On 29/10/24 07:35, Qu Wenruo wrote:
[BUG] During the development to make btrfs pass generic/563 (which needs to make btrfs to support partial folios), generic/095 causes hangs during tests. The call trace for the hanging process looks like this: __switch_to+0xf8/0x168 __schedule+0x328/0x8a8 schedule+0x54/0x140 io_schedule+0x44/0x68 folio_wait_bit_common+0x198/0x3f8 __folio_lock+0x24/0x40 extent_write_cache_pages+0x2e0/0x4c0 [btrfs] btrfs_writepages+0x94/0x158 [btrfs] do_writepages+0x74/0x190 filemap_fdatawrite_wbc+0x88/0xc8 __filemap_fdatawrite_range+0x6c/0xa8 filemap_fdatawrite_range+0x1c/0x30 btrfs_start_ordered_extent+0x264/0x2e0 [btrfs] btrfs_lock_and_flush_ordered_range+0x8c/0x160 [btrfs] __get_extent_map+0xa0/0x220 [btrfs] btrfs_do_readpage+0x1bc/0x5d8 [btrfs] btrfs_read_folio+0x50/0xa0 [btrfs] filemap_read_folio+0x54/0x110 filemap_update_page+0x2e0/0x3b8 filemap_get_pages+0x228/0x4d8 filemap_read+0x11c/0x3b8 btrfs_file_read_iter+0x74/0x90 [btrfs] new_sync_read+0xd0/0x1d0 vfs_read+0x1a0/0x1f0 [CAUSE] The root cause is a btrfs specific behavior that during a folio read, we can trigger writeback of the same folio, which will try to lock the same folio already locked by the read process. The fix is already sent to the mailing list: https://lore.kernel.org/linux-btrfs/62bf73ada7be2888d45a787c2b6fd252103a5d25.1729725088.git.wqu@xxxxxxxx/ This problem can only happen if all the following conditions are met: - The sector size of btrfs is smaller than page size To have partial uptodate folios. - Btrfs won't read the full folio if buffered write is block aligned This is done by the not yet merged patch: https://lore.kernel.org/linux-btrfs/ac2639ec4e9ac176d33e95ef7ecf008fa6be5461.1727833878.git.wqu@xxxxxxxx/ [TEST CASE] During the debugging of that generic/095 hang, I extracted a minimal reproducer which is much smaller and faster, although it still requires several runs to trigger a hang. The test case will run the fio workload 32 times by default, which is more than enough to trigger the hang. Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> --- Changelog: v2: - Fix the duplicated _fixed_by_kernel_commit line - Fix a typo in the commit message - Update the comments to show the workload and how it hangs btrfs --- tests/generic/366 | 106 ++++++++++++++++++++++++++++++++++++++++++ tests/generic/366.out | 2 + 2 files changed, 108 insertions(+) create mode 100755 tests/generic/366 create mode 100644 tests/generic/366.out diff --git a/tests/generic/366 b/tests/generic/366 new file mode 100755 index 00000000..9d31c1c8 --- /dev/null +++ b/tests/generic/366 @@ -0,0 +1,106 @@ +#! /bin/bash +# SPDX-License-Identifier: GPL-2.0 +# Copyright (C) 2024 SUSE Linux Products GmbH. All Rights Reserved. +# +# FS QA Test 366 +# +# Test if mixed direct read, direct write and buffered write on the same file will +# hang the filesystem. +# +# This is exposed by an incoming btrfs feature, which allows a folio to be +# partial uptodate if the buffered write range is block aligned but not yet +# full folio aligned. +# +# Such behavior makes btrfs to hang reliably under generic/095. +# This is the extracted minimal reproducer for 4k block size and 64K page size. +# +. ./common/preamble +_begin_fstest auto quick + +. ./common/filter + +_require_scratch +_require_odirect +_require_aio + +_fixed_by_kernel_commit xxxxxxxxxxxx \ + "btrfs: avoid deadlock when reading a partial uptodate folio" + +iterations=$((32 * LOAD_FACTOR)) + +fio_config=$tmp.fio +fio_out=$tmp.fio.out +blksz=`$here/src/min_dio_alignment $SCRATCH_MNT $SCRATCH_DEV` +cat >$fio_config <<EOF +[global] +bs=8k +iodepth=1 +randrepeat=1 +size=256k +directory=$SCRATCH_MNT +numjobs=1 +[job1] +ioengine=sync +bs=512 +direct=1 +rw=randread +filename=file1 +[job2] +ioengine=libaio +rw=randwrite +direct=1 +filename=file1 +[job3] +ioengine=posixaio +rw=randwrite +filename=file1 +EOF +_require_fio $fio_config + +for (( i = 0; i < $iterations; i++)); do + _scratch_mkfs >>$seqres.full 2>&1 + _scratch_mount + # There's a known EIO failure to report collisions between directio and buffered + # writes to userspace, refer to upstream linux 5a9d929d6e13. So ignore EIO error + # at here. + # + # And for btrfs if sector size < page size, if we have a partial + # uptodate folio caused by a buffered write, e.g: + # + # 0 16K 32K 48K 64K + # | |///////////| + # \- sector Uptodate|Dirty + # + # Then writeback happens and finished, but btrfs' ordered extent not + # yet finished. + # In that case, the folio can be released from the page cache (since + # the folio is not under IO/lock). + # + # Then new buffered writes into the folio happened, re-dirty the folio: + # 0 16K 32K 48K 64K + # |//////////| |///////////| + # \- sector Uptodate|Dirty \- No sector flags + # extent map PINNED + # OE still here + # + # Now read is triggered on that folio. + # Btrfs will need to wait for any existing ordered extents in the folio range, + # that wait will also trigger writeback if the folio is dirty. + # That writeback will happen for range [48K, 64K), but since the whole folio + # is locked for read, writeback will also try to lock the same folio, causing + # a deadlock. + $FIO_PROG $fio_config --ignore_error=,EIO --output=$fio_out + # umount before checking dmesg in case umount triggers any WARNING or Oops + _scratch_unmount + + _check_dmesg _filter_aiodio_dmesg + + echo "=== fio $i/$iterations ===" >> $seqres.full + cat $fio_out >> $seqres.full +done + +echo "Silence is golden" +
Thanks! Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>
+# success, all done +status=0 +exit diff --git a/tests/generic/366.out b/tests/generic/366.out new file mode 100644 index 00000000..1fe90e06 --- /dev/null +++ b/tests/generic/366.out @@ -0,0 +1,2 @@ +QA output created by 366 +Silence is golden
