtree_namelookup -> lfsr_mtree_namelookup
- added lfsr_file_lookupleaf
- lfsr_file_lookupnext -> lfsr_file_lookupnext
- added lfsr_file_commitleaf
- lfsr_file_commit -> lfsr_file_commit
Also added lookupnext to Mdir/Mtree in the dbg scripts.
Unfortunately this did add both code and stack, but only because of the
optional mdir returns in the mtree lookups:
code stack ctx
before: 35520 2440 636
after: 35548 (+0.1%) 2472 (+1.3%) 636 (+0.0%)
So mbid=0 now implies the mdir is not inlined.
Downsides:
- A bit more work to calculate
- May lose information due to masking everything when mtree.weight==0
- Risk of confusion when in-lfs.c state doesn't match (mbid=-1 is
implied by mtree.weight==0)
Upsides:
- Includes more information about the topology of the mtree
- Avoids multiple dbgmbids for the same physical mdir
Also added lfsr_dbgmbid and lfsr_dbgmrid to help make logging
easier/more consistent.
And updated dbg scripts.
So now:
(block_size)
mbits = nlog2(----------) = nlog2(block_size) - 3
( 8 )
Instead of:
( (block_size))
mbits = nlog2(floor(----------)) = nlog2(block_size & ~0x7) - 3
( ( 8 ))
This makes the post-log - 3 formula simpler, which we probably want to
prefer as it avoids a division. And ceiling is arguably more intuitive
corner case behavior.
This may seem like a minor detail, but because mbits is purely
block_size derived and not configurable, any quirks here will become
a permanent compatibility requirement.
And hey, it saves a couple bytes (I'm not really sure why, the division
should've been optimized to a shift):
code stack ctx
before: 35528 2440 636
after: 35520 (-0.0%) 2440 (+0.0%) 636 (+0.0%)
This is limited to dbgle32.py, dbgleb128.py, and dbgtag.py for now.
This more closely matches how littlefs behaves, in that we read a
bounded number of bytes before leb128 decoding. This minimizes bugs
related to leb128 overflow and avoids reading inherently undecodable
data.
The previous unbounded behavior is still available with -w0.
Note this gives dbgle32.py much more flexibility in that it can now
decode other integer widths. Uh, ignore the name for now. At least it's
self documenting that the default is 32-bits...
---
Also fixed a bug in fromleb128 where size was reported incorrectly on
offset + truncated leb128.
Do you see the O(n^2) behavior in this loop?
j = 0
while j < len(data):
word, d = fromleb(data[j:])
j += d
The slice, data[j:], creates a O(n) copy every iteration of the loop.
A bit tricky. Or at least I found it tricky to notice. Maybe because
array indexing being cheap is baked into my brain...
Long story short, this repeated slicing resulted in O(n^2) behavior in
Rbyd.fetch and probably some other functions. Even though we don't care
_too_ much about performance in these scripts, having Rbyd.fetch run in
O(n^2) isn't great.
Tweaking all from* functions to take an optional index solves this, at
least on paper.
---
In practice I didn't actually find any measurable performance gain. I
guess array slicing in Python is optimized enough that the constant
factor takes over?
(Maybe it's being helped by us limiting Rbyd.fetch to block_size in most
scripts? I haven't tested NAND block sizes yet...)
Still, it's good to at least know this isn't a bottleneck.
This just gives dbgtag.py a few more bells and whistles that may be
useful:
- Can now parse multiple tags from hex:
$ ./scripts/dbgtag.py -x 71 01 01 01 12 02 02 02
71 01 01 01 altrgt 0x101 w1 -1
12 02 02 02 shrubdir w2 2
Note this _does_ skip attached data, which risks some confusion but
not skipping attached data will probably end up printing a bunch of
garbage for most use cases:
$ ./scripts/dbgtag.py -x 01 01 01 04 02 02 02 02 03 03 03 03
01 01 01 04 gdelta 0x01 w1 4
03 03 03 03 struct 0x03 w3 3
- Included hex in output. This is helpful for learning about the tag
encoding and also helps identify tags when parsing multiple tags.
I considered also included offsets, which might help with
understanding attached data, but decided it would be too noisy. At
some point you should probably jump to dbgrbyd.py anyways...
- Added -i/--input to read tags from a file. This is roughly the same as
-x/--hex, but allows piping from other scripts:
$ ./scripts/dbgcat.py disk -b4096 0 -n4,8 | ./scripts/dbgtag.py -i-
80 03 00 08 magic 8
Note this reads the entire file in before processing. We'd need to fit
everything into RAM anyways to figure out padding.
- Added TreeArt __bool__ and __len__.
This was causing a crash in _treeartfrommtreertree when rtree was
empty.
The code was not updated in the set -> TreeArt class transition, and
went unnoticed because it's unlikely to be hit unless the filesystem
is corrupt.
Fortunately(?) realtime rendering creates a bunch of transiently
corrupt filesystem images.
- Tweaked lookupleaf to not include mroots in their own paths.
This matches the behavior of leaf mdirs, and is intentionally
different from btree's lookupleaf which needs to lookup the leaf rattr
to terminate.
- Tweaked leaves to not remove the last path entry if it is an mdir.
This hid the previous lookupleaf inconsistency. We only remove the
last rbyd from the path because it is redundant, and for mdirs/mroots
it should never be redundant.
I ended up just replacing the corrupt check with an explicit check
that the rbyd is redundant. This should be more precise and avoid
issues like this in the future.
Also adopted explicit redundant checks in Btree.leaves and
Lfs.File.leaves.
Reading Wikipedia:
> Later terminals added the ability to directly specify the "bright"
> colors with 90–97 and 100–107.
So if we want to stick to one pattern, we should probably go with
brightness as a separate modifier.
This shouldn't noticeably change any script, unless your terminal
interprets 90-97m colors differently from 1;30-37m, in which case things
should be more consistent now.
Jumping from a simple Python implementation to the fully hardware
accelerated crc32c library basically deletes any crc32c related
bottlenecks:
crc32c.py disk (1MiB) w/ crc32c lib: 0m0.027s
crc32c.py disk (1MiB) w/o crc32c lib: 0m0.844s
This uses the same try-import trick we use for inotify_simple, so we get
the speed improvement without losing portability.
---
In dbgbmap.py:
dbgbmap.py w/ crc32c lib: 0m0.273s
dbgbmap.py w/o crc32c lib: 0m0.697s
dbgbmap.py w/ crc32c lib --no-ckdata: 0m0.269s
dbgbmap.py w/o crc32c lib --no-ckdata: 0m0.490s
dbgbmap.old.py: 0m0.231s
The bulk of the runtime is still in Rbyd.fetch, but this is now
dominated by leb128 decoding, which makes sense. We do ~twice as many
fetches in the new dbgbmap.py in order to calculate the gcksum (which
we then ignore...).
This better matches what you would expect from a function called
bd.read, at least in the context of littlefs, while also decreasing the
state (seek) we have to worry about.
Note that bd.readblock already behaved mostly like this, and is
preferred by every class except for Bptr.
So no more __getitem__, __contains__, or __iter__ for Rbyd, Btree, Mdir,
Mtree, Lfs.File, etc.
These were way too error-prone, especially when accidental unpacking
triggered unintended disk traversal and weird error states. We didn't
even use the implicit behavior because we preferred the full name for
heavy disk operations.
The motivation for this was Python not catching this bug, which is a bit
silly:
rid, rattr, *path_ = rbyd
This is a rework of dbgbmap.py to match dbgbmapd3.py, adopt the new
Rbyd/Lfs class abstractions, as well as Canvas, -k/--keep-open, etc.
Some of the main changes:
- dbgbmap.py now reports corrupt/conflict blocks, which can be useful
for debugging.
Note though that you will probably get false positives if running with
-k/--keep-open while something is writing to the disk. littlefs is
powerloss safe, not multi-write safe! Very different problem!
- dbgbmap.py now groups by blocks before mapping to the space filling
curve. This matches dbgbmapd3.py and I think is more intuitive now
that we have a bmap tiling algorithm.
-%/--usage still works, but is rendered as a second space filling
curve _inside_ the block tile. Different blocks can end up with
slightly different sizes due to rounding, but it's not the end of the
world.
I wasn't originally going to keep it around, but ended up caving, so
you can still get the original byte-level curve via -u/--contiguous.
- Like the other ascii rendering script, dbgbmap.py now supports
-k/--keep-open and friends as a thin main wrapper. This just makes it
a bit easier to watch a realtime bmap without needing to use watch.py.
- --mtree-only is supported, but filtering via --mdirs/--btrees/--data
is _not_ supported. This was too much complexity for a minor feature,
and doesn't cover other niche blocks like corrupted/conflict or parity
in the future.
- Things are more customizable thanks to the Attr class. For an example
you can now use the littlefs mount string as the title via
--title-littlefs.
- Support for --to-scale and -t/--tiny mode, if you want to scale based
on block_size.
One of the bigger differences dbgbmapd3.py -> dbgbmap.py is that
dbgbmap.py still supports -%/--usage. Should we backport -%/--usage to
dbgbmapd3.py? Uhhhh...
This ends up a funny example of raster graphics vs vector graphics. A
pixel-level space filling curve is easy with raster graphics, but with
an svg you'd need some sort of pixel -> path wrapping algorithm...
So no -%/--usage in dbgbmapd3.py for now.
Also just ripped out all of the -@/--blocks byte-level range stuff. Way
too complicated for what it was worth. -@/--blocks is limited to simple
block ranges now. High-level scripts should stick to high-level options.
One last thing to note is the adoption of "if '%' in label__" checks
before applying punescape. I wasn't sure if we should support punescape
in dbgbmap.py, since it's quite a bit less useful here, and may be
costly due to the lazy attr generation. Adding this simple check avoids
the cost and consistency question, so I adopted it in all scripts.
This matches the coloring in dbglfs.py for other erroneous conditions,
and also matches how we color hidden items when shown.
Also fixed some minor bugs in grm printing.
This can be useful when you just want to check for errors.
The only exception being dbgblock.py/dbgcat.py, since these don't really
have a concept of an error.
For more aggressive checking of filesystem state. These should match the
behavior of LFS_M_CKMETA/CKDATA in lfs.c.
Also tweaked dbgbmapd3.py (and eventually dbgmap.py) to match, though we
don't need new flags there since we're already checking every block in
the filesystem.
- Added Lfs.traverse for full filesystem traversal
- Added Rbyd.shrub flag so we can tell if an Rbyd is a shrub
- Removed redundant leaves from paths in leaf iters
Like codemapd3.py this include an interactive UI for viewing the
underlying filesystem graph, including:
- mode-tree - Shows all reachable blocks from a given block
- mode-branches - Shows immediate children of a given block
- mode-references - Shows parents of a given block
- mode-redund - Shows sibling blocks in redund groups (This is
currently just mdir pairs, but the plan is to add more)
This is _not_ a full filesystem explorer, so we don't embed all block
data/metadata in the svg. That's probably a project for another time.
However we do include interesting bits such as trunk addresses,
checksums, etc.
An example:
# create an filesystem image
$ make test-runner -j
$ ./scripts/test.py -B test_files_many -a -ddisk -O- \
-DBLOCK_SIZE=1024 \
-DCHUNK=10 \
-DSIZE=2050 \
-DN=128 \
-DBLOCK_RECYCLES=1
... snip ...
done: 2/2 passed, 0/2 failed, 164pls!, in 0.16s
# generate bmap svg
$ ./scripts/dbgbmapd3.py disk -b1024 -otest.svg \
-W1400 -H750 -Z --dark
updated test.svg, littlefs v0.0 1024x1024 0x{26e,26f}.d8 w64.128, cksu
m 41ea791e
And open test.svg in a browser of your choice.
Here's what the current colors mean:
- yellow => mdirs
- blue => btree nodes
- green => data blocks
- red => corrupt/conflict issue
- gray => unused blocks
But like codemapd3.py the output is decently customizable. See -h/--help
for more info.
And, just like codemapd3.py, this is based on ideas from d3 and
brendangregg's flamegraphs:
- d3 - https://d3js.org
- brendangregg's flamegraphs - https://github.com/brendangregg/FlameGraph
Note we don't actually use d3... the name might be a bit confusing...
---
One interesting change from the previous dbgbmap.py is the addition of
"corrupt" (bad checksum) and "conflict" (multiple parents) blocks, which
can help find bugs.
You may find the "conflict" block reporting a bit strange. Yes it's
useful for finding block allocation failures, but won't naturally formed
dags in file btrees also be reported as "conflicts"?
Yes, but the long-term plan is to move away from dags and make littlefs
a pure tree (for block allocator and error correction reasons). This
hasn't been implemented yet, so for now dags will result in false
positives.
---
Implementation wise, this script was pretty straightforward given prior
dbglfs.py and codemapd3.py work.
However there was an interesting case of https://xkcd.com/1425:
- Traverse the filesystem and build a graph - easy
- Tile a rectangle with n nice looking rectangles - uhhh
I toyed around with an analytical approach (something like block width =
sqrt(canvas_width*canvas_height/n) * block_aspect_ratio), but ended up
settling on an algorithm that divides the number of columns by 2 until
we hit our target aspect ratio.
This algorithm seems to work quite well, runs in only O(log n), and
perfectly tiles the grid for powers-of-two. Honestly the result is
better than I was expecting.
This fixes an issue where shrub trunks were never printed even with
-i/--internal.
While only showing mdir/shrub/btree/bptr addresses on block changes is
nice in theory, it results in shrub trunks never being printed because
the mdir -> shrub block doesn't change.
Also checking for changes in block type avoids this.
I'm trying to avoid having classes with different implementations across
scripts, as it makes updating things error-prone, but at same time
copying all the tree renderers to all dbg scripts would be a bit much.
Monkey-patching the TreeArt class in relevant scripts seems like a
reasonable compromise.
These are pretty script specific, so probably shouldn't be in the
abstract littlefs classes. This also avoids the tree renderers getting
copied into scripts that don't need them (mtree -> dbglfs.py, dbgbmap.py
in the future, etc).
This also makes TreeArt consistent with JumpArt and LifetimeArt.
So, instead of trying to be clever with python's tuple globbing, just
rely on lazy tuple unpacking and a whole bunch of if statements.
This is more verbose, but less magical. And generally, the less magic
there is, the easier things are to read.
This also drops the always-tupled lookup_ variants, which were
cluttering up the various namespaces.
Also tweaked how we fetch shrubs, adding Rbyd.fetchshrub and
Btree.fetchshrub instead of overloading the bd argument.
Oh, and also added --trunk to dbgmtree.py and dbglfs.py. Actually
_using_ --trunk isn't advised, since it will probably just result in a
corrupted filesystem, but these scripts are for accessing things that
aren't normally allowed anyways.
The reason for dropping the list/tuple distinction is because it was a
big ugly hack, unpythonic, and likely to catch users (and myself) by
surprise. Now, Rbyd.fetch and friends always require separate
block/trunk arguments, and the exercise of deciding which trunk to use
is left up to the caller.
The inconsistency between inner/non-inner (-i/--inner) views was a bit
too confusing.
At least now the bptr rendering in dbglfs.py matches behavior, showing
the bptr tag -> bptr jump even when not showing inner nodes.
If the point of these renderers is to show all jumps necessary to reach
a given piece of data, hiding bptr jumps only sometimes is somewhat
counterproductive...
I'm starting to regret these reworks. They've been a big time sink. But
at least these should be much easier to extend with the future planned
auxiliary trees?
New classes:
- Bptr - A representation of littlefs's data-only block pointers.
Extra fun is the lazily checked Bptr.__bool__ method, which should
prevent slowing down scripts that don't actually verify checksums.
- Config - The set of littlefs config entries.
- Gstate - The set of littlefs gstate.
I may have had too much fun with Config and Gstate. Not only do these
provide lookup functions for config/gstate, but known config/gstate
get lazily parsed classes that can provide easy access to the relevant
metadata.
These even abuse Python's __subclasses__, so all you need to do to add
a new known config/gstate is extend the relevant Config.Config/
Gstate.Gstate class.
The __subclasses__ API is a weird but powerful one.
- Lfs - The big one, a high-level abstraction of littlefs itself.
Contains subclasses for known files: Lfs.Reg, Lfs.Dir, Lfs.Stickynote,
etc, which can be accessed by path, did+name, mid, etc. It even
supports iterating over orphaned files, though it's expensive (but
incredibly valuable for debugging!).
Note that all file types can currently have attached bshrubs/btrees.
In the existing implementation only reg files should actually end up
with bshrubs/btrees, but the whole point of these scripts is to debug
things that _shouldn't_ happen.
I intentionally gave up on providing depth bounds in Lfs. Too
complicated for something so high-level.
On noteworthy change is not recursing into directories by default. This
hopefully avoids overloading new users and matches the behavior of most
other Linux/Unix tools.
This adopts -r/--recurse/--file-depth for controlling how far to recurse
down directories, and -z/--depth/--tree-depth for controlling how far to
recurse down tree structures (mostly files). I like this API. It's
consistent with -z/--depth in the other dbg scripts, and -r/--recurse is
probably intuitive for most Linux/Unix users.
To make this work we did need to change -r/--raw -> -x/--raw. But --raw
is already a bit of a weird name for what really means "include a hex
dump".
Note that -z/--depth/--tree-depth does _not_ imply --files. Right now
only files can contain tree structures, but this will change when we get
around to adding the auxiliary trees.
This also adds the ability to specify a file path to use as the root
directory, though we need the leading slash to disambiguate file paths
and mroot addresses.
---
Also tagrepr has been tweaked to include the global/delta names,
toggleable with the optional global_ kwarg.
Rattr now has its own lazy parsers for did + name. A more organized
codebase would probably have a separate Name type, but it just wasn't
worth the hassle.
And the abstraction classes have all been tweaked to require the
explicit Rbyd.repr() function for a CLI-friendly representation. Relying
on __str__ hurt readability and debugging, especially since Python
prefers __str__ over __repr__ when printing things.
The main difference between -t/--tree and -R/--tree-rbyd is that only
the latter shows all internal jumps (unconditional alt->alt), so it
makes sense to also hide internal branches (rbyd->rbyd).
Note that we already hide the rbyd->block branches in dbglfs.py.
Also added color-ignoring comparison operators to our internal
TreeBranch struct. This fixes an issue where our non-inner branch
merging logic could end up with identical branches with different
colors, resulting in different colorings per run. Not the end of the
world, but something we want to avoid.
This requires an additional traversal of the mtree just to precalculate
the mrid width (mbits provides an upper-bound, but the actual number of
mrids in any given mdir may be much less), but it makes the output look
nicer.
This is where the high-level structure of littlefs starts to reveal
itself.
This is also where a lot of really annoying Mtree vs Btree API questions
come to a head, like should Mtree.lookup return an Mdir or an Rattr?
What about Btree.lookup? What gets included in the returned path in all
of these? Well, at least this is an interesting exercise in rethinking
littlefs's internal APIs...
New classes:
- Mid - A representation of littlefs's metadata ids. I've just gone
ahead and included the block_size-dependent mbits as a field in every
Mid instance to try to make Mid operations easier.
It's not like we care about one extra word of storage in Python.
- Mdir - Again, we intentionally _don't_ inherit Rbyd to try to reduce
type errors, though Mdirs really are just Rbyds in this design.
- Mtree - The skeleton of littlefs. Tricky bits include traversing the
mroot chain and handling mroot-inlined mdirs. Note mroots are included
in the mdir/mid iteration methods.
Getting the tree renderers all working again was a real pain in the ass.
Just some minor tweaks:
- rbydaddr: Return list instead of tuple, note we rely on the type
distinction in Rbyd.fetch now.
- tagrepr: Rename w -> weight.
altas, and to a lesser extend altns, are just too problematic for our
rbyd-append algorithm.
Main issue is these break our "narrowing" invariant, where each alt only
ever decreases the bounds.
I wanted to use altas to simplify lfsr_rbyd_appendcompaction, but
decided it wasn't worth it. Handling them correctly would require adding
a number of special cases to lfsr_rbyd_appendrat, adding complexity to
an already incredibly complex function.
---
Fortunately, we don't really need altns/altas on-disk, but we _do_ need
a way to mark alts as unreachable internally in order to know when we
can collapse alts when recoloring (at this point bounds information is
lost).
I was originally going to use the alt's sign bit for this, but it turns
out we already have this information thanks to setting jump=0 to assert
that an alt is unreachable. So no explicit flag needed!
This ends up saving a surprising amount of code for what is only a
couple lines of changes:
code stack ctx
before: 38512 2624 640
after: 38440 (-0.2%) 2624 (+0.0%) 640 (+0.0%)
This was quite a puzzle.
The problem: How do we detect corrupt mdirs?
Seems like a simple question, but we can't just rely on mdir cksums. Our
mdirs are independently updateable logs, and logs have this annoying
tendency to "rollback" to previously valid states when corrupted.
Rollback issues aren't littlefs-specific, but what _is_ littlefs-
specific is that when one mdir rolls back, it can disagree with other
mdirs, resulting in wildly incorrect filesystem state.
To solve this, or at least protect against disagreeable mdirs, we need
to somehow include the state of all other mdirs in each mdir commit.
---
The first thought: Why not use gstate?
We already have a system for storing distributed state. If we add the
xor of all of our mdir cksums, we can rebuild it during mount and verify
that nothing changed:
.--------. .--------. .--------. .--------.
.| mdir 0 | .| mdir 1 | .| mdir 2 | .| mdir 3 |
|| | || | || | || |
|| gdelta | || gdelta | || gdelta | || gdelta |
|'-----|--' |'-----|--' |'-----|--' |'-----|--'
'------|-' '------|-' '------|-' '------|-'
'--.------' '--.------' '--.------' '--.------'
cksum | cksum | cksum | cksum |
| | v | v | v |
'---------> xor -------> xor -------> xor -------> gcksum
| v v v =?
'---------> xor -------> xor -------> xor ---> gcksum
Unfortunately it's not that easy. Consider what this looks like
mathematically (g is our gcksum, c_i is an mdir cksum, d_i is a
gcksumdelta, and +/-/sum is xor):
g = sum(c_i) = sum(d_i)
If we solve for a new gcksumdelta, d_i:
d_i = g' - g
d_i = g + c_i - g
d_i = c_i
The gcksum cancels itself out! We're left with an equation that depends
only on the current mdir, which doesn't help us at all.
Next thought: What if we permute the gcksum with a function t before
distributing it over our gcksumdeltas?
.--------. .--------. .--------. .--------.
.| mdir 0 | .| mdir 1 | .| mdir 2 | .| mdir 3 |
|| | || | || | || |
|| gdelta | || gdelta | || gdelta | || gdelta |
|'-----|--' |'-----|--' |'-----|--' |'-----|--'
'------|-' '------|-' '------|-' '------|-'
'--.------' '--.------' '--.------' '--.------'
cksum | cksum | cksum | cksum |
| | v | v | v |
'---------> xor -------> xor -------> xor -------> gcksum
| | | | .--t--'
| | | | '-> t(gcksum)
| v v v =?
'---------> xor -------> xor -------> xor ---> t(gcksum)
In math terms:
t(g) = t(sum(c_i)) = sum(d_i)
In order for this to work, t needs to be non-linear. If t is linear, the
same thing happens:
d_i = t(g') - t(g)
d_i = t(g + c_i) - t(g)
d_i = t(g) + t(c_i) - t(g)
d_i = t(c_i)
This was quite funny/frustrating (funnistrating?) during development,
because it means a lot of seemingly obvious functions don't work!
- t(g) = g - Doesn't work
- t(g) = crc32c(g) - Doesn't work because crc32cs are linear
- t(g) = g^2 in GF(2^n) - g^2 is linear in GF(2^n)!?
Fortunately, powers coprime with 2 finally give us a non-linear function
in GF(2^n), so t(g) = g^3 works:
d_i = g'^3 - g^3
d_i = (g + c_i)^3 - g^3
d_i = (g^2 + gc_i + gc_i + c_i^2)(g + c_i) - g^3
d_i = (g^2 + c_i^2)(g + c_i) - g^3
d_i = g^3 + gc_i^2 + g^2c_i + c_i^3 - g^3
d_i = gc_i^2 + g^2c_i + c_i^3
---
Bleh, now we need to implement finite-field operations? Well, not
entirely!
Note that our algorithm never uses division. This means we don't need a
full finite-field (+, -, *, /), but can get away with a finite-ring (+,
-, *). And conveniently for us, our crc32c polynomial defines a ring
epimorphic to a 31-bit finite-field.
All we need to do is define crc32c multiplication as polynomial
multiplication mod our crc32c polynomial:
crc32cmul(a, b) = pmod(pmul(a, b), P)
And since crc32c is more-or-less just pmod(x, P), this lets us take
advantage of any crc32c hardware/tables that may be available.
---
Bunch of notes:
- Our 2^n-bit crc-ring maps to a 2^n-1-bit finite-field because our crc
polynomial is defined as P(x) = Q(x)(x + 1), where Q(x) is a 2^n-1-bit
irreducible polynomial.
This is a common crc construction as it provides optimal odd-bit/2-bit
error detection, so it shouldn't be too difficult to adapt to other
crc sizes.
- t(g) = g^3 is not the only function that works, but it turns out to be
a pretty good one:
- 3 and 2^(2^n-1)-1 are coprime, which means our function t(g) = g^3
provides a one-to-one mapping in the underlying fields of all crc
rings of size 2^(2^n).
We know 3 and 2^(2^n-1)-1 are coprime because 2^(2^n-1)-1 =
2^(2^n)-1 (a Fermat number) - 2^(2^n-1) (a power-of-2), and 3
divides Fermat numbers >=3 (A023394) and is not 2.
- Our delta, when viewed as a polynomial in g: d(g) = gc^2 + g^2c +
c^3, has degree 2, which implies there are at most 2 solutions or
1-bit of information loss in the underlying field.
This is optimal since the original definition already had 2
solutions before we even chose a function:
d(g) = t(g + c) - t(g)
d(g) = t(g + c) - t((g + c) - c)
d(g) = t((g + c) + c) - t(g + c)
d(g) = d(g + c)
Though note the mapping of our crc-ring to the underlying field
already represents 1-bit of information loss.
- If you're using a cryptographic hash or other non-crc, you should
probably just use an equal sized finite-field.
Though note changing from a 2^n-1-bit field to a 2^n-bit field does
change the math a bit, with t(g) = g^7 being a better non-linear
function:
- 7 is the smallest odd-number coprime with 2^n-1, a Fermat number,
which makes t(g) = g^7 a one-to-one mapping.
3 humorously divides all 2^n-1 Fermat numbers.
- Expanding delta with t(g) = g^7 gives us a 6 degree polynomial,
which implies at most 6 solutions or ~3-bits of information loss.
This isn't actually the best you can do, some exhaustive searching
over small fields (<=2^16) suggests t(g) = g^(2^(n-1)-1) _might_ be
optimal, but that's a heck of a lot more multiplications.
- Because our crc32cs preserve parity/are epimorphic to parity bits,
addition (xor) and multiplication (crc32cmul) also preserve parity,
which can be used to show our entire gcksum system preserves parity.
This is quite neat, and means we are guaranteed to detect any odd
number of bit-errors across the entire filesystem.
- Another idea was to use two different addition operations: xor and
overflowing addition (or mod a prime).
This probably would have worked, but lacks the rigor of the above
solution.
- You might think an RS-like construction would help here, where g =
sum(c_ia^i), but this suffers from the same problem:
d_i = g' - g
d_i = g + c_ia^i - g
d_i = c_ia^i
Nothing here depends on anything outside of the current mdir.
- Another question is should we be using an RS-like construction anyways
to include location information in our gcksum?
Maybe in another system, but I don't think it's necessary in littlefs.
While our mdir are independently updateable, they aren't _entirely_
independent. The location of each mdir is stored in either the mtree
or a parent mdir, so it always gets mixed into the gcksum somewhere.
The only exception being the mrootanchor which is always at the fixed
blocks 0x{0,1}.
- This does _not_ catch "global-rollback" issues, where the most recent
commit in the entire filesystem is corrupted, revealing an older, but
still valid, filesystem state.
But as far as I am aware this is just a fundamental limitation of
powerloss-resilient filesystems, short of doing destructive
operations.
At the very least, exposing the gcksum would allow the user to store
it externally and prevent this issue.
---
Implementation details:
- Our gcksumdelta depends on the rbyd's cksum, so there's a catch-22 if
we include it in the rbyd itself.
We can avoid this by including it in the commit tags (actually the
separate canonical cksum makes this easier than it would have been
earlier), but this does mean LFSR_TAG_GCKSUMDELTA is not an
LFSR_TAG_GDELTA subtype. Unfortunate but not a dealbreaker.
- Reading/writing the gcksumdelta gets a bit annoying with it not being
in the rbyd. For now I've extended the low-level lfsr_rbyd_fetch_/
lfsr_rbyd_appendcksum_ to accept an optional gcksumdelta pointer,
which is a bit awkward, but I don't know of a better solution.
- Unlike the grm, _every_ mdir commit involves the gcksum, which means
we either need to propagate the gcksumdelta up the mroot chain
correctly, or somehow keep track of partially flushed gcksumdeltas.
To make this work I modified the low-level lfsr_mdir_commit__
functions to accept start_rid=-2 to indicate when gcksumdeltas should
be flushed.
It's a bit of a hack, but I think it might make sense to extend this
to all gdeltas eventually.
The gcksum cost both code and RAM, but I think it's well worth it for
removing an entire category of filesystem corruption:
code stack ctx
before: 37796 2608 620
after: 38428 (+1.7%) 2640 (+1.2%) 644 (+3.9%)
Now that we perturb commit cksums with the odd-parity zero, the q-bit no
longer serves a purpose other than extra debug info. But this is a
double-edged sword, because redundant info just means another thing that
can go wrong.
For example, should we assert? If the q-bit doesn't reflect the
previous-perturb state it's a bug, but the only thing that would break
would be the q-bit itself. And if we don't assert what's the point of
keeping the q-bit around?
Dropping the q-bit avoids answering this question and saves a bit of
code:
code stack ctx
before: 37772 2608 620
after: 37768 (-0.0%) 2608 (+0.0%) 620 (+0.0%)
I've been unhappy with LFSR_TAG_ORPHAN for a while now. While it's true
these represent orphaned files, they also represent zombied files. And
as long as a reference to the file exists in-RAM, I find it hard to say
these files are truely "orphaned".
We're also just using the term "orphan" for too many things.
Really this tag just represents an mid reservation. The term stickynote
works well enough for this, and fits in with the other internal tag,
LFSR_TAG_BOOKMARK.
Moved local import hack behind if __name__ == "__main__"
These scripts aren't really intended to be used as python libraries.
Still, it's useful to import them for debugging and to get access to
their juicy internals.
This seems like a more fitting name now that this script has evolved
into more of a general purpose high-level CSV tool.
Unfortunately this does conflict with the standard csv module in Python,
breaking every script that imports csv (which is most of them).
Fortunately, Python is flexible enough to let us remove the current
directory before imports with a bit of an ugly hack:
# prevent local imports
__import__('sys').path.pop(0)
These scripts are intended to be standalone anyways, so this is probably
a good pattern to adopt.
This extends Rbyd.fetch to accept another rbyd, in which case we inherit
the RAM-backed block without rereading it from disk. This avoids an
issue where shrubs can become corrupted if the disk is being
simultaneously written and debugged.
Normally we can detect the checksum mismatch and toss out the rbyd
during fetch, but shrub pointers don't include a checksum since they
assume the containing rbyd has already been checksummed.
It's interesting to note this even avoids the memory copy thanks to
Python's reference counting.
If we're fetching branches anyways, we might as well check that the
checksums match. This helps protect against infinite loops in B-tree
branches.
Also fixed an issue where we weren't xoring perturb state on finding an
explicit trunk.
Note this is equivalent to LFS_M_CKFETCHES in lfs.c.
---
This doesn't mean we always need LFS_M_CKFETCHES. Our dbg scripts just
need to be a little bit tougher because 1. running tests with -j creates
wildly corrupted and entangled littlefs images, and 2. Rbyd.fetch is
almost too forgiving in choosing the nearest trunk.
These work by keeping a set of all seen mroots as we descend down the
mroot chain. Simple, but it works.
The downside of this approach is that the mroot set grows unbounded, but
it's unlikely we'll ever have enough mroots in a system for this to
really matter.
This fixes scripts like dbgbmap.py getting stuck on intentional mroot
cycles created for testing. It's not a problem for a foreground script
to get stuck in an infinite loop, since you can just kill it, but a
background script getting stuck at 100% CPU is a bit more annoying.
This matches the style used in C, which is good for consistency:
a_really_long_function_name(
double_indent_after_first_newline(
single_indent_nested_newlines))
We were already doing this for multiline control-flow statements, simply
because I'm not sure how else you could indent this without making
things really confusing:
if a_really_long_function_name(
double_indent_after_first_newline(
single_indent_nested_newlines)):
do_the_thing()
This was the only real difference style-wise between the Python code and
C code, so now both should be following roughly the same style (80 cols,
double-indent multiline exprs, prefix multiline binary ops, etc).
Mainly to avoid conflicts with match results m, this frees up the single
letter variables m for other purposes.
Choosing a two letter alias was surprisingly difficult, but mt is nice
in that it somewhat matches it (for itertools) and ft (for functools).
It would be nice to have a full 8-bit range for both user attrs and
system attrs, for both backwards compatibility and maximizing the
available attr space, but I think it just doesn't make sense from an API
perspective.
Sure we could finagle the user/sys bit into a flags argument, or provide
separate lfsr_getuattr/getsattr functions, but asking users to use a
9-bit int for higher-level operations (dynamic attrs, iteration, etc) is
a bit much...
So this reduces the two attr ranges down to 7-bits, requiring 8-bits
total to store all possible attr types in the current system:
TAG_ATTR 0x0400 v--- -1-a -aaa aaaa
TAG_UATTR 0x04aa v--- -1-- -aaa aaaa
TAG_SATTR 0x05aa v--- -1-1 -aaa aaaa
This really just affects scripts, since we haven't actually implemented
attributes yet.
Worst case we still have the 9-bit encoding space carved out, so we can
always add an additional set of attrs in the future if we start running
into attr pressure.
Or, you know, just turn on the subtype leb128 encoding the 8th subtype
bit is reserved for. Then you'd only be limited by internal driver
details, probably 24-bits per attr range if we make tags 32-bits
internally. Though this would probably come with quite a code cost...
Updating the canonical checksum should only depend on if the tag is a
trunkish tag (not a checksum tag), and not if the tag is in the current
trunk. The trunk parameter to lfsr_rbyd_fetch should have no effect on
the canonical checksum.
Fixed in boath lfsr_rbyd_fetch and scripts.
Curiously no code changes:
code stack
before: 36416 2616
after: 36416 (+0.0%) 2616 (+0.0%
I've been scratching my head over our rbyd perturb scheme. It's gotten
rather clunky with needing to xor valid bits and whatnot.
But it's tricky with needing erased-state to be included in parity bits,
while at the same time excluded from our canonical checksum. If only
there was some way to flip the checksums parity without changing its
value...
Enter the crc32c odd-parity zero: 0xfca42daf!
This bends the definition of zero a bit, but it is one of two numbers in
our crc32c-ring with a very interesting property:
crc32c(m) == crc32c(m xor 0xfca42daf) xor 0xfca42daf // odd-p zero
crc32c(m) == crc32c(m xor 0x00000000) xor 0x00000000 // even-p zero
Recall that crc32c's polynomial, 0x11edc6f41, is composed of two
polynomials: 0x3, the parity polynomial, and 0xf5b4253f, a maximally
sized irreducible polynomial. Because our polynomial breaks down into
two smaller polynomials, our crc32c space turns out to not be a field,
but rather a ring containing two smaller sub-fields. Because these
sub-fields are defined by their polynomials, one is the 31-bit crc
defined by the polynomial 0xf5b4253f, while the other is the current
parity.
We can move in the parity sub-field without changing our position in the
31-bit crc sub-field by xoring with a number that is one in the parity
sub-field, but zero in the 31-bit crc sub-field.
This number happens to be 0xf5b4253f (0xfca42daf bit-reversed)!
(crcs being bit-reversed will never not be annoying)
So long story short, xoring any crc32c with 0xfca42daf will change its
parity but not its value.
---
An that's basically our new perturb scheme. If we need to perturb, xor
with 0xfca42daf to change the parity, and after calculating/validating
the checksum, xor with 0xfca42daf to get our canonical checksum.
Isn't that neat!
There was one small hiccup: At first I assumed you could continue
including the valid bits in the checksum, which would have been nice for
bulk checksumming. But this doesn't work because while valid bits cancel
out so the parity doesn't change, changing valid bits _does_ change the
underlying 31-bit crc, poisoning our checksum and making everything a
mess.
So we still need to mask out valid bits, which is a bit annoying.
But then I stumbled on the funny realization that by masking our valid
bits, we accidentally end up with a fully functional parity scheme.
Because valid bits _don't_ include the previous valid bit, we can figure
out the parity for not only the entire commit, but also each individual
tag:
80 03 00 08 6c 69 74 74 6c 65 66 73 80
^'----------------.---------------' ^
| | |
v + parity = v'
Or more simply:
80 03 00 08 6c 69 74 74 6c 65 66 73 80
'----------------.----------------' ^
| |
parity = v'
Double neat!
Some other notes:
- By keeping the commit checksum perturbed, but not the canonical
checksum, the perturb state is self-validating. We no longer need to
explicitly check the previous-perturb-bit (q) to avoid the perturb
hole we ran into previously.
I'm still keeping the previous-perturb-bit (q) around, since it's
useful for debugging. We still need to know the perturb state
internally at all times in order to xor out the canonical checksum
correctly anyways.
- Thanks to all of our perturb iterations, we now know how to remove the
valid bits from the checksum easily:
cksum ^= 0x00000080 & (tag >> 8)
This makes the whole omitting-valid-bits thing less of a pain point.
- It wasn't actually worth it to perturb the checksum when building
commits, vs manually flipping each valid bit, as this would have made
our internal appendattr API really weird.
At least the perturbed checksum made fetch a bit simpler.
Not sure exactly how to draw this with our perturb scheme diagrams,
maybe something like this?
.---+---+---+---. \ \ \ \
|v| tag | | | | |
+---+---+---+---+ | | | |
| commit | | | | |
| | +-. | | |
+---+---+---+---+ / | | | |
|v|qp-------------->p>p-->p .
+---+---+---+---+ | . . .
| cksum | | . . .
+---+---+---+---+ | . . .
| padding | | . . .
| | | . . .
+---+---+---+---+ | | | |
|v------------------' | | |
+---+---+---+---+ | | |
| commit | +-. | +- rbyd
| | | | | | cksum
+---+---+---+---+ / | +-. /
|v----------------------' | |
+-------+---+---+ / |
| cksum ----------------'
+---+---+---+---+
| padding |
| |
+---+---+---+---+
| erased |
| |
. .
. .
---
Code changes were minimal, saving a tiny bit of code:
code stack
before: 36368 2664
after: 36352 (-0.0%) 2672 (+0.3%)
There was a stack bump in lfsr_bd_readtag, but as far as I can tell it's
just compiler noise? I poked around a bit but couldn't figure out why it
changed...
