linux-kernel - Re: [f2fs-dev] [PATCH v2] f2fs: avoid sb_start

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Date:   Wed, 2 Mar 2022 15:01:26 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: avoid sb_start_intwrite during
 eviction

On 2022/3/2 13:45, Jaegeuk Kim wrote:
> On 03/01, Jaegeuk Kim wrote:
>> On 03/01, Jaegeuk Kim wrote:
>>> On 03/02, Chao Yu wrote:
>>>> On 2022/3/1 12:48, Jaegeuk Kim wrote:
>>>>> 1. waiting for f2fs_evict_inode
>>>>> [ 5560.043945]  __wait_on_freeing_inode+0xac/0xf0
>>>>> [ 5560.045540]  ? var_wake_function+0x30/0x30
>>>>> [ 5560.047036]  find_inode_fast+0x6d/0xc0
>>>>> [ 5560.048473]  iget_locked+0x79/0x230
>>>>> [ 5560.049933]  f2fs_iget+0x27/0x1200 [f2fs]
>>>>> [ 5560.051496]  f2fs_lookup+0x18c/0x3e0 [f2fs]
>>>>> [ 5560.053069]  __lookup_slow+0x84/0x150
>>>>> [ 5560.054503]  walk_component+0x141/0x1b0
>>>>> [ 5560.055938]  link_path_walk.part.0+0x23b/0x360
>>>>> [ 5560.057541]  ? end_bio_bh_io_sync+0x37/0x50
>>>>> [ 5560.059086]  path_parentat+0x3c/0x90
>>>>> [ 5560.060492]  filename_parentat+0xd7/0x1e0
>>>>> [ 5560.062002]  ? blk_mq_free_request+0x127/0x150
>>>>> [ 5560.063576]  do_renameat2+0xc1/0x5b0
>>>>>    --> sb_start_write(m->mnt_sb); ->  __sb_start_write(sb, SB_FREEZE_WRITE);
>>>>>
>>>>> [ 5560.064999]  ? __check_object_size+0x13f/0x150
>>>>> [ 5560.066559]  ? strncpy_from_user+0x44/0x150
>>>>> [ 5560.068038]  ? getname_flags.part.0+0x4c/0x1b0
>>>>> [ 5560.069617]  __x64_sys_renameat2+0x51/0x60
>>>>>
>>>>> 2. waiting for sb_start_intwrite -> __sb_start_write(sb, SB_FREEZE_FS);
>>>>
>>>> It's still not clear that why __sb_start_write(sb, SB_FREEZE_FS) will be blocked,
>>>> as SB_FREEZE_FS and SB_FREEZE_WRITE points to different locks.
>>>
>>> It seems I missed another call, thaw_super(), got SB_FREEZE_FS and then being
>>> stuck to grab SB_FREEZE_WRITE.
>>
>> Ah, sorry. freeze_super().
> 
> Messed up. So, the lock order is SB_FREEZE_WRITE -> SB_FREEZE_FS in both cases.

Yeah, I noticed this, w/ such lock order, Thread C (freeze_super) will be blocked
on SB_FREEZE_WRITE, and then Thread B won't be block on SB_FREEZE_FS, right?

Thanks,

> 
>>
>>>
>>>>
>>>> Thread A				Thread B				Thread C
>>>> - rename
>>>>   - sb_start_write
>>>>    - __sb_start_write(SB_FREEZE_WRITE)
>>>> ...
>>>>       - f2fs_lookup
>>>> ...
>>>>         - __wait_on_freeing_inode
>>>> 					- drop_slab
>>>> 					 - prune_icache_sb
>>>> 					  - inode_lru_isolate
>>>> 					   :inode->i_state |= I_FREEING
>>>> 										- Is there any flow that it has already held
>>>> 										 SB_FREEZE_FS and try to lock SB_FREEZE_WRITE?
>>>> 					   - f2fs_evict_inode
>>>> 					    - __sb_start_write(SB_FREEZE_FS)
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>> [ 5560.152447]  percpu_rwsem_wait+0xaf/0x160
>>>>> [ 5560.154000]  ? percpu_down_write+0xd0/0xd0
>>>>> [ 5560.155498]  __percpu_down_read+0x4e/0x60
>>>>> [ 5560.157000]  f2fs_evict_inode+0x5a3/0x610 [f2fs]
>>>>> [ 5560.158648]  ? var_wake_function+0x30/0x30
>>>>> [ 5560.160341]  evict+0xd2/0x180
>>>>> [ 5560.161728]  prune_icache_sb+0x81/0xb0
>>>>>    --> inode_lru_isolate() -> inode->i_state |= I_FREEING;
>>>>>
>>>>> [ 5560.163179]  super_cache_scan+0x169/0x1f0
>>>>> [ 5560.164675]  do_shrink_slab+0x145/0x2b0
>>>>> [ 5560.166121]  shrink_slab+0x186/0x2d0
>>>>> [ 5560.167481]  drop_slab_node+0x4a/0x90
>>>>> [ 5560.168876]  drop_slab+0x3e/0x80
>>>>> [ 5560.170178]  drop_caches_sysctl_handler+0x75/0x90
>>>>> [ 5560.171761]  proc_sys_call_handler+0x149/0x280
>>>>> [ 5560.173328]  proc_sys_write+0x13/0x20
>>>>> [ 5560.174667]  new_sync_write+0x117/0x1b0
>>>>> [ 5560.176120]  vfs_write+0x1d5/0x270
>>>>> [ 5560.177409]  ksys_write+0x67/0xe0
>>>>>
>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
>>>>> ---
>>>>>    Note, I found this call stack.
>>>>>
>>>>>    fs/f2fs/inode.c | 2 --
>>>>>    1 file changed, 2 deletions(-)
>>>>>
>>>>> diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
>>>>> index ab8e0c06c78c..882db4bd917b 100644
>>>>> --- a/fs/f2fs/inode.c
>>>>> +++ b/fs/f2fs/inode.c
>>>>> @@ -778,7 +778,6 @@ void f2fs_evict_inode(struct inode *inode)
>>>>>    	f2fs_remove_ino_entry(sbi, inode->i_ino, UPDATE_INO);
>>>>>    	f2fs_remove_ino_entry(sbi, inode->i_ino, FLUSH_INO);
>>>>> -	sb_start_intwrite(inode->i_sb);
>>>>>    	set_inode_flag(inode, FI_NO_ALLOC);
>>>>>    	i_size_write(inode, 0);
>>>>>    retry:
>>>>> @@ -809,7 +808,6 @@ void f2fs_evict_inode(struct inode *inode)
>>>>>    		if (dquot_initialize_needed(inode))
>>>>>    			set_sbi_flag(sbi, SBI_QUOTA_NEED_REPAIR);
>>>>>    	}
>>>>> -	sb_end_intwrite(inode->i_sb);
>>>>>    no_delete:
>>>>>    	dquot_drop(inode);
>>>
>>>
>>> _______________________________________________
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>>> Linux-f2fs-devel@...ts.sourceforge.net
>>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>>
>>
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>> Linux-f2fs-devel@...ts.sourceforge.net
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