linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: avoid race between zero

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Date:   Sun, 29 Jul 2018 18:38:16 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: avoid race between zero_range and
 background GC

On 07/28, Jaegeuk Kim wrote:
> On 07/29, Chao Yu wrote:
> > On 2018/7/29 10:59, Jaegeuk Kim wrote:
> > > On 07/29, Chao Yu wrote:
> > >> On 2018/7/29 10:02, Jaegeuk Kim wrote:
> > >>> On 07/27, Chao Yu wrote:
> > >>>> On 2018/7/27 18:29, Jaegeuk Kim wrote:
> > >>>>> On 07/26, Chao Yu wrote:
> > >>>>>> Thread A				Background GC
> > >>>>>> - f2fs_zero_range
> > >>>>>>  - truncate_pagecache_range
> > >>>>>> 					- gc_data_segment
> > >>>>>> 					 - get_read_data_page
> > >>>>>> 					  - move_data_page
> > >>>>>> 					   - set_page_dirty
> > >>>>>> 					   - set_cold_data
> > >>>>>>  - f2fs_do_zero_range
> > >>>>>>   - dn->data_blkaddr = NEW_ADDR;
> > >>>>>>   - f2fs_set_data_blkaddr
> > >>>>>>
> > >>>>>> Actually, we don't need to set dirty & checked flag on the page, since
> > >>>>>> all valid data in the page should be zeroed by zero_range().
> > >>>>>
> > >>>>> But, it doesn't matter too much, right?
> > >>>>
> > >>>> No, if the dirtied page is writebacked after f2fs_do_zero_range(), result of
> > >>>> zero_range() should be wrong, as zeroed page contains valid user data.
> > >>>
> > >>> How about truncating page caches after block address change or doing it twice
> > >>> before and after?
> > >>
> > >> Thread A				Background GC
> > >> - f2fs_zero_range
> > >>  - truncate_pagecache_range
> > >> 					- gc_data_segment
> > >> 					 - get_read_data_page
> > >> 					  - move_data_page
> > >> 					   - set_page_dirty
> > >> 					   - set_cold_data
> > >>  - f2fs_do_zero_range
> > >>   - dn->data_blkaddr = NEW_ADDR;
> > >>   - f2fs_set_data_blkaddr
> > >> 					bdi-flusher
> > >> 					- __write_data_page
> > >> 					 - f2fs_update_data_blkaddr
> > >> 					 : data_blkaddr has been updated here.
> > >>  - truncate_pagecache_range
> > >>  : data & dnode has been writebacked before page cache truncation?
> > >>
> > >> How about this case?
> > > 
> > > So, truncating pages under dnode lock can address it?
> > 
> > Normally, our lock dependency is
> > 
> > ->writepage()
> > lock data page -> lock dnode page
> > 
> > here
> > lock dnode page -> truncate_pagecache_range::lock data page
> > 
> > Will easily cause deadlock?
> 
> Yeah. Can we add an inode flag to bypass GC in this case, then?

Hmm, BTW, how about using i_gc_rwsem[WRITE] in a very narrow scope?

for (index = pg_start; index < pg_end;) {
	f2fs_lock_op();
	down_write(i_gc_rwsem[WRITE]);
	truncate_page_cache_range(index, index + 4k);
	f2fs_do_zero_range(&dn, index, end);
	up_write(i_gc_rwsem[WRITE]);
	f2fs_unlock_op();
	f2fs_balance_fs();
}

> 
> > 
> > Thanks,
> > 
> > > 
> > >>
> > >> Thanks,
> > >>
> > >>>
> > >>>>
> > >>>>>
> > >>>>>> Use i_gc_rwsem[WRITE] to avoid such race condition.
> > >>>>>
> > >>>>> Hope to avoid abusing i_gc_rwsem[] tho.
> > >>>>
> > >>>> Agreed, let's try avoiding until we have to use it.
> > >>>>
> > >>>> Thanks,
> > >>>>
> > >>>>>
> > >>>>>>
> > >>>>>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> > >>>>>> ---
> > >>>>>>  fs/f2fs/file.c | 2 ++
> > >>>>>>  1 file changed, 2 insertions(+)
> > >>>>>>
> > >>>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > >>>>>> index 267ec3794e1e..7bd2412a8c37 100644
> > >>>>>> --- a/fs/f2fs/file.c
> > >>>>>> +++ b/fs/f2fs/file.c
> > >>>>>> @@ -1309,6 +1309,7 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
> > >>>>>>  	if (ret)
> > >>>>>>  		return ret;
> > >>>>>>  
> > >>>>>> +	down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> > >>>>>>  	down_write(&F2FS_I(inode)->i_mmap_sem);
> > >>>>>>  	ret = filemap_write_and_wait_range(mapping, offset, offset + len - 1);
> > >>>>>>  	if (ret)
> > >>>>>> @@ -1389,6 +1390,7 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
> > >>>>>>  	}
> > >>>>>>  out_sem:
> > >>>>>>  	up_write(&F2FS_I(inode)->i_mmap_sem);
> > >>>>>> +	up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> > >>>>>>  
> > >>>>>>  	return ret;
> > >>>>>>  }
> > >>>>>> -- 
> > >>>>>> 2.18.0.rc1
> 
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