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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>
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@ -279,6 +279,12 @@ static int csum_tree_block(struct btrfs_fs_info *fs_info,
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len = buf->len - offset;
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while (len > 0) {
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/*
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* Note: we don't need to check for the err == 1 case here, as
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* with the given combination of 'start = BTRFS_CSUM_SIZE (32)'
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* and 'min_len = 32' and the currently implemented mapping
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* algorithm we cannot cross a page boundary.
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*/
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err = map_private_extent_buffer(buf, offset, 32,
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&kaddr, &map_start, &map_len);
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if (err)
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