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

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

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

Thanks,
Qu