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| author | Qu Wenruo <wqu@suse.com> | 2018-05-17 07:10:01 +0200 |
|---|---|---|
| committer | David Sterba <dsterba@suse.com> | 2018-05-29 18:13:00 +0200 |
| commit | 2a1f7c0cbddae68a34096d056fc7ff7b171bab17 (patch) | |
| tree | 16ceb972e6301404c21e50c45af6dc2ae8e05037 /fs/btrfs | |
| parent | btrfs: compression: Add linux/sizes.h for compression.h (diff) | |
| download | linux-2a1f7c0cbddae68a34096d056fc7ff7b171bab17.tar.xz linux-2a1f7c0cbddae68a34096d056fc7ff7b171bab17.zip | |
btrfs: lzo: document the compressed data format
Although it's not that complex, but such comment could still save
several minutes for newer reader/reviewer instead of inferring that from
the code.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor wording updates ]
Signed-off-by: David Sterba <dsterba@suse.com>
Diffstat (limited to 'fs/btrfs')
| -rw-r--r-- | fs/btrfs/lzo.c | 37 |
1 files changed, 37 insertions, 0 deletions
diff --git a/fs/btrfs/lzo.c b/fs/btrfs/lzo.c index 0667ea07f766..9591927022ba 100644 --- a/fs/btrfs/lzo.c +++ b/fs/btrfs/lzo.c @@ -17,6 +17,43 @@ #define LZO_LEN 4 +/* + * Btrfs LZO compression format + * + * Regular and inlined LZO compressed data extents consist of: + * + * 1. Header + * Fixed size. LZO_LEN (4) bytes long, LE32. + * Records the total size (including the header) of compressed data. + * + * 2. Segment(s) + * Variable size. Each segment includes one segment header, followd by data + * payload. + * One regular LZO compressed extent can have one or more segments. + * For inlined LZO compressed extent, only one segment is allowed. + * One segment represents at most one page of uncompressed data. + * + * 2.1 Segment header + * Fixed size. LZO_LEN (4) bytes long, LE32. + * Records the total size of the segment (not including the header). + * Segment header never crosses page boundary, thus it's possible to + * have at most 3 padding zeros at the end of the page. + * + * 2.2 Data Payload + * Variable size. Size up limit should be lzo1x_worst_compress(PAGE_SIZE) + * which is 4419 for a 4KiB page. + * + * Example: + * Page 1: + * 0 0x2 0x4 0x6 0x8 0xa 0xc 0xe 0x10 + * 0x0000 | Header | SegHdr 01 | Data payload 01 ... | + * ... + * 0x0ff0 | SegHdr N | Data payload N ... |00| + * ^^ padding zeros + * Page 2: + * 0x1000 | SegHdr N+1| Data payload N+1 ... | + */ + struct workspace { void *mem; void *buf; /* where decompressed data goes */ |