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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>
This commit is contained in:
Qu Wenruo 2018-05-17 13:10:01 +08:00 committed by David Sterba
parent d5c1d68fde
commit 2a1f7c0cbd
1 changed files with 37 additions and 0 deletions
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37
fs/btrfs/lzo.c
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@ -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 */