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

[Nikolay Borisov <nborisov@suse.com>]

On 6.11.18 г. 16:40 ч., 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")
> ---
>  fs/btrfs/disk-io.c | 11 +----------
>  1 file changed, 1 insertion(+), 10 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 00ee5e37e989..279c6dbcc736 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -477,9 +477,9 @@ static int btree_read_extent_buffer_pages(struct btrfs_fs_info *fs_info,
>  	int mirror_num = 0;
>  	int failed_mirror = 0;
>  
> -	clear_bit(EXTENT_BUFFER_CORRUPT, &eb->bflags);
>  	io_tree = &BTRFS_I(fs_info->btree_inode)->io_tree;
>  	while (1) {
> +		clear_bit(EXTENT_BUFFER_CORRUPT, &eb->bflags);
>  		ret = read_extent_buffer_pages(io_tree, eb, WAIT_COMPLETE,
>  					       mirror_num);
>  		if (!ret) {

Qu, 

Do you think it makes sense to do refactoring like below in
 a follow up patch: 

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 279c6dbcc736..9891e13a2b6f 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -482,16 +482,11 @@ static int btree_read_extent_buffer_pages(struct btrfs_fs_info *fs_info,
                clear_bit(EXTENT_BUFFER_CORRUPT, &eb->bflags);
                ret = read_extent_buffer_pages(io_tree, eb, WAIT_COMPLETE,
                                               mirror_num);
-               if (!ret) {
-                       if (verify_parent_transid(io_tree, eb,
-                                                  parent_transid, 0))
-                               ret = -EIO;
-                       else if (verify_level_key(fs_info, eb, level,
-                                                 first_key, parent_transid))
-                               ret = -EUCLEAN;
-                       else
+               if (!ret &&
+                   !verify_parent_transid(io_tree, eb, parent_transid, 0) &&
+                   !verify_level_key(fs_info, eb, level, first_key,
+                                    parent_transid))
                                break;
-               }


since the ret value doesn't really have any meaning or perhaps the 
verify_level_key and ret = -EUCLEAN could be reteinaed as well as the 
if (ret == EUCLEAN) break logic ? 

> @@ -493,15 +493,6 @@ static int btree_read_extent_buffer_pages(struct btrfs_fs_info *fs_info,
>  				break;
>  		}
>  
> -		/*
> -		 * This buffer's crc is fine, but its contents are corrupted, so
> -		 * there is no reason to read the other copies, they won't be
> -		 * any less wrong.
> -		 */
> -		if (test_bit(EXTENT_BUFFER_CORRUPT, &eb->bflags) ||
> -		    ret == -EUCLEAN)
> -			break;
> -
>  		num_copies = btrfs_num_copies(fs_info,
>  					      eb->start, eb->len);
>  		if (num_copies == 1)
>