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

Jens Axboe
2023-08-20T14:14:31Z

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);

--
Jens Axboe