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

[Qu Wenruo <wqu@suse.de>]

On 2018/11/7 上午12:07, Nikolay Borisov wrote:
> 
> 
> 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 ? 

Yes, that's a valid cleanup.

Thanks,
Qu

> 
>> @@ -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)
>>