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Date: Mon, 15 Jun 2020 16:53:38 -0400
From: Waiman Long <longman@...hat.com>
To: "Darrick J. Wong" <darrick.wong@...cle.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
Dave Chinner <david@...morbit.com>, Qian Cai <cai@....pw>,
Eric Sandeen <sandeen@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] xfs: Fix false positive lockdep warning with
sb_internal & fs_reclaim
On 6/15/20 12:43 PM, Darrick J. Wong wrote:
> On Mon, Jun 15, 2020 at 12:08:30PM -0400, Waiman Long wrote:
>> Depending on the workloads, the following circular locking dependency
>> warning between sb_internal (a percpu rwsem) and fs_reclaim (a pseudo
>> lock) may show up:
>>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 5.0.0-rc1+ #60 Tainted: G W
>> ------------------------------------------------------
>> fsfreeze/4346 is trying to acquire lock:
>> 0000000026f1d784 (fs_reclaim){+.+.}, at:
>> fs_reclaim_acquire.part.19+0x5/0x30
>>
>> but task is already holding lock:
>> 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650
>>
>> which lock already depends on the new lock.
>> :
>> Possible unsafe locking scenario:
>>
>> CPU0 CPU1
>> ---- ----
>> lock(sb_internal);
>> lock(fs_reclaim);
>> lock(sb_internal);
>> lock(fs_reclaim);
>>
>> *** DEADLOCK ***
>>
>> 4 locks held by fsfreeze/4346:
>> #0: 00000000b478ef56 (sb_writers#8){++++}, at: percpu_down_write+0xb4/0x650
>> #1: 000000001ec487a9 (&type->s_umount_key#28){++++}, at: freeze_super+0xda/0x290
>> #2: 000000003edbd5a0 (sb_pagefaults){++++}, at: percpu_down_write+0xb4/0x650
>> #3: 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650
>>
>> stack backtrace:
>> Call Trace:
>> dump_stack+0xe0/0x19a
>> print_circular_bug.isra.10.cold.34+0x2f4/0x435
>> check_prev_add.constprop.19+0xca1/0x15f0
>> validate_chain.isra.14+0x11af/0x3b50
>> __lock_acquire+0x728/0x1200
>> lock_acquire+0x269/0x5a0
>> fs_reclaim_acquire.part.19+0x29/0x30
>> fs_reclaim_acquire+0x19/0x20
>> kmem_cache_alloc+0x3e/0x3f0
>> kmem_zone_alloc+0x79/0x150
>> xfs_trans_alloc+0xfa/0x9d0
>> xfs_sync_sb+0x86/0x170
>> xfs_log_sbcount+0x10f/0x140
>> xfs_quiesce_attr+0x134/0x270
>> xfs_fs_freeze+0x4a/0x70
>> freeze_super+0x1af/0x290
>> do_vfs_ioctl+0xedc/0x16c0
>> ksys_ioctl+0x41/0x80
>> __x64_sys_ioctl+0x73/0xa9
>> do_syscall_64+0x18f/0xd23
>> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> This is a false positive as all the dirty pages are flushed out before
>> the filesystem can be frozen.
>>
>> Perhaps breaking the fs_reclaim pseudo lock into a per filesystem lock
>> may fix the issue. However, that will greatly complicate the logic and
>> may not be worth it.
>>
>> Another way to fix it is to disable the taking of the fs_reclaim
>> pseudo lock when in the freezing code path as a reclaim on the
>> freezed filesystem is not possible. By using the newly introduced
>> PF_MEMALLOC_NOLOCKDEP flag, lockdep checking is disabled in
>> xfs_trans_alloc() if XFS_TRANS_NO_WRITECOUNT flag is set.
>>
>> In the freezing path, there is another path where memory allocation
>> is being done without the XFS_TRANS_NO_WRITECOUNT flag:
>>
>> xfs_fs_freeze()
>> => xfs_quiesce_attr()
>> => xfs_log_quiesce()
>> => xfs_log_unmount_write()
>> => xlog_unmount_write()
>> => xfs_log_reserve()
>> => xlog_ticket_alloc()
>>
>> In this case, we just disable fs reclaim for this particular 600 bytes
>> memory allocation.
>>
>> Without this patch, the command sequence below will show that the lock
>> dependency chain sb_internal -> fs_reclaim exists.
>>
>> # fsfreeze -f /home
>> # fsfreeze --unfreeze /home
>> # grep -i fs_reclaim -C 3 /proc/lockdep_chains | grep -C 5 sb_internal
>>
>> After applying the patch, such sb_internal -> fs_reclaim lock dependency
>> chain can no longer be found. Because of that, the locking dependency
>> warning will not be shown.
>>
>> Suggested-by: Dave Chinner <david@...morbit.com>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>> fs/xfs/xfs_log.c | 9 +++++++++
>> fs/xfs/xfs_trans.c | 30 ++++++++++++++++++++++++++----
>> 2 files changed, 35 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
>> index 00fda2e8e738..33244680d0d4 100644
>> --- a/fs/xfs/xfs_log.c
>> +++ b/fs/xfs/xfs_log.c
>> @@ -830,8 +830,17 @@ xlog_unmount_write(
>> xfs_lsn_t lsn;
>> uint flags = XLOG_UNMOUNT_TRANS;
>> int error;
>> + unsigned long pflags;
>>
>> + /*
>> + * xfs_log_reserve() allocates memory. This can lead to fs reclaim
>> + * which may conflicts with the unmount process. To avoid that,
>> + * disable fs reclaim for this allocation.
>> + */
>> + current_set_flags_nested(&pflags, PF_MEMALLOC_NOFS);
>> error = xfs_log_reserve(mp, 600, 1, &tic, XFS_LOG, 0);
>> + current_restore_flags_nested(&pflags, PF_MEMALLOC_NOFS);
>> +
>> if (error)
>> goto out_err;
>>
>> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
>> index 3c94e5ff4316..ddb10ad3f51f 100644
>> --- a/fs/xfs/xfs_trans.c
>> +++ b/fs/xfs/xfs_trans.c
>> @@ -255,7 +255,27 @@ xfs_trans_alloc(
>> struct xfs_trans **tpp)
>> {
>> struct xfs_trans *tp;
>> - int error;
>> + int error = 0;
>> + unsigned long pflags = -1;
>> +
>> + /*
>> + * When XFS_TRANS_NO_WRITECOUNT is set, it means there are no dirty
>> + * data pages in the filesystem at this point.
> That's not true. Look at the other callers of xfs_trans_alloc_empty.
Yes, I am aware of that. I can change it to check the freeze state.
>
> Also: Why not set PF_MEMALLOC_NOFS at the start of the freeze call
> chain?
I guess we can do that, but it eliminates a potential source for memory
reclaim leading to freeze error when not much free memory is left. We
can go this route if you think this is not a problem.
Cheers,
Longman
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