Commit d2e174d5 authored Nov 28, 2018 by Johannes Thumshirn Committed by David Sterba Dec 17, 2018

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>

parent cc2c39d6

fs/btrfs/disk-io.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -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)

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