Relaxed alignment requirements for lfs_malloc

The only reason we needed this alignment was for the lookahead buffer.

Now that the lookahead buffer is relaxed to operate on bytes, we can
relax our malloc alignment requirement all the way down to the byte
level, since we mainly use lfs_malloc to allocate byte-level buffers.

This does introduce a risk that we might need word-level mallocs in the
future. If that happens we will need to decide if changing the malloc
alignment is a breaking change, or gate alignment requirements behind
user provided defines.

Found by HiFiPhile.

lfs.c

@@ -596,42 +596,48 @@ static int lfs_rawunmount(lfs_t *lfs);
 #ifndef LFS_READONLY
 static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
     lfs_t *lfs = (lfs_t*)p;
-    lfs_block_t off = ((block - lfs->free.off)
+    lfs_block_t off = ((block - lfs->lookahead.start)
             + lfs->block_count) % lfs->block_count;
 
-    if (off < lfs->free.size) {
-        lfs->free.buffer[off / 32] |= 1U << (off % 32);
+    if (off < lfs->lookahead.size) {
+        lfs->lookahead.buffer[off / 8] |= 1U << (off % 8);
     }
 
     return 0;
 }
 #endif
 
-// indicate allocated blocks have been committed into the filesystem, this
-// is to prevent blocks from being garbage collected in the middle of a
-// commit operation
-static void lfs_alloc_ack(lfs_t *lfs) {
-    lfs->free.ack = lfs->block_count;
+// allocations should call this when all allocated blocks are committed to
+// the filesystem
+//
+// after a checkpoint, the block allocator may realloc any untracked blocks
+static void lfs_alloc_ckpoint(lfs_t *lfs) {
+    lfs->lookahead.ckpoint = lfs->block_count;
 }
 
 // drop the lookahead buffer, this is done during mounting and failed
 // traversals in order to avoid invalid lookahead state
 static void lfs_alloc_drop(lfs_t *lfs) {
-    lfs->free.size = 0;
-    lfs->free.i = 0;
-    lfs_alloc_ack(lfs);
+    lfs->lookahead.size = 0;
+    lfs->lookahead.next = 0;
+    lfs_alloc_ckpoint(lfs);
 }
 
 #ifndef LFS_READONLY
 static int lfs_fs_rawgc(lfs_t *lfs) {
-    // Move free offset at the first unused block (lfs->free.i)
-    // lfs->free.i is equal lfs->free.size when all blocks are used
-    lfs->free.off = (lfs->free.off + lfs->free.i) % lfs->block_count;
-    lfs->free.size = lfs_min(8*lfs->cfg->lookahead_size, lfs->free.ack);
-    lfs->free.i = 0;
+    // move lookahead buffer to the first unused block
+    //
+    // note we limit the lookahead buffer to at most the amount of blocks
+    // checkpointed, this prevents the math in lfs_alloc from underflowing
+    lfs->lookahead.start = (lfs->lookahead.start + lfs->lookahead.next) 
+            % lfs->block_count;
+    lfs->lookahead.next = 0;
+    lfs->lookahead.size = lfs_min(
+            8*lfs->cfg->lookahead_size,
+            lfs->lookahead.ckpoint);
 
     // find mask of free blocks from tree
-    memset(lfs->free.buffer, 0, lfs->cfg->lookahead_size);
+    memset(lfs->lookahead.buffer, 0, lfs->cfg->lookahead_size);
     int err = lfs_fs_rawtraverse(lfs, lfs_alloc_lookahead, lfs, true);
     if (err) {
         lfs_alloc_drop(lfs);
@@ -645,35 +651,48 @@ static int lfs_fs_rawgc(lfs_t *lfs) {
 #ifndef LFS_READONLY
 static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
     while (true) {
-        while (lfs->free.i != lfs->free.size) {
-            lfs_block_t off = lfs->free.i;
-            lfs->free.i += 1;
-            lfs->free.ack -= 1;
-
-            if (!(lfs->free.buffer[off / 32] & (1U << (off % 32)))) {
+        // scan our lookahead buffer for free blocks
+        while (lfs->lookahead.next < lfs->lookahead.size) {
+            if (!(lfs->lookahead.buffer[lfs->lookahead.next / 8]
+                    & (1U << (lfs->lookahead.next % 8)))) {
                 // found a free block
-                *block = (lfs->free.off + off) % lfs->block_count;
+                *block = (lfs->lookahead.start + lfs->lookahead.next)
+                        % lfs->block_count;
 
-                // eagerly find next off so an alloc ack can
-                // discredit old lookahead blocks
-                while (lfs->free.i != lfs->free.size &&
-                        (lfs->free.buffer[lfs->free.i / 32]
-                            & (1U << (lfs->free.i % 32)))) {
-                    lfs->free.i += 1;
-                    lfs->free.ack -= 1;
+                // eagerly find next free block to maximize how many blocks
+                // lfs_alloc_ckpoint makes available for scanning
+                while (true) {
+                    lfs->lookahead.next += 1;
+                    lfs->lookahead.ckpoint -= 1;
+
+                    if (lfs->lookahead.next >= lfs->lookahead.size
+                            || !(lfs->lookahead.buffer[lfs->lookahead.next / 8]
+                                & (1U << (lfs->lookahead.next % 8)))) {
+                        return 0;
+                    }
                 }
-
-                return 0;
             }
+
+            lfs->lookahead.next += 1;
+            lfs->lookahead.ckpoint -= 1;
         }
 
-        // check if we have looked at all blocks since last ack
-        if (lfs->free.ack == 0) {
-            LFS_ERROR("No more free space %"PRIu32,
-                    lfs->free.i + lfs->free.off);
+        // In order to keep our block allocator from spinning forever when our
+        // filesystem is full, we mark points where there are no in-flight
+        // allocations with a checkpoint before starting a set of allocations.
+        //
+        // If we've looked at all blocks since the last checkpoint, we report
+        // the filesystem as out of storage.
+        //
+        if (lfs->lookahead.ckpoint <= 0) {
+            LFS_ERROR("No more free space 0x%"PRIx32,
+                    (lfs->lookahead.start + lfs->lookahead.next)
+                        % lfs->cfg->block_count);
             return LFS_ERR_NOSPC;
         }
 
+        // No blocks in our lookahead buffer, we need to scan the filesystem for
+        // unused blocks in the next lookahead window.
         int err = lfs_fs_rawgc(lfs);
         if(err) {
             return err;
@@ -2586,7 +2605,7 @@ static int lfs_rawmkdir(lfs_t *lfs, const char *path) {
     }
 
     // build up new directory
-    lfs_alloc_ack(lfs);
+    lfs_alloc_ckpoint(lfs);
     lfs_mdir_t dir;
     err = lfs_dir_alloc(lfs, &dir);
     if (err) {
@@ -3272,7 +3291,7 @@ relocate:
 #ifndef LFS_READONLY
 static int lfs_file_outline(lfs_t *lfs, lfs_file_t *file) {
     file->off = file->pos;
-    lfs_alloc_ack(lfs);
+    lfs_alloc_ckpoint(lfs);
     int err = lfs_file_relocate(lfs, file);
     if (err) {
         return err;
@@ -3535,7 +3554,7 @@ static lfs_ssize_t lfs_file_flushedwrite(lfs_t *lfs, lfs_file_t *file,
                 }
 
                 // extend file with new blocks
-                lfs_alloc_ack(lfs);
+                lfs_alloc_ckpoint(lfs);
                 int err = lfs_ctz_extend(lfs, &file->cache, &lfs->rcache,
                         file->block, file->pos,
                         &file->block, &file->off);
@@ -3578,7 +3597,7 @@ relocate:
         data += diff;
         nsize -= diff;
 
-        lfs_alloc_ack(lfs);
+        lfs_alloc_ckpoint(lfs);
     }
 
     return size;
@@ -4180,15 +4199,14 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     lfs_cache_zero(lfs, &lfs->rcache);
     lfs_cache_zero(lfs, &lfs->pcache);
 
-    // setup lookahead, must be multiple of 64-bits, 32-bit aligned
+    // setup lookahead buffer, note mount finishes initializing this after
+    // we establish a decent pseudo-random seed
     LFS_ASSERT(lfs->cfg->lookahead_size > 0);
-    LFS_ASSERT(lfs->cfg->lookahead_size % 8 == 0 &&
-            (uintptr_t)lfs->cfg->lookahead_buffer % 4 == 0);
     if (lfs->cfg->lookahead_buffer) {
-        lfs->free.buffer = lfs->cfg->lookahead_buffer;
+        lfs->lookahead.buffer = lfs->cfg->lookahead_buffer;
     } else {
-        lfs->free.buffer = lfs_malloc(lfs->cfg->lookahead_size);
-        if (!lfs->free.buffer) {
+        lfs->lookahead.buffer = lfs_malloc(lfs->cfg->lookahead_size);
+        if (!lfs->lookahead.buffer) {
             err = LFS_ERR_NOMEM;
             goto cleanup;
         }
@@ -4245,7 +4263,7 @@ static int lfs_deinit(lfs_t *lfs) {
     }
 
     if (!lfs->cfg->lookahead_buffer) {
-        lfs_free(lfs->free.buffer);
+        lfs_free(lfs->lookahead.buffer);
     }
 
     return 0;
@@ -4265,12 +4283,12 @@ static int lfs_rawformat(lfs_t *lfs, const struct lfs_config *cfg) {
         LFS_ASSERT(cfg->block_count != 0);
 
         // create free lookahead
-        memset(lfs->free.buffer, 0, lfs->cfg->lookahead_size);
-        lfs->free.off = 0;
-        lfs->free.size = lfs_min(8*lfs->cfg->lookahead_size,
+        memset(lfs->lookahead.buffer, 0, lfs->cfg->lookahead_size);
+        lfs->lookahead.start = 0;
+        lfs->lookahead.size = lfs_min(8*lfs->cfg->lookahead_size,
                 lfs->block_count);
-        lfs->free.i = 0;
-        lfs_alloc_ack(lfs);
+        lfs->lookahead.next = 0;
+        lfs_alloc_ckpoint(lfs);
 
         // create root dir
         lfs_mdir_t root;
@@ -4478,7 +4496,7 @@ static int lfs_rawmount(lfs_t *lfs, const struct lfs_config *cfg) {
 
     // setup free lookahead, to distribute allocations uniformly across
     // boots, we start the allocator at a random location
-    lfs->free.off = lfs->seed % lfs->block_count;
+    lfs->lookahead.start = lfs->seed % lfs->block_count;
     lfs_alloc_drop(lfs);
 
     return 0;
@@ -5451,10 +5469,10 @@ static int lfs1_mount(lfs_t *lfs, struct lfs1 *lfs1,
         lfs->lfs1->root[1] = LFS_BLOCK_NULL;
 
         // setup free lookahead
-        lfs->free.off = 0;
-        lfs->free.size = 0;
-        lfs->free.i = 0;
-        lfs_alloc_ack(lfs);
+        lfs->lookahead.start = 0;
+        lfs->lookahead.size = 0;
+        lfs->lookahead.next = 0;
+        lfs_alloc_ckpoint(lfs);
 
         // load superblock
         lfs1_dir_t dir;

lfs.h

@@ -226,7 +226,7 @@ struct lfs_config {
     // Size of the lookahead buffer in bytes. A larger lookahead buffer
     // increases the number of blocks found during an allocation pass. The
     // lookahead buffer is stored as a compact bitmap, so each byte of RAM
-    // can track 8 blocks. Must be a multiple of 8.
+    // can track 8 blocks.
     lfs_size_t lookahead_size;
 
     // Optional statically allocated read buffer. Must be cache_size.
@@ -237,9 +237,8 @@ struct lfs_config {
     // By default lfs_malloc is used to allocate this buffer.
     void *prog_buffer;
 
-    // Optional statically allocated lookahead buffer. Must be lookahead_size
-    // and aligned to a 32-bit boundary. By default lfs_malloc is used to
-    // allocate this buffer.
+    // Optional statically allocated lookahead buffer. Must be lookahead_size.
+    // By default lfs_malloc is used to allocate this buffer.
     void *lookahead_buffer;
 
     // Optional upper limit on length of file names in bytes. No downside for
@@ -430,13 +429,13 @@ typedef struct lfs {
     lfs_gstate_t gdisk;
     lfs_gstate_t gdelta;
 
-    struct lfs_free {
-        lfs_block_t off;
+    struct lfs_lookahead {
+        lfs_block_t start;
         lfs_block_t size;
-        lfs_block_t i;
-        lfs_block_t ack;
-        uint32_t *buffer;
-    } free;
+        lfs_block_t next;
+        lfs_block_t ckpoint;
+        uint8_t *buffer;
+    } lookahead;
 
     const struct lfs_config *cfg;
     lfs_size_t block_count;

lfs_util.h

@@ -215,7 +215,9 @@ static inline uint32_t lfs_tobe32(uint32_t a) {
 uint32_t lfs_crc(uint32_t crc, const void *buffer, size_t size);
 
 // Allocate memory, only used if buffers are not provided to littlefs
-// Note, memory must be 64-bit aligned
+//
+// littlefs current has no alignment requirements, as it only allocates
+// byte-level buffers.
 static inline void *lfs_malloc(size_t size) {
 #ifndef LFS_NO_MALLOC
     return malloc(size);

tests/test_orphans.toml

@@ -98,7 +98,7 @@ code = '''
     lfs_mount(&lfs, cfg) => 0;
     // create an orphan
     lfs_mdir_t orphan;
-    lfs_alloc_ack(&lfs);
+    lfs_alloc_ckpoint(&lfs);
     lfs_dir_alloc(&lfs, &orphan) => 0;
     lfs_dir_commit(&lfs, &orphan, NULL, 0) => 0;
 
@@ -170,7 +170,7 @@ code = '''
     lfs_mount(&lfs, cfg) => 0;
     // create an orphan
     lfs_mdir_t orphan;
-    lfs_alloc_ack(&lfs);
+    lfs_alloc_ckpoint(&lfs);
     lfs_dir_alloc(&lfs, &orphan) => 0;
     lfs_dir_commit(&lfs, &orphan, NULL, 0) => 0;