Re: [PATCH v3] f2fs: Reduce the scope of setting fsck tag when de->name_len is zero

[Chao Yu]

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@xxxxxxxxxx>
> Date: 2021-07-20 18:23:15
> To:  Yangtao Li <frank.li@xxxxxxxx>,jaegeuk@xxxxxxxxxx
> Cc:  linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx,linux-kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxxx>
> > > ---
> > >    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，