On 2018/11/6 下午11:14, Nikolay Borisov wrote:
>
>
> On 6.11.18 г. 16:53 ч., Qu Wenruo wrote:
>>
>>
>> On 2018/11/6 下午10:40, 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@xxxxxxxx>
>>> Suggested-by: Qu Wenruo <wqu@xxxxxxxx>
>>
>> Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>
>>
>> However there is still a tiny piece missing.
>>
>> Tree checker is done after some basic checks, including:
>> 1) bytenr
>> 2) level
>> 3) fsid
>> 4) csum
>>
>> 1~2) can be easily skipped just by pure luck.
>>
>> But 3) and especially 4) are not that easy to hit.
>> Not to mention meeting both 3) and 4), since csum range covers fsid.
>>
>> So I must say you're a really super lucky guy!
>
> s/lucky/unlucky/ :)
>
> I wonder if reads just land in some random memory on the stale disk
> hence it's really a matter of what's "there" so that reads fail with
> random reasons?
Even for random memory, you're still too lucky to hit it.
Thanks,
Qu
>
>>
>> Thanks,
>> Qu
>>
>>
>>> 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) {
>>> @@ -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)
>>>
>>
