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

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Date:   Sat, 28 Jul 2018 22:14:59 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     Chao Yu <yuchao0@...wei.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] f2fs: avoid race between zero_range and background GC

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?

> 
> 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