Re: [f2fs-dev] [PATCH v3] f2fs: avoid readahead race condition

[Jaegeuk Kim]

On 07/01, Chao Yu wrote:
> On 2020/7/1 9:59, Chao Yu wrote:
> > On 2020/7/1 4:56, Jaegeuk Kim wrote:
> >> On 06/30, Nathan Chancellor wrote:
> >>> On Mon, Jun 29, 2020 at 01:27:20PM -0700, Jaegeuk Kim wrote:
> >>>> If two readahead threads having same offset enter in readpages, every read
> >>>> IOs are split and issued to the disk which giving lower bandwidth.
> >>>>
> >>>> This patch tries to avoid redundant readahead calls.
> >>>>
> >>>> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> >>>> ---
> >>>> v3:
> >>>>  - use READ|WRITE_ONCE
> >>>> v2:
> >>>>   - add missing code to bypass read
> >>>>  
> >>>>  fs/f2fs/data.c  | 18 ++++++++++++++++++
> >>>>  fs/f2fs/f2fs.h  |  1 +
> >>>>  fs/f2fs/super.c |  2 ++
> >>>>  3 files changed, 21 insertions(+)
> >>>>
> >>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> >>>> index 995cf78b23c5e..360b4c9080d97 100644
> >>>> --- a/fs/f2fs/data.c
> >>>> +++ b/fs/f2fs/data.c
> >>>> @@ -2296,6 +2296,7 @@ static int f2fs_mpage_readpages(struct inode *inode,
> >>>>  	unsigned nr_pages = rac ? readahead_count(rac) : 1;
> >>>>  	unsigned max_nr_pages = nr_pages;
> >>>>  	int ret = 0;
> >>>> +	bool drop_ra = false;
> >>>>  
> >>>>  	map.m_pblk = 0;
> >>>>  	map.m_lblk = 0;
> >>>> @@ -2306,10 +2307,24 @@ static int f2fs_mpage_readpages(struct inode *inode,
> >>>>  	map.m_seg_type = NO_CHECK_TYPE;
> >>>>  	map.m_may_create = false;
> >>>>  
> >>>> +	/*
> >>>> +	 * Two readahead threads for same address range can cause race condition
> >>>> +	 * which fragments sequential read IOs. So let's avoid each other.
> >>>> +	 */
> >>>> +	if (rac && readahead_count(rac)) {
> >>>> +		if (READ_ONCE(F2FS_I(inode)->ra_offset) == readahead_index(rac))
> >>>> +			drop_ra = true;
> >>>> +		else
> >>>> +			WRITE_ONCE(F2FS_I(inode)->ra_offset,
> >>>> +						readahead_index(rac));
> >>>> +	}
> >>>> +
> >>>>  	for (; nr_pages; nr_pages--) {
> >>>>  		if (rac) {
> >>>>  			page = readahead_page(rac);
> >>>>  			prefetchw(&page->flags);
> >>>> +			if (drop_ra)
> >>>> +				goto next_page;
> >>>
> >>> When CONFIG_F2FS_FS_COMPRESSION is not set (i.e. x86_64 defconfig +
> >>> CONFIG_F2FS_FS=y):
> >>>
> >>> $ make -skj"$(nproc)" O=out distclean defconfig fs/f2fs/data.o
> >>> ../fs/f2fs/data.c: In function âf2fs_mpage_readpagesâ:
> >>> ../fs/f2fs/data.c:2327:5: error: label ânext_pageâ used but not defined
> >>>  2327 |     goto next_page;
> >>>       |     ^~~~
> >>> ...
> >>
> >> Thanks. I pushed the fix for -next.
> >> https://lore.kernel.org/linux-f2fs-devel/1be18397-7fc6-703e-121b-e210e101357f@xxxxxxxxxxxxx/T/#t
> 
> It will hang the kernel because we missed to unlock those cached pages,
> I changed to 'goto set_error_page', the issue was gone.

How about v4?

> 
> Thanks,
> 
> > 
> > Reviewed-by: Chao Yu <yuchao0@xxxxxxxxxx>
> > 
> > Thanks,
> > 
> >>
> >> Thanks,
> >>
> >>>
> >>> Cheers,
> >>> Nathan
> >>>
> >>>
> >>> _______________________________________________
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> >>> Linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> >>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
> >>
> >>
> >> _______________________________________________
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> >> Linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> >> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
> >>
> > 
> > 
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> >