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smb: client: compress: add code docs to lz77.c 

Document parts of the code, especially the apparently
non-sense parts.

Other:
- change pointer increment constants to sizeof() values

Signed-off-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>
master
Enzo Matsumiya 2026-04-13 16:07:11 -03:00 committed by Steve French
parent 4460e9c68d
commit 71179a5ee9
1 changed files with 73 additions and 8 deletions
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81
fs/smb/client/compress/lz77.c
View File

@ -15,6 +15,20 @@
/*
* Compression parameters.
*
* LZ77_MATCH_MAX_DIST: Farthest back a match can be from current position (can be 1 - 8K).
* LZ77_HASH_LOG:
* LZ77_HASH_SIZE: ilog2 hash size (recommended to be 13 - 18, default 15 (hash size
* 32k)).
* LZ77_RSTEP_SIZE: Number of bytes to read from input buffer for hashing and initial
* match check (default 4 bytes, this effectivelly makes this the min
* match len).
* LZ77_MSTEP_SIZE: Number of bytes to extend-compare a found match (default 8 bytes).
* LZ77_SKIP_TRIGGER: ilog2 value for adaptive skipping, i.e. to progressively skip input
* bytes when we can't find matches. Default is 4.
* Higher values (>0) will decrease compression time, but will result
* in worse compression ratio. Lower values will give better
* compression ratio (more matches found), but will increase time.
*/
#define LZ77_MATCH_MAX_DIST SZ_8K
#define LZ77_HASH_LOG 15
@ -86,6 +100,19 @@ static __always_inline u32 lz77_match_len(const void *match, const void *cur, co
return (cur - start);
}
/**
* lz77_encode_match() - Match encoding.
* @dst: compressed buffer
* @nib: pointer to an address in @dst
* @dist: match distance
* @len: match length
*
* Assumes all args were previously checked.
*
* Return: @dst advanced to new position
*
* Ref: MS-XCA 2.3.4 "Plain LZ77 Compression Algorithm Details" - "Processing"
*/
static __always_inline void *lz77_encode_match(void *dst, void **nib, u16 dist, u32 len)
{
len -= 3;
@ -95,12 +122,12 @@ static __always_inline void *lz77_encode_match(void *dst, void **nib, u16 dist,
if (len < 7) {
lz77_write16(dst, dist + len);
return dst + 2;
return dst + sizeof(u16);
}
dist |= 7;
lz77_write16(dst, dist);
dst += 2;
dst += sizeof(u16);
len -= 7;
if (!*nib) {
@ -130,16 +157,32 @@ static __always_inline void *lz77_encode_match(void *dst, void **nib, u16 dist,
if (len <= 0xffff) {
lz77_write16(dst, len);
return dst + 2;
return dst + sizeof(u16);
}
lz77_write16(dst, 0);
dst += 2;
dst += sizeof(u16);
lz77_write32(dst, len);
return dst + 4;
return dst + sizeof(u32);
}
/**
* lz77_encode_literals() - Literals encoding.
* @start: where to start copying literals (uncompressed buffer)
* @end: when to stop copying (uncompressed buffer)
* @dst: compressed buffer
* @f: pointer to current flag value
* @fc: pointer to current flag count
* @fp: pointer to current flag address
*
* Batch copy literals from @start to @dst, updating flag values accordingly.
* Assumes all args were previously checked.
*
* Return: @dst advanced to new position
*
* MS-XCA 2.3.4 "Plain LZ77 Compression Algorithm Details" - "Processing"
*/
static __always_inline void *lz77_encode_literals(const void *start, const void *end, void *dst,
long *f, u32 *fc, void **fp)
{
@ -160,7 +203,7 @@ static __always_inline void *lz77_encode_literals(const void *start, const void
lz77_write32(*fp, *f);
*fc = 0;
*fp = dst;
dst += 4;
dst += sizeof(u32);
}
} while (start < end);
@ -188,7 +231,7 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen
rlim = end - LZ77_MSTEP_SIZE; /* read limit (for lz77_match_len()) */
dstp = dst;
flag_pos = dstp;
dstp += 4;
dstp += sizeof(u32);
nib = NULL;
htable = kvcalloc(LZ77_HASH_SIZE, sizeof(*htable), GFP_KERNEL);
@ -197,6 +240,10 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen
LZ77_PREFETCH(srcp + LZ77_RSTEP_SIZE);
/*
* Adjust @srcp so we don't get a false positive match on first iteration.
* Then prepare hash for first loop iteration (don't advance @srcp again).
*/
hash = lz77_hash(lz77_read32(srcp++));
htable[hash] = 0;
hash = lz77_hash(lz77_read32(srcp));
@ -219,6 +266,14 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen
srcp = next;
next += step;
/*
* Adaptive skipping.
*
* Increment @step every (1 << LZ77_SKIP_TRIGGER, 16 in our case) bytes
* without a match.
* Reset to 1 when a match is found.
*/
step = (skip++ >> LZ77_SKIP_TRIGGER);
if (unlikely(next > rlim))
goto out;
@ -229,6 +284,16 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen
} while (likely(match + LZ77_MATCH_MAX_DIST < srcp) ||
lz77_read32(match) != lz77_read32(srcp));
/*
* Match found. Warm/cold path; begin parsing @srcp and writing to @dstp:
* - flush literals
* - compute match length (*)
* - encode match
*
* (*) Current minimum match length is defined by the memory read size above, so
* here we already know that we have 4 matching bytes, but it's just faster to
* redundantly compute it again in lz77_match_len() than to adjust pointers/len.
*/
dstp = lz77_encode_literals(anchor, srcp, dstp, &flag, &flag_count, &flag_pos);
len = lz77_match_len(match, srcp, end);
dstp = lz77_encode_match(dstp, &nib, srcp - match, len);
@ -243,7 +308,7 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen
lz77_write32(flag_pos, flag);
flag_count = 0;
flag_pos = dstp;
dstp += 4;
dstp += sizeof(u32);
}
if (unlikely(srcp > rlim))