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. -- Jens Axboe
