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Date: Wed, 15 May 2024 04:42:58 +0000
From: Jaegeuk Kim <jaegeuk@kernel.org>
To: Chao Yu <chao@kernel.org>
Cc: linux-f2fs-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] f2fs: fix to avoid racing in between read and OPU dio
 write
Message-ID: <ZkQ9Uo5713Xpr2n7@google.com>
References: <20240510023906.281700-1-chao@kernel.org>
 <ZkOMwKAcKmEPQ4Xz@google.com>
 <fc0d8b1f-0c54-4447-8ceb-3722645f71c2@kernel.org>
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On 05/15, Chao Yu wrote:
> On 2024/5/15 0:09, Jaegeuk Kim wrote:
> > On 05/10, Chao Yu wrote:
> > > If lfs mode is on, buffered read may race w/ OPU dio write as below,
> > > it may cause buffered read hits unwritten data unexpectly, and for
> > > dio read, the race condition exists as well.
> > > 
> > > Thread A                      Thread B
> > > - f2fs_file_write_iter
> > >   - f2fs_dio_write_iter
> > >    - __iomap_dio_rw
> > >     - f2fs_iomap_begin
> > >      - f2fs_map_blocks
> > >       - __allocate_data_block
> > >        - allocated blkaddr #x
> > >         - iomap_dio_submit_bio
> > >                                - f2fs_file_read_iter
> > >                                 - filemap_read
> > >                                  - f2fs_read_data_folio
> > >                                   - f2fs_mpage_readpages
> > >                                    - f2fs_map_blocks
> > >                                     : get blkaddr #x
> > >                                    - f2fs_submit_read_bio
> > >                                IRQ
> > >                                - f2fs_read_end_io
> > >                                 : read IO on blkaddr #x complete
> > > IRQ
> > > - iomap_dio_bio_end_io
> > >   : direct write IO on blkaddr #x complete
> > > 
> > > This patch introduces a new per-inode i_opu_rwsem lock to avoid
> > > such race condition.
> > 
> > Wasn't this supposed to be managed by user-land?
> 
> Actually, the test case is:
> 
> 1. mount w/ lfs mode
> 2. touch file;
> 3. initialize file w/ 4k zeroed data; fsync;
> 4. continue triggering dio write 4k zeroed data to file;
> 5. and meanwhile, continue triggering buf/dio 4k read in file,
> use md5sum to verify the 4k data;
> 
> It expects data is all zero, however it turned out it's not.

Can we check outstanding write bios instead of abusing locks?

> 
> Thanks,
> 
> > 
> > > 
> > > Fixes: f847c699cff3 ("f2fs: allow out-place-update for direct IO in LFS mode")
> > > Signed-off-by: Chao Yu <chao@kernel.org>
> > > ---
> > > v2:
> > > - fix to cover dio read path w/ i_opu_rwsem as well.
> > >   fs/f2fs/f2fs.h  |  1 +
> > >   fs/f2fs/file.c  | 28 ++++++++++++++++++++++++++--
> > >   fs/f2fs/super.c |  1 +
> > >   3 files changed, 28 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > > index 30058e16a5d0..91cf4b3d6bc6 100644
> > > --- a/fs/f2fs/f2fs.h
> > > +++ b/fs/f2fs/f2fs.h
> > > @@ -847,6 +847,7 @@ struct f2fs_inode_info {
> > >        /* avoid racing between foreground op and gc */
> > >        struct f2fs_rwsem i_gc_rwsem[2];
> > >        struct f2fs_rwsem i_xattr_sem; /* avoid racing between reading and changing EAs */
> > > +     struct f2fs_rwsem i_opu_rwsem;  /* avoid racing between buf read and opu dio write */
> > > 
> > >        int i_extra_isize;              /* size of extra space located in i_addr */
> > >        kprojid_t i_projid;             /* id for project quota */
> > > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > > index 72ce1a522fb2..4ec260af321f 100644
> > > --- a/fs/f2fs/file.c
> > > +++ b/fs/f2fs/file.c
> > > @@ -4445,6 +4445,7 @@ static ssize_t f2fs_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
> > >        const loff_t pos = iocb->ki_pos;
> > >        const size_t count = iov_iter_count(to);
> > >        struct iomap_dio *dio;
> > > +     bool do_opu = f2fs_lfs_mode(sbi);
> > >        ssize_t ret;
> > > 
> > >        if (count == 0)
> > > @@ -4457,8 +4458,14 @@ static ssize_t f2fs_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
> > >                        ret = -EAGAIN;
> > >                        goto out;
> > >                }
> > > +             if (do_opu && !f2fs_down_read_trylock(&fi->i_opu_rwsem)) {
> > > +                     f2fs_up_read(&fi->i_gc_rwsem[READ]);
> > > +                     ret = -EAGAIN;
> > > +                     goto out;
> > > +             }
> > >        } else {
> > >                f2fs_down_read(&fi->i_gc_rwsem[READ]);
> > > +             f2fs_down_read(&fi->i_opu_rwsem);
> > >        }
> > > 
> > >        /*
> > > @@ -4477,6 +4484,7 @@ static ssize_t f2fs_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
> > >                ret = iomap_dio_complete(dio);
> > >        }
> > > 
> > > +     f2fs_up_read(&fi->i_opu_rwsem);
> > >        f2fs_up_read(&fi->i_gc_rwsem[READ]);
> > > 
> > >        file_accessed(file);
> > > @@ -4523,7 +4531,13 @@ static ssize_t f2fs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> > >        if (f2fs_should_use_dio(inode, iocb, to)) {
> > >                ret = f2fs_dio_read_iter(iocb, to);
> > >        } else {
> > > +             bool do_opu = f2fs_lfs_mode(F2FS_I_SB(inode));
> > > +
> > > +             if (do_opu)
> > > +                     f2fs_down_read(&F2FS_I(inode)->i_opu_rwsem);
> > >                ret = filemap_read(iocb, to, 0);
> > > +             if (do_opu)
> > > +                     f2fs_up_read(&F2FS_I(inode)->i_opu_rwsem);
> > >                if (ret > 0)
> > >                        f2fs_update_iostat(F2FS_I_SB(inode), inode,
> > >                                                APP_BUFFERED_READ_IO, ret);
> > > @@ -4748,14 +4762,22 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
> > >                        ret = -EAGAIN;
> > >                        goto out;
> > >                }
> > > +             if (do_opu && !f2fs_down_write_trylock(&fi->i_opu_rwsem)) {
> > > +                     f2fs_up_read(&fi->i_gc_rwsem[READ]);
> > > +                     f2fs_up_read(&fi->i_gc_rwsem[WRITE]);
> > > +                     ret = -EAGAIN;
> > > +                     goto out;
> > > +             }
> > >        } else {
> > >                ret = f2fs_convert_inline_inode(inode);
> > >                if (ret)
> > >                        goto out;
> > > 
> > >                f2fs_down_read(&fi->i_gc_rwsem[WRITE]);
> > > -             if (do_opu)
> > > +             if (do_opu) {
> > >                        f2fs_down_read(&fi->i_gc_rwsem[READ]);
> > > +                     f2fs_down_write(&fi->i_opu_rwsem);
> > > +             }
> > >        }
> > > 
> > >        /*
> > > @@ -4779,8 +4801,10 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
> > >                ret = iomap_dio_complete(dio);
> > >        }
> > > 
> > > -     if (do_opu)
> > > +     if (do_opu) {
> > > +             f2fs_up_write(&fi->i_opu_rwsem);
> > >                f2fs_up_read(&fi->i_gc_rwsem[READ]);
> > > +     }
> > >        f2fs_up_read(&fi->i_gc_rwsem[WRITE]);
> > > 
> > >        if (ret < 0)
> > > diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> > > index daf2c4dbe150..b4ed3b094366 100644
> > > --- a/fs/f2fs/super.c
> > > +++ b/fs/f2fs/super.c
> > > @@ -1428,6 +1428,7 @@ static struct inode *f2fs_alloc_inode(struct super_block *sb)
> > >        init_f2fs_rwsem(&fi->i_gc_rwsem[READ]);
> > >        init_f2fs_rwsem(&fi->i_gc_rwsem[WRITE]);
> > >        init_f2fs_rwsem(&fi->i_xattr_sem);
> > > +     init_f2fs_rwsem(&fi->i_opu_rwsem);
> > > 
> > >        /* Will be used by directory only */
> > >        fi->i_dir_level = F2FS_SB(sb)->dir_level;
> > > --
> > > 2.40.1