Re: [PATCH] btrfs: Always try all copies when reading extent buffers

[David Sterba <dsterba@suse.cz>]

On Tue, Nov 06, 2018 at 04:40:20PM +0200, Nikolay Borisov wrote:
> When a metadata read is served the endio routine btree_readpage_end_io_hook
> is called which eventually runs the tree-checker. If tree-checker fails
> to validate the read eb then it sets EXTENT_BUFFER_CORRUPT flag. This
> leads to btree_read_extent_buffer_pages wrongly assuming that all
> available copies of this extent buffer are wrong and failing prematurely.
> Fix this modify btree_read_extent_buffer_pages to read all copies of
> the data.
> 
> This failure was exhibitted in xfstests btrfs/124 which would
> spuriously fail its balance operations. The reason was that when balance
> was run following re-introduction of the missing raid1 disk
> __btrfs_map_block would map the read request to stripe 0, which
> corresponded to devid 2 (the disk which is being removed in the test):
> 
>     item 2 key (FIRST_CHUNK_TREE CHUNK_ITEM 3553624064) itemoff 15975 itemsize 112
> 	length 1073741824 owner 2 stripe_len 65536 type DATA|RAID1
> 	io_align 65536 io_width 65536 sector_size 4096
> 	num_stripes 2 sub_stripes 1
> 		stripe 0 devid 2 offset 2156920832
> 		dev_uuid 8466c350-ed0c-4c3b-b17d-6379b445d5c8
> 		stripe 1 devid 1 offset 3553624064
> 		dev_uuid 1265d8db-5596-477e-af03-df08eb38d2ca
> 
> This caused read requests for a checksum item that to be routed to the
> stale disk which triggered the aforementioned logic involving
> EXTENT_BUFFER_CORRUPT flag. This then triggered cascading failures of
> the balance operation.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> Suggested-by: Qu Wenruo <wqu@suse.com>
> Fixes: a826d6dcb32d ("Btrfs: check items for correctness as we search")

Added to misc-next, thanks.