Re: [PATCH] btrfs: qgroup: Search commit root for rescan to avoid missing extent

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2018年05月12日 01:08, Jeff Mahoney wrote:
> On 5/3/18 3:20 AM, Qu Wenruo wrote:
>> When doing qgroup rescan using the following script (modified from
>> btrfs/017 test case), we can sometimes hit qgroup corruption.
>>
>> ------
>> umount $dev &> /dev/null
>> umount $mnt &> /dev/null
>>
>> mkfs.btrfs -f -n 64k $dev
>> mount $dev $mnt
>>
>> extent_size=8192
>>
>> xfs_io -f -d -c "pwrite 0 $extent_size" $mnt/foo > /dev/null
>> btrfs subvolume snapshot $mnt $mnt/snap
>>
>> xfs_io -f -c "reflink $mnt/foo" $mnt/foo-reflink > /dev/null
>> xfs_io -f -c "reflink $mnt/foo" $mnt/snap/foo-reflink > /dev/null
>> xfs_io -f -c "reflink $mnt/foo" $mnt/snap/foo-reflink2 > /dev/unll
>> btrfs quota enable $mnt
>>
>>  # -W is the new option to only wait rescan while not starting new one
>> btrfs quota rescan -W $mnt
>> btrfs qgroup show -prce $mnt
>>
>>  # Need to patch btrfs-progs to report qgroup mismatch as error
>> btrfs check $dev || _fail
>> ------
>>
>> For fast machine, we can hit some corruption which missed accounting
>> tree blocks:
>> ------
>> qgroupid         rfer         excl     max_rfer     max_excl parent  child
>> --------         ----         ----     --------     -------- ------  -----
>> 0/5           8.00KiB        0.00B         none         none ---     ---
>> 0/257         8.00KiB        0.00B         none         none ---     ---
>> ------
>>
>> This is due to the fact that we're always searching commit root for
>> btrfs_find_all_roots() at qgroup_rescan_leaf(), but the leaf we get is
>> from current transaction, not commit root.
>>
>> And if our tree blocks get modified in current transaction, we won't
>> find any owner in commit root, thus causing the corruption.
>>
>> Fix it by searching commit root for extent tree for
>> qgroup_rescan_leaf().
>>
>> Reported-by: Nikolay Borisov <nborisov@suse.com>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>
>> Please keep in mind that it is possible to hit another type of race
>> which double accounting tree blocks:
>> ------
>> qgroupid         rfer         excl     max_rfer     max_excl parent  child
>> --------         ----         ----     --------     -------- ------  -----
>> 0/5          136.00KiB     128.00KiB         none         none ---     ---
>> 0/257        136.00KiB     128.00KiB         none         none ---     ---
>> ------
>> For this type of corruption, this patch could reduce the possibility,
>> but the root cause is race between transaction commit and qgroup rescan,
>> which needs to be addressed in another patch.
>> ---
>>  fs/btrfs/qgroup.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
>> index 4baa4ba2d630..829e8fe5c97e 100644
>> --- a/fs/btrfs/qgroup.c
>> +++ b/fs/btrfs/qgroup.c
>> @@ -2681,6 +2681,11 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
>>  	path = btrfs_alloc_path();
>>  	if (!path)
>>  		goto out;
>> +	/*
>> +	 * Rescan should only search for commit root, and any later difference
>> +	 * should be recorded by qgroup
>> +	 */
>> +	path->search_commit_root = 1;
>>  
>>  	err = 0;
>>  	while (!err && !btrfs_fs_closing(fs_info)) {
>>
> 
> If we're searching the commit root here, do we need the tree mod
> sequence number dance in qgroup_rescan_leaf anymore?

No, so I'll remove it in next version.

Thanks for pointing this out,
Qu

> 
> -Jeff
> 


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