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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>
This commit is contained in:
Johannes Thumshirn 2018-11-28 09:54:56 +01:00 committed by David Sterba
parent cc2c39d605
commit d2e174d5d3
1 changed files with 6 additions and 0 deletions
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6
fs/btrfs/disk-io.c
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@ -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)