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

[Jaegeuk Kim <jaegeuk@kernel.org>]

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@kernel.org>
> > ---
> > 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 ‘f2fs_mpage_readpages’:
> ../fs/f2fs/data.c:2327:5: error: label ‘next_page’ 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@infradead.org/T/#t

Thanks,

> 
> Cheers,
> Nathan
> 
> 
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