[PATCH v5.10-v5.4] btrfs: get rid of warning on transaction commit when using flushoncommit

[PATCH v5.10-v5.4] btrfs: get rid of warning on transaction commit when using flushoncommit

[hsimeliere.opensource]

From: Filipe Manana <fdmanana@suse.com>

commit a0f0cf8341e34e5d2265bfd3a7ad68342da1e2aa upstream.

When using the flushoncommit mount option, during almost every transaction
commit we trigger a warning from __writeback_inodes_sb_nr():

  $ cat fs/fs-writeback.c:
  (...)
  static void __writeback_inodes_sb_nr(struct super_block *sb, ...
  {
        (...)
        WARN_ON(!rwsem_is_locked(&sb->s_umount));
        (...)
  }
  (...)

The trace produced in dmesg looks like the following:

  [947.473890] WARNING: CPU: 5 PID: 930 at fs/fs-writeback.c:2610 __writeback_inodes_sb_nr+0x7e/0xb3
  [947.481623] Modules linked in: nfsd nls_cp437 cifs asn1_decoder cifs_arc4 fscache cifs_md4 ipmi_ssif
  [947.489571] CPU: 5 PID: 930 Comm: btrfs-transacti Not tainted 95.16.3-srb-asrock-00001-g36437ad63879 #186
  [947.497969] RIP: 0010:__writeback_inodes_sb_nr+0x7e/0xb3
  [947.502097] Code: 24 10 4c 89 44 24 18 c6 (...)
  [947.519760] RSP: 0018:ffffc90000777e10 EFLAGS: 00010246
  [947.523818] RAX: 0000000000000000 RBX: 0000000000963300 RCX: 0000000000000000
  [947.529765] RDX: 0000000000000000 RSI: 000000000000fa51 RDI: ffffc90000777e50
  [947.535740] RBP: ffff888101628a90 R08: ffff888100955800 R09: ffff888100956000
  [947.541701] R10: 0000000000000002 R11: 0000000000000001 R12: ffff888100963488
  [947.547645] R13: ffff888100963000 R14: ffff888112fb7200 R15: ffff888100963460
  [947.553621] FS:  0000000000000000(0000) GS:ffff88841fd40000(0000) knlGS:0000000000000000
  [947.560537] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [947.565122] CR2: 0000000008be50c4 CR3: 000000000220c000 CR4: 00000000001006e0
  [947.571072] Call Trace:
  [947.572354]  <TASK>
  [947.573266]  btrfs_commit_transaction+0x1f1/0x998
  [947.576785]  ? start_transaction+0x3ab/0x44e
  [947.579867]  ? schedule_timeout+0x8a/0xdd
  [947.582716]  transaction_kthread+0xe9/0x156
  [947.585721]  ? btrfs_cleanup_transaction.isra.0+0x407/0x407
  [947.590104]  kthread+0x131/0x139
  [947.592168]  ? set_kthread_struct+0x32/0x32
  [947.595174]  ret_from_fork+0x22/0x30
  [947.597561]  </TASK>
  [947.598553] ---[ end trace 644721052755541c ]---

This is because we started using writeback_inodes_sb() to flush delalloc
when committing a transaction (when using -o flushoncommit), in order to
avoid deadlocks with filesystem freeze operations. This change was made
by commit ce8ea7cc6eb313 ("btrfs: don't call btrfs_start_delalloc_roots
in flushoncommit"). After that change we started producing that warning,
and every now and then a user reports this since the warning happens too
often, it spams dmesg/syslog, and a user is unsure if this reflects any
problem that might compromise the filesystem's reliability.

We can not just lock the sb->s_umount semaphore before calling
writeback_inodes_sb(), because that would at least deadlock with
filesystem freezing, since at fs/super.c:freeze_super() sync_filesystem()
is called while we are holding that semaphore in write mode, and that can
trigger a transaction commit, resulting in a deadlock. It would also
trigger the same type of deadlock in the unmount path. Possibly, it could
also introduce some other locking dependencies that lockdep would report.

To fix this call try_to_writeback_inodes_sb() instead of
writeback_inodes_sb(), because that will try to read lock sb->s_umount
and then will only call writeback_inodes_sb() if it was able to lock it.
This is fine because the cases where it can't read lock sb->s_umount
are during a filesystem unmount or during a filesystem freeze - in those
cases sb->s_umount is write locked and sync_filesystem() is called, which
calls writeback_inodes_sb(). In other words, in all cases where we can't
take a read lock on sb->s_umount, writeback is already being triggered
elsewhere.

An alternative would be to call btrfs_start_delalloc_roots() with a
number of pages different from LONG_MAX, for example matching the number
of delalloc bytes we currently have, in which case we would end up
starting all delalloc with filemap_fdatawrite_wbc() and not with an
async flush via filemap_flush() - that is only possible after the rather
recent commit e076ab2a2ca70a ("btrfs: shrink delalloc pages instead of
full inodes"). However that creates a whole new can of worms due to new
lock dependencies, which lockdep complains, like for example:

[ 8948.247280] ======================================================
[ 8948.247823] WARNING: possible circular locking dependency detected
[ 8948.248353] 5.17.0-rc1-btrfs-next-111 #1 Not tainted
[ 8948.248786] ------------------------------------------------------
[ 8948.249320] kworker/u16:18/933570 is trying to acquire lock:
[ 8948.249812] ffff9b3de1591690 (sb_internal#2){.+.+}-{0:0}, at: find_free_extent+0x141e/0x1590 [btrfs]
[ 8948.250638]
               but task is already holding lock:
[ 8948.251140] ffff9b3e09c717d8 (&root->delalloc_mutex){+.+.}-{3:3}, at: start_delalloc_inodes+0x78/0x400 [btrfs]
[ 8948.252018]
               which lock already depends on the new lock.

[ 8948.252710]
               the existing dependency chain (in reverse order) is:
[ 8948.253343]
               -> #2 (&root->delalloc_mutex){+.+.}-{3:3}:
[ 8948.253950]        __mutex_lock+0x90/0x900
[ 8948.254354]        start_delalloc_inodes+0x78/0x400 [btrfs]
[ 8948.254859]        btrfs_start_delalloc_roots+0x194/0x2a0 [btrfs]
[ 8948.255408]        btrfs_commit_transaction+0x32f/0xc00 [btrfs]
[ 8948.255942]        btrfs_mksubvol+0x380/0x570 [btrfs]
[ 8948.256406]        btrfs_mksnapshot+0x81/0xb0 [btrfs]
[ 8948.256870]        __btrfs_ioctl_snap_create+0x17f/0x190 [btrfs]
[ 8948.257413]        btrfs_ioctl_snap_create_v2+0xbb/0x140 [btrfs]
[ 8948.257961]        btrfs_ioctl+0x1196/0x3630 [btrfs]
[ 8948.258418]        __x64_sys_ioctl+0x83/0xb0
[ 8948.258793]        do_syscall_64+0x3b/0xc0
[ 8948.259146]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 8948.259709]
               -> #1 (&fs_info->delalloc_root_mutex){+.+.}-{3:3}:
[ 8948.260330]        __mutex_lock+0x90/0x900
[ 8948.260692]        btrfs_start_delalloc_roots+0x97/0x2a0 [btrfs]
[ 8948.261234]        btrfs_commit_transaction+0x32f/0xc00 [btrfs]
[ 8948.261766]        btrfs_set_free_space_cache_v1_active+0x38/0x60 [btrfs]
[ 8948.262379]        btrfs_start_pre_rw_mount+0x119/0x180 [btrfs]
[ 8948.262909]        open_ctree+0x1511/0x171e [btrfs]
[ 8948.263359]        btrfs_mount_root.cold+0x12/0xde [btrfs]
[ 8948.263863]        legacy_get_tree+0x30/0x50
[ 8948.264242]        vfs_get_tree+0x28/0xc0
[ 8948.264594]        vfs_kern_mount.part.0+0x71/0xb0
[ 8948.265017]        btrfs_mount+0x11d/0x3a0 [btrfs]
[ 8948.265462]        legacy_get_tree+0x30/0x50
[ 8948.265851]        vfs_get_tree+0x28/0xc0
[ 8948.266203]        path_mount+0x2d4/0xbe0
[ 8948.266554]        __x64_sys_mount+0x103/0x140
[ 8948.266940]        do_syscall_64+0x3b/0xc0
[ 8948.267300]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 8948.267790]
               -> #0 (sb_internal#2){.+.+}-{0:0}:
[ 8948.268322]        __lock_acquire+0x12e8/0x2260
[ 8948.268733]        lock_acquire+0xd7/0x310
[ 8948.269092]        start_transaction+0x44c/0x6e0 [btrfs]
[ 8948.269591]        find_free_extent+0x141e/0x1590 [btrfs]
[ 8948.270087]        btrfs_reserve_extent+0x14b/0x280 [btrfs]
[ 8948.270588]        cow_file_range+0x17e/0x490 [btrfs]
[ 8948.271051]        btrfs_run_delalloc_range+0x345/0x7a0 [btrfs]
[ 8948.271586]        writepage_delalloc+0xb5/0x170 [btrfs]
[ 8948.272071]        __extent_writepage+0x156/0x3c0 [btrfs]
[ 8948.272579]        extent_write_cache_pages+0x263/0x460 [btrfs]
[ 8948.273113]        extent_writepages+0x76/0x130 [btrfs]
[ 8948.273573]        do_writepages+0xd2/0x1c0
[ 8948.273942]        filemap_fdatawrite_wbc+0x68/0x90
[ 8948.274371]        start_delalloc_inodes+0x17f/0x400 [btrfs]
[ 8948.274876]        btrfs_start_delalloc_roots+0x194/0x2a0 [btrfs]
[ 8948.275417]        flush_space+0x1f2/0x630 [btrfs]
[ 8948.275863]        btrfs_async_reclaim_data_space+0x108/0x1b0 [btrfs]
[ 8948.276438]        process_one_work+0x252/0x5a0
[ 8948.276829]        worker_thread+0x55/0x3b0
[ 8948.277189]        kthread+0xf2/0x120
[ 8948.277506]        ret_from_fork+0x22/0x30
[ 8948.277868]
               other info that might help us debug this:

[ 8948.278548] Chain exists of:
                 sb_internal#2 --> &fs_info->delalloc_root_mutex --> &root->delalloc_mutex

[ 8948.279601]  Possible unsafe locking scenario:

[ 8948.280102]        CPU0                    CPU1
[ 8948.280508]        ----                    ----
[ 8948.280915]   lock(&root->delalloc_mutex);
[ 8948.281271]                                lock(&fs_info->delalloc_root_mutex);
[ 8948.281915]                                lock(&root->delalloc_mutex);
[ 8948.282487]   lock(sb_internal#2);
[ 8948.282800]
                *** DEADLOCK ***

[ 8948.283333] 4 locks held by kworker/u16:18/933570:
[ 8948.283750]  #0: ffff9b3dc00a9d48 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x1d2/0x5a0
[ 8948.284609]  #1: ffffa90349dafe70 ((work_completion)(&fs_info->async_data_reclaim_work)){+.+.}-{0:0}, at: process_one_work+0x1d2/0x5a0
[ 8948.285637]  #2: ffff9b3e14db5040 (&fs_info->delalloc_root_mutex){+.+.}-{3:3}, at: btrfs_start_delalloc_roots+0x97/0x2a0 [btrfs]
[ 8948.286674]  #3: ffff9b3e09c717d8 (&root->delalloc_mutex){+.+.}-{3:3}, at: start_delalloc_inodes+0x78/0x400 [btrfs]
[ 8948.287596]
              stack backtrace:
[ 8948.287975] CPU: 3 PID: 933570 Comm: kworker/u16:18 Not tainted 5.17.0-rc1-btrfs-next-111 #1
[ 8948.288677] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 8948.289649] Workqueue: events_unbound btrfs_async_reclaim_data_space [btrfs]
[ 8948.290298] Call Trace:
[ 8948.290517]  <TASK>
[ 8948.290700]  dump_stack_lvl+0x59/0x73
[ 8948.291026]  check_noncircular+0xf3/0x110
[ 8948.291375]  ? start_transaction+0x228/0x6e0 [btrfs]
[ 8948.291826]  __lock_acquire+0x12e8/0x2260
[ 8948.292241]  lock_acquire+0xd7/0x310
[ 8948.292714]  ? find_free_extent+0x141e/0x1590 [btrfs]
[ 8948.293241]  ? lock_is_held_type+0xea/0x140
[ 8948.293601]  start_transaction+0x44c/0x6e0 [btrfs]
[ 8948.294055]  ? find_free_extent+0x141e/0x1590 [btrfs]
[ 8948.294518]  find_free_extent+0x141e/0x1590 [btrfs]
[ 8948.294957]  ? _raw_spin_unlock+0x29/0x40
[ 8948.295312]  ? btrfs_get_alloc_profile+0x124/0x290 [btrfs]
[ 8948.295813]  btrfs_reserve_extent+0x14b/0x280 [btrfs]
[ 8948.296270]  cow_file_range+0x17e/0x490 [btrfs]
[ 8948.296691]  btrfs_run_delalloc_range+0x345/0x7a0 [btrfs]
[ 8948.297175]  ? find_lock_delalloc_range+0x247/0x270 [btrfs]
[ 8948.297678]  writepage_delalloc+0xb5/0x170 [btrfs]
[ 8948.298123]  __extent_writepage+0x156/0x3c0 [btrfs]
[ 8948.298570]  extent_write_cache_pages+0x263/0x460 [btrfs]
[ 8948.299061]  extent_writepages+0x76/0x130 [btrfs]
[ 8948.299495]  do_writepages+0xd2/0x1c0
[ 8948.299817]  ? sched_clock_cpu+0xd/0x110
[ 8948.300160]  ? lock_release+0x155/0x4a0
[ 8948.300494]  filemap_fdatawrite_wbc+0x68/0x90
[ 8948.300874]  ? do_raw_spin_unlock+0x4b/0xa0
[ 8948.301243]  start_delalloc_inodes+0x17f/0x400 [btrfs]
[ 8948.301706]  ? lock_release+0x155/0x4a0
[ 8948.302055]  btrfs_start_delalloc_roots+0x194/0x2a0 [btrfs]
[ 8948.302564]  flush_space+0x1f2/0x630 [btrfs]
[ 8948.302970]  btrfs_async_reclaim_data_space+0x108/0x1b0 [btrfs]
[ 8948.303510]  process_one_work+0x252/0x5a0
[ 8948.303860]  ? process_one_work+0x5a0/0x5a0
[ 8948.304221]  worker_thread+0x55/0x3b0
[ 8948.304543]  ? process_one_work+0x5a0/0x5a0
[ 8948.304904]  kthread+0xf2/0x120
[ 8948.305184]  ? kthread_complete_and_exit+0x20/0x20
[ 8948.305598]  ret_from_fork+0x22/0x30
[ 8948.305921]  </TASK>

It all comes from the fact that btrfs_start_delalloc_roots() takes the
delalloc_root_mutex, in the transaction commit path we are holding a
read lock on one of the superblock's freeze semaphores (via
sb_start_intwrite()), the async reclaim task can also do a call to
btrfs_start_delalloc_roots(), which ends up triggering writeback with
calls to filemap_fdatawrite_wbc(), resulting in extent allocation which
in turn can call btrfs_start_transaction(), which will result in taking
the freeze semaphore via sb_start_intwrite(), forming a nasty dependency
on all those locks which can be taken in different orders by different
code paths.

So just adopt the simple approach of calling try_to_writeback_inodes_sb()
at btrfs_start_delalloc_flush().

Link: https://lore.kernel.org/linux-btrfs/20220130005258.GA7465@cuci.nl/
Link: https://lore.kernel.org/linux-btrfs/43acc426-d683-d1b6-729d-c6bc4a2fff4d@gmail.com/
Link: https://lore.kernel.org/linux-btrfs/6833930a-08d7-6fbc-0141-eb9cdfd6bb4d@gmail.com/
Link: https://lore.kernel.org/linux-btrfs/20190322041731.GF16651@hungrycats.org/
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
[ add more link reports ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Hugo SIMELIERE <hsimeliere.opensource@witekio.com>
---
 fs/btrfs/transaction.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 8878aa7cbdc5..c797563fa9cf 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -2021,16 +2021,24 @@ static inline int btrfs_start_delalloc_flush(struct btrfs_trans_handle *trans)
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 
 	/*
-	 * We use writeback_inodes_sb here because if we used
+	 * We use try_to_writeback_inodes_sb() here because if we used
 	 * btrfs_start_delalloc_roots we would deadlock with fs freeze.
 	 * Currently are holding the fs freeze lock, if we do an async flush
 	 * we'll do btrfs_join_transaction() and deadlock because we need to
 	 * wait for the fs freeze lock.  Using the direct flushing we benefit
 	 * from already being in a transaction and our join_transaction doesn't
 	 * have to re-take the fs freeze lock.
+	 *
+	 * Note that try_to_writeback_inodes_sb() will only trigger writeback
+	 * if it can read lock sb->s_umount. It will always be able to lock it,
+	 * except when the filesystem is being unmounted or being frozen, but in
+	 * those cases sync_filesystem() is called, which results in calling
+	 * writeback_inodes_sb() while holding a write lock on sb->s_umount.
+	 * Note that we don't call writeback_inodes_sb() directly, because it
+	 * will emit a warning if sb->s_umount is not locked.
 	 */
 	if (btrfs_test_opt(fs_info, FLUSHONCOMMIT)) {
-		writeback_inodes_sb(fs_info->sb, WB_REASON_SYNC);
+		try_to_writeback_inodes_sb(fs_info->sb, WB_REASON_SYNC);
 	} else {
 		struct btrfs_pending_snapshot *pending;
 		struct list_head *head = &trans->transaction->pending_snapshots;
-- 
2.43.0

Re: [PATCH v5.10-v5.4] btrfs: get rid of warning on transaction commit when using flushoncommit

[Greg KH]

On Tue, Oct 08, 2024 at 12:58:34PM +0200, hsimeliere.opensource@witekio.com wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> commit a0f0cf8341e34e5d2265bfd3a7ad68342da1e2aa upstream.

Now queued up, thanks.

greg k-h

Re: [PATCH v5.10-v5.4] btrfs: get rid of warning on transaction commit when using flushoncommit

[David Sterba]

On Tue, Oct 08, 2024 at 01:06:10PM +0200, Greg KH wrote:
> On Tue, Oct 08, 2024 at 12:58:34PM +0200, hsimeliere.opensource@witekio.com wrote:
> > From: Filipe Manana <fdmanana@suse.com>
> > 
> > commit a0f0cf8341e34e5d2265bfd3a7ad68342da1e2aa upstream.
> 
> Now queued up, thanks.

I haven't sent this patch to backport for < 5.15 because IIRC there were
some other changes needed. I'm not sure this patch is on itself is
sufficient to fix the warning and correct regarding the flushing logic.
So this needs an analysis first, if someobdy really wants to get i to
stable trees < 5.10. For now I suggest not to add it.

(For completeness, 5.15.27 and anything newer has the fix).

Re: [PATCH v5.10-v5.4] btrfs: get rid of warning on transaction commit when using flushoncommit

[Greg KH]

On Tue, Oct 08, 2024 at 04:58:02PM +0200, David Sterba wrote:
> On Tue, Oct 08, 2024 at 01:06:10PM +0200, Greg KH wrote:
> > On Tue, Oct 08, 2024 at 12:58:34PM +0200, hsimeliere.opensource@witekio.com wrote:
> > > From: Filipe Manana <fdmanana@suse.com>
> > > 
> > > commit a0f0cf8341e34e5d2265bfd3a7ad68342da1e2aa upstream.
> > 
> > Now queued up, thanks.
> 
> I haven't sent this patch to backport for < 5.15 because IIRC there were
> some other changes needed. I'm not sure this patch is on itself is
> sufficient to fix the warning and correct regarding the flushing logic.
> So this needs an analysis first, if someobdy really wants to get i to
> stable trees < 5.10. For now I suggest not to add it.
> 
> (For completeness, 5.15.27 and anything newer has the fix).
> 

Ok, now dropped, thanks.

greg k-h