Re: [PATCH 2/2] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim

[Waiman Long <longman@redhat.com>]

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@fromorbit.com>
>> Signed-off-by: Waiman Long <longman@redhat.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