On 28/10/24 13:41, Qu Wenruo wrote:
[BUG]
During the development to make btrfs pass generic/563 (which needs to
make btrfs to support partial updaote folios), generic/095 causes hangs
s/updaote/update
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>
---
tests/generic/366 | 80 +++++++++++++++++++++++++++++++++++++++++++
tests/generic/366.out | 2 ++
2 files changed, 82 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..2ebc5728
--- /dev/null
+++ b/tests/generic/366
@@ -0,0 +1,80 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2024 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test 366
+#
+# Test if certain fio load will hang the filesystem.
+#
It is a good idea to also specify the type of workload being used
with fio, for example, as in generic/095
----
# Concurrent mixed I/O (buffer I/O, aiodio, mmap, splice) on the same files
----
+# 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 fa630df665aa \
+_fixed_by_kernel_commit xxxxxxxxxxxx \
+ "btrfs: avoid deadlock when reading a partial uptodate folio"
An extra _fixed_by_kernel_commit.
+
+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
+
So this fio drops splice and extra sync and posixaio workloads
from generic/095. And iterates the workload.
+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.
This can be updated with a short snippet of how the workload
helped reproduce the bug.
+ $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"
+
+# success, all done
+status=0
+exit
looks good.
Thanks.
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
