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Fix several wear-leveling issues found in lfs_alloc_reset #487

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Merged
merged 5 commits into from
Dec 4, 2020
Merged

Fix several wear-leveling issues found in lfs_alloc_reset #487

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480cdd9
Fixed incorrect modulus in lfs_alloc_reset
geky Nov 14, 2020
1ae4b36
Removed unnecessary randomization of offsets in lfs_alloc_reset
geky Nov 20, 2020
f215027
Switched to CRC as seed collection function instead of xor
geky Nov 20, 2020
0ea2871
Fixed typo in scripts/readtree.py
geky Nov 22, 2020
0aba71d
Fixed single unchecked bit during commit verification
geky Nov 22, 2020
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39 changes: 26 additions & 13 deletions lfs.c
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Original file line number Diff line number Diff line change
Expand Up @@ -452,14 +452,16 @@ static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
return 0;
}

// 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->cfg->block_count;
}

// Invalidate the lookahead buffer. This is done during mounting and
// failed traversals
static void lfs_alloc_reset(lfs_t *lfs) {
lfs->free.off = lfs->seed % lfs->cfg->block_size;
// 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);
Expand Down Expand Up @@ -505,7 +507,7 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
memset(lfs->free.buffer, 0, lfs->cfg->lookahead_size);
int err = lfs_fs_traverseraw(lfs, lfs_alloc_lookahead, lfs, true);
if (err) {
lfs_alloc_reset(lfs);
lfs_alloc_drop(lfs);
return err;
}
}
Expand Down Expand Up @@ -870,8 +872,10 @@ static lfs_stag_t lfs_dir_fetchmatch(lfs_t *lfs,
ptag ^= (lfs_tag_t)(lfs_tag_chunk(tag) & 1U) << 31;

// toss our crc into the filesystem seed for
// pseudorandom numbers
lfs->seed ^= crc;
// pseudorandom numbers, note we use another crc here
// as a collection function because it is sufficiently
// random and convenient
lfs->seed = lfs_crc(lfs->seed, &crc, sizeof(crc));

// update with what's found so far
besttag = tempbesttag;
Expand Down Expand Up @@ -1261,12 +1265,13 @@ static int lfs_dir_commitattr(lfs_t *lfs, struct lfs_commit *commit,
}

static int lfs_dir_commitcrc(lfs_t *lfs, struct lfs_commit *commit) {
const lfs_off_t off1 = commit->off;
const uint32_t crc1 = commit->crc;
// align to program units
const lfs_off_t end = lfs_alignup(off1 + 2*sizeof(uint32_t),
const lfs_off_t end = lfs_alignup(commit->off + 2*sizeof(uint32_t),
lfs->cfg->prog_size);

lfs_off_t off1 = 0;
uint32_t crc1 = 0;

// create crc tags to fill up remainder of commit, note that
// padding is not crced, which lets fetches skip padding but
// makes committing a bit more complicated
Expand Down Expand Up @@ -1302,6 +1307,12 @@ static int lfs_dir_commitcrc(lfs_t *lfs, struct lfs_commit *commit) {
return err;
}

// keep track of non-padding checksum to verify
if (off1 == 0) {
off1 = commit->off + sizeof(uint32_t);
crc1 = commit->crc;
}

commit->off += sizeof(tag)+lfs_tag_size(tag);
commit->ptag = tag ^ ((lfs_tag_t)reset << 31);
commit->crc = 0xffffffff; // reset crc for next "commit"
Expand All @@ -1315,7 +1326,7 @@ static int lfs_dir_commitcrc(lfs_t *lfs, struct lfs_commit *commit) {

// successful commit, check checksums to make sure
lfs_off_t off = commit->begin;
lfs_off_t noff = off1 + sizeof(uint32_t);
lfs_off_t noff = off1;
while (off < end) {
uint32_t crc = 0xffffffff;
for (lfs_off_t i = off; i < noff+sizeof(uint32_t); i++) {
Expand Down Expand Up @@ -3797,8 +3808,10 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
lfs->gstate.tag += !lfs_tag_isvalid(lfs->gstate.tag);
lfs->gdisk = lfs->gstate;

// setup free lookahead
lfs_alloc_reset(lfs);
// setup free lookahead, to distribute allocations uniformly across
// boots, we start the allocator at a random location
lfs->free.off = lfs->seed % lfs->cfg->block_count;
lfs_alloc_drop(lfs);

LFS_TRACE("lfs_mount -> %d", 0);
return 0;
Expand Down
2 changes: 1 addition & 1 deletion scripts/readtree.py
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Original file line number Diff line number Diff line change
Expand Up @@ -106,7 +106,7 @@ def main(args):
struct.unpack('<HH', superblock[1].data[0:4].ljust(4, b'\xff'))))
print("%-47s%s" % ("littlefs v%s.%s" % version,
"data (truncated, if it fits)"
if not any([args.no_truncate, args.tags, args.log, args.all]) else ""))
if not any([args.no_truncate, args.log, args.all]) else ""))

# print gstate
print("gstate 0x%s" % ''.join('%02x' % c for c in gstate))
Expand Down