linux-kernel - Re: [PATCH] F2FS: invalidate META

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Date:   Mon, 01 Nov 2021 16:23:32 +0900
From:   Hyeong-Jun Kim <hj514.kim@...sung.com>
To:     Chao Yu <chao@...nel.org>, Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] F2FS: invalidate META_MAPPING before IPU/DIO write

On Mon, 2021-11-01 at 15:12 +0800, Chao Yu wrote:
> On 2021/11/1 15:09, Hyeong-Jun Kim wrote:
> > On Mon, 2021-11-01 at 14:28 +0800, Chao Yu wrote:
> > > On 2021/11/1 13:42, Hyeong-Jun Kim wrote:
> > > > Encrypted pages during GC are read and cached in META_MAPPING.
> > > > However, due to cached pages in META_MAPPING, there is an issue
> > > > where
> > > > newly written pages are lost by IPU or DIO writes.
> > > > 
> > > > Thread A                              Thread B
> > > > - f2fs_gc(): blk 0x10 -> 0x20 (a)
> > > >                                         - IPU or DIO write on
> > > > blk
> > > > 0x20 (b)
> > > > - f2fs_gc(): blk 0x20 -> 0x30 (c)
> > > > 
> > > > (a) page for blk 0x20 is cached in META_MAPPING and page for
> > > > blk
> > > > 0x10
> > > >       is invalidated from META_MAPPING.
> > > > (b) write new data to blk 0x200 using IPU or DIO, but outdated
> > > > data
> > > >       still remains in META_MAPPING.
> > > > (c) f2fs_gc() try to move blk from 0x20 to 0x30 using cached
> > > > page
> > > > in
> > > >       META_MAPPING. In conclusion, the newly written data in
> > > > (b) is
> > > > lost.
> > > 
> > > In c), f2fs_gc() will readahead encrypted block from disk via
> > > ra_data_block() anyway,
> > > not matter cached encrypted page of meta inode is uptodate or
> > > not, so
> > > it's safe, right?
> > 
> > Right,
> > However, if DIO write is performed between phase 3 and phase 4 of
> > f2fs_gc(),
> > the cached page of meta_mapping will be out-dated, though it read
> > data
> > from
> > disk via ra_data_block() in phase 3.
> > 
> > What do you think?
> 
> Due to i_gc_rwsem lock coverage, the race condition should not happen
> right now?
> 
- Thread A                                       - Thread B
/* phase 3 */
down_write(i_gc_rwsem)
ra_data_block()
up_write(i_gc_rwsem)
                                                       
 f2fs_direct_IO() : 
                                                       
 down_read(i_gc_rwsem)
                                                       
 __blockdev_direct_IO()
                                                           ...
                                                         
 get_ddata_block_dio_write()
                                                           ...
                                                         
 f2fs_dio_submit_bio()
                                                       
 up_read(i_gc_rwsem)
/* phase 4 */
down_write(i_gc_rwsem)
move_data_block()
up_write(i_gc_rwsem)

It looks, i_gc_rwsem could not protect page update between phase 3 and
4.

Am I missing anything?

Thanks

> Thanks,
> 
> > Thanks,
> > > Am I missing anything?
> > > 
> > > Thanks,
> > > 
> > > > To address this issue, invalidating pages in META_MAPPING
> > > > before
> > > > IPU or
> > > > DIO write.
> > > > 
> > > > Signed-off-by: Hyeong-Jun Kim <
> > > > hj514.kim@...sung.com
> > > > 
> > > > 
> > > > ---
> > > >    fs/f2fs/data.c    | 2 ++
> > > >    fs/f2fs/segment.c | 3 +++
> > > >    2 files changed, 5 insertions(+)
> > > > 
> > > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > > > index 74e1a350c1d8..9f754aaef558 100644
> > > > --- a/fs/f2fs/data.c
> > > > +++ b/fs/f2fs/data.c
> > > > @@ -1708,6 +1708,8 @@ int f2fs_map_blocks(struct inode *inode,
> > > > struct f2fs_map_blocks *map,
> > > >    		 */
> > > >    		f2fs_wait_on_block_writeback_range(inode,
> > > >    						map->m_pblk,
> > > > map-
> > > > > m_len);
> > > > 
> > > > +		invalidate_mapping_pages(META_MAPPING(sbi),
> > > > +						map->m_pblk,
> > > > map-
> > > > > m_pblk);
> > > > 
> > > >    
> > > >    		if (map->m_multidev_dio) {
> > > >    			block_t blk_addr = map->m_pblk;
> > > > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > > > index 526423fe84ce..f57c55190f9e 100644
> > > > --- a/fs/f2fs/segment.c
> > > > +++ b/fs/f2fs/segment.c
> > > > @@ -3652,6 +3652,9 @@ int f2fs_inplace_write_data(struct
> > > > f2fs_io_info *fio)
> > > >    		goto drop_bio;
> > > >    	}
> > > >    
> > > > +	invalidate_mapping_pages(META_MAPPING(fio->sbi),
> > > > +				fio->new_blkaddr, fio-
> > > > >new_blkaddr);
> > > > +
> > > >    	stat_inc_inplace_blocks(fio->sbi);
> > > >    
> > > >    	if (fio->bio && !(SM_I(sbi)->ipu_policy & (1 <<
> > > > F2FS_IPU_NOCACHE)))
> > > > 
> > > 
> > > 
> 
>