linux-kernel - Re: [PATCH v3] f2fs: Reduce the scope of setting fsck tag when de->name

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Date:   Wed, 4 Aug 2021 11:32:59 +0800
From:   Chao Yu <chao@...nel.org>
To:     李扬韬 <frank.li@...o.com>
Cc:     jaegeuk@...nel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] f2fs: Reduce the scope of setting fsck tag when
 de->name_len is zero

Hello, Yangtao,

I did some cleanup in your patch, and resend for you, please
note that. :)

Thanks,

On 2021/7/20 20:31, 李扬韬 wrote:
> HI Chao，
> 
> From: Chao Yu <chao@...nel.org>
> Date: 2021-07-20 18:23:15
> To:  Yangtao Li <frank.li@...o.com>,jaegeuk@...nel.org
> Cc:  linux-f2fs-devel@...ts.sourceforge.net,linux-kernel@...r.kernel.org
> Subject: Re: [PATCH v3] f2fs: Reduce the scope of setting fsck tag when de->name_len is zero>On 2021/7/20 15:06, Yangtao Li wrote:
>>> I recently found a case where de->name_len is 0 in f2fs_fill_dentries() easily reproduced,
>>> and finally set the fsck flag.
>>>
>>> Thread A					Thread B
>>>
>>> f2fs_readdir
>>> 	f2fs_read_inline_dir
>>> 		ctx->pos = d.max
>>> 						f2fs_add_dentry
>>> 							f2fs_add_inline_entry
>>> 								do_convert_inline_dir
>>> 							f2fs_add_regular_entry
>>> f2fs_readdir
>>> 	f2fs_fill_dentries
>>> 		set_sbi_flag(sbi, SBI_NEED_FSCK)
>>>
>>> Process A opens the folder, and has been reading without closing it. During this period,
>>> Process B created a file under the folder (occupying multiple f2fs_dir_entry, exceeding
>>> the d.max of the inline dir). After creation, process A uses the d.max of inline dir to
>>> read it again, and it will read that de->name_len is 0.
>>>
>>> And Chao pointed out that w/o inline conversion, the race condition still can happen as below
>>>
>>> dir_entry1: A
>>> dir_entry2: B
>>> dir_entry3: C
>>> free slot: _
>>> ctx->pos: ^
>>>
>>> Before:
>>> AAAABBBB___
>>> 	 ^
>>
>> please use blank instead of tab before '^'
> 
> I don't know exactly what happened. In fact, in v2, spaces were used. Then it was changed to tab in v3.
> 
>>
>>> Thread B delete dir_entry2, and create dir_entry3.
>>>
>>> After:
>>> AAAACCCCC__
>>> 	 ^
>>
>> Ditto
>>
>>>
>>> In these scenarios, the file system is not damaged, and it's hard to avoid it. But we can bypass
>>> tagging FSCK flag if:
>>> a) bit_pos (:= ctx->pos % d->max) is non-zero & b) before bit_pos moves to first
>>> valid dir_entry.
>>>
>>> Signed-off-by: Yangtao Li <frank.li@...o.com>
>>> ---
>>>    fs/f2fs/dir.c | 14 +++++++++-----
>>>    1 file changed, 9 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
>>> index 456651682daf..bfe942733b5e 100644
>>> --- a/fs/f2fs/dir.c
>>> +++ b/fs/f2fs/dir.c
>>> @@ -1000,6 +1000,7 @@ int f2fs_fill_dentries(struct dir_context *ctx, struct f2fs_dentry_ptr *d,
>>>    	struct f2fs_sb_info *sbi = F2FS_I_SB(d->inode);
>>>    	struct blk_plug plug;
>>>    	bool readdir_ra = sbi->readdir_ra == 1;
>>> +	bool found_valid_dirent  = false;
>>
>> One more blank before '='.
>>
>> bool found_valid_dirent = false;
>>
> 
> OK.
> 
> Thx，
> 
> 
>