Re: Possible io_uring related race leads to btrfs data csum mismatch

[Jens Axboe <axboe@kernel.dk>]

On 8/20/23 6:38 PM, Qu Wenruo wrote:
> 
> 
> On 2023/8/20 22:11, Jens Axboe wrote:
>> On 8/20/23 7:26 AM, Jens Axboe wrote:
>>> On 8/19/23 6:22 PM, Qu Wenruo wrote:
>>>>
>>>>
>>>> On 2023/8/20 07:59, Qu Wenruo wrote:
>>>>> Hi Jens
>>>>>
>>>>> I tried more on my side to debug the situation, and found a very weird
>>>>> write behavior:
>>>>>
>>>>>       Some unexpected direct IO happened, without corresponding
>>>>>       fsstress workload.
>>>>>
>>>>> The workload is:
>>>>>
>>>>>       $fsstress -p 7 -n 50 -s 1691396493 -w -d $mnt -v > /tmp/fsstress
>>>>>
>>>>> Which I can reliably reproduce the problem locally, around 1/50
>>>>> possibility.
>>>>> In my particular case, it results data corruption at root 5 inode 283
>>>>> offset 8192.
>>>>>
>>>>> Then I added some trace points for the following functions:
>>>>>
>>>>> - btrfs_do_write_iter()
>>>>>     Two trace points, one before btrfs_direct_write(), and one
>>>>>     before btrfs_buffered_write(), outputting the aligned and unaligned
>>>>>     write range, root/inode number, type of the write (buffered or
>>>>>     direct).
>>>>>
>>>>> - btrfs_finish_one_ordered()
>>>>>     This is where btrfs inserts its ordered extent into the subvolume
>>>>>     tree.
>>>>>     This happens when a range of pages finishes its writeback.
>>>>>
>>>>> Then here comes the fsstress log for inode 283 (no btrfs root number):
>>>>>
>>>>> 0/22: clonerange d0/f2[283 1 0 0 0 0] [0,0] -> d0/f2[283 1 0 0 0 0]
>>>>> [307200,0]
>>>>> 0/23: copyrange d0/f2[283 1 0 0 0 0] [0,0] -> d0/f2[283 1 0 0 0 0]
>>>>> [1058819,0]
>>>>> 0/25: write d0/f2[283 2 0 0 0 0] [393644,88327] 0
>>>>> 0/29: fallocate(INSERT_RANGE) d0/f3 [283 2 0 0 176 481971]t 884736
>>>>> 585728 95
>>>>> 0/30: uring_write d0/f3[283 2 0 0 176 481971] [1400622, 56456(res=56456)] 0
>>>>> 0/31: writev d0/f3[283 2 0 0 296 1457078] [709121,8,964] 0
>>>>> 0/33: do_aio_rw - xfsctl(XFS_IOC_DIOINFO) d0/f2[283 2 308134 1763236 320
>>>>> 1457078] return 25, fallback to stat()
>>>>> 0/34: dwrite - xfsctl(XFS_IOC_DIOINFO) d0/f3[283 2 308134 1763236 320
>>>>> 1457078] return 25, fallback to stat()
>>>>> 0/34: dwrite d0/f3[283 2 308134 1763236 320 1457078] [589824,16384] 0
>>>>> 0/38: dwrite - xfsctl(XFS_IOC_DIOINFO) d0/f3[283 2 308134 1763236 496
>>>>> 1457078] return 25, fallback to stat()
>>>>> 0/38: dwrite d0/f3[283 2 308134 1763236 496 1457078] [2084864,36864] 0
>>>>> 0/39: write d0/d4/f6[283 2 308134 1763236 496 2121728] [2749000,60139] 0
>>>>> 0/40: fallocate(ZERO_RANGE) d0/f3 [283 2 308134 1763236 688 2809139]t
>>>>> 3512660 81075 0
>>>>> 0/43: splice d0/f5[293 1 0 0 1872 2678784] [552619,59420] -> d0/f3[283 2
>>>>> 308134 1763236 856 3593735] [5603798,59420] 0
>>>>> 0/48: fallocate(KEEP_SIZE|PUNCH_HOLE) d0/f3 [283 1 308134 1763236 976
>>>>> 5663218]t 1361821 480392 0
>>>>> 0/49: clonerange d0/f3[283 1 308134 1763236 856 5663218] [2461696,53248]
>>>>> -> d0/f5[293 1 0 0 1872 2678784] [942080,53248]
>>>>>
>>>>> Note one thing, there is no direct/buffered write into inode 283 offset
>>>>> 8192.
>>>>>
>>>>> But from the trace events for root 5 inode 283:
>>>>>
>>>>>    btrfs_do_write_iter: r/i=5/283 buffered fileoff=393216(393644)
>>>>> len=90112(88327)
>>>>>    btrfs_do_write_iter: r/i=5/283 buffered fileoff=1396736(1400622)
>>>>> len=61440(56456)
>>>>>    btrfs_do_write_iter: r/i=5/283 buffered fileoff=708608(709121)
>>>>> len=12288(7712)
>>>>>
>>>>>    btrfs_do_write_iter: r/i=5/283 direct fileoff=8192(8192)
>>>>> len=73728(73728) <<<<<
>>>>>
>>>>>    btrfs_do_write_iter: r/i=5/283 direct fileoff=589824(589824)
>>>>> len=16384(16384)
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=8192 len=73728
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=589824 len=16384
>>>>>    btrfs_do_write_iter: r/i=5/283 direct fileoff=2084864(2084864)
>>>>> len=36864(36864)
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=2084864 len=36864
>>>>>    btrfs_do_write_iter: r/i=5/283 buffered fileoff=2748416(2749000)
>>>>> len=61440(60139)
>>>>>    btrfs_do_write_iter: r/i=5/283 buffered fileoff=5603328(5603798)
>>>>> len=61440(59420)
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=393216 len=90112
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=708608 len=12288
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=1396736 len=61440
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=3592192 len=4096
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=2748416 len=61440
>>>>>    btrfs_finish_one_ordered: r/i=5/283 fileoff=5603328 len=61440
>>>>>
>>>>> Note that phantom direct IO call, which is in the corrupted range.
>>>>>
>>>>> If paired with fsstress, that phantom write happens between the two
>>>>> operations:
>>>>>
>>>>> 0/31: writev d0/f3[283 2 0 0 296 1457078] [709121,8,964] 0
>>>>> 0/34: dwrite d0/f3[283 2 308134 1763236 320 1457078] [589824,16384] 0
>>>>
>>>> Just to be more accurate, there is a 0/33 operation, which is:
>>>>
>>>> 0/33: do_aio_rw - xfsctl(XFS_IOC_DIOINFO) d0/f2[285 2 308134 1763236 320
>>>> 1457078] return 25, fallback to stat()
>>>> 0/33: awrite - io_getevents failed -4
>>>>
>>>> The failed one doesn't have inode number thus it didn't get caught by grep.
>>>>
>>>> Return value -4 means -INTR, not sure who sent the interruption.
>>>> But if this interruption happens before the IO finished, we can call
>>>> free() on the buffer, and if we're unlucky enough, the freed memory can
>>>> be re-allocated for some other usage, thus modifying the pages before
>>>> the writeback finished.
>>>>
>>>> I think this is the direct cause of the data corruption, page
>>>> modification before direct IO finished.
>>>>
>>>> But unfortunately I still didn't get why the interruption can happen,
>>>> nor how can we handle such interruption?
>>>> (I guess just retry?)
>>>
>>> It's because you are mixing aio/io_uring, and the default settings for
>>> io_uring is to use signal based notifications for queueing task_work.
>>> This then causes a spurious -EINTR, which stops your io_getevents()
>>> wait. Looks like this is a bug in fsstress, it should just retry the
>>> wait if this happens. You can also configure the ring to not use signal
>>> based notifications, but that bug needs fixing regardless.
>>
>> Something like this will probably fix it.
>>
>>
>> diff --git a/ltp/fsstress.c b/ltp/fsstress.c
>> index 6641a525fe5d..05fbfd3f8cf8 100644
>> --- a/ltp/fsstress.c
>> +++ b/ltp/fsstress.c
>> @@ -2072,6 +2072,23 @@ void inode_info(char *str, size_t sz, struct stat64 *s, int verbose)
>>                (long long) s->st_blocks, (long long) s->st_size);
>>   }
>>
>> +static int io_get_single_event(struct io_event *event)
>> +{
>> +    int ret;
>> +
>> +    do {
>> +        /*
>> +         * We can get -EINTR if competing with io_uring using signal
>> +         * based notifications. For that case, just retry the wait.
>> +         */
>> +        ret = io_getevents(io_ctx, 1, 1, event, NULL);
>> +        if (ret != -EINTR)
>> +            break;
>> +    } while (1);
>> +
>> +    return ret;
>> +}
>> +
>>   void
>>   afsync_f(opnum_t opno, long r)
>>   {
>> @@ -2111,7 +2128,7 @@ afsync_f(opnum_t opno, long r)
>>           close(fd);
>>           return;
>>       }
>> -    if ((e = io_getevents(io_ctx, 1, 1, &event, NULL)) != 1) {
>> +    if ((e = io_get_single_event(&event)) != 1) {
>>           if (v)
>>               printf("%d/%lld: afsync - io_getevents failed %d\n",
>>                      procid, opno, e);
>> @@ -2220,10 +2237,10 @@ do_aio_rw(opnum_t opno, long r, int flags)
>>       if ((e = io_submit(io_ctx, 1, iocbs)) != 1) {
>>           if (v)
>>               printf("%d/%lld: %s - io_submit failed %d\n",
>> -                   procid, opno, iswrite ? "awrite" : "aread", e);
>> +                    procid, opno, iswrite ? "awrite" : "aread", e);
>>           goto aio_out;
>>       }
>> -    if ((e = io_getevents(io_ctx, 1, 1, &event, NULL)) != 1) {
>> +    if ((e = io_get_single_event(&event)) != 1) {
>>           if (v)
>>               printf("%d/%lld: %s - io_getevents failed %d\n",
>>                      procid, opno, iswrite ? "awrite" : "aread", e);
>>
> Exactly what I sent for fsstress:
> https://lore.kernel.org/linux-btrfs/20230820010219.12907-1-wqu@suse.com/T/#u

It's not really, as you only did the one case of io_getevents(). What
happens if the other one gets EINTR and aborts, now we do a rw operation
and the first event returned is the one from the fsync?

You should not just fix up the one that you happened to hit, fix up both
of them.

-- 
Jens Axboe