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| author | Johannes Thumshirn <jthumshirn@suse.de> | 2018-11-28 11:54:56 +0300 |
|---|---|---|
| committer | David Sterba <dsterba@suse.com> | 2018-12-17 16:51:41 +0300 |
| commit | d2e174d5d3eedb3b9dd97167307bf1f01bd2fa5c (patch) | |
| tree | 35131555f323b709ffa0926e0d673f156b3c4428 /fs/btrfs/disk-io.c | |
| parent | cc2c39d605d23e7f1fbc376bf011fd06e46ee79f (diff) | |
| download | linux-d2e174d5d3eedb3b9dd97167307bf1f01bd2fa5c.tar.xz | |
btrfs: document extent mapping assumptions in checksum
Document why map_private_extent_buffer() cannot return '1' (i.e. the map
spans two pages) for the csum_tree_block() case.
The current algorithm for detecting a page boundary crossing in
map_private_extent_buffer() will return a '1' *IFF* the extent buffer's
offset in the page + the offset passed in by csum_tree_block() and the
minimal length passed in by csum_tree_block() - 1 are bigger than
PAGE_SIZE.
We always pass BTRFS_CSUM_SIZE (32) as offset and a minimal length of 32
and the current extent buffer allocator always guarantees page aligned
extends, so the above condition can't be true.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>
Diffstat (limited to 'fs/btrfs/disk-io.c')
| -rw-r--r-- | fs/btrfs/disk-io.c | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 4bc270ef29b4..14d355d0cb7a 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -279,6 +279,12 @@ static int csum_tree_block(struct btrfs_fs_info *fs_info, len = buf->len - offset; while (len > 0) { + /* + * Note: we don't need to check for the err == 1 case here, as + * with the given combination of 'start = BTRFS_CSUM_SIZE (32)' + * and 'min_len = 32' and the currently implemented mapping + * algorithm we cannot cross a page boundary. + */ err = map_private_extent_buffer(buf, offset, 32, &kaddr, &map_start, &map_len); if (err) |