Re: [PATCH v2] f2fs: avoid readahead race condition

[Eric Biggers]

On Mon, Jun 29, 2020 at 11:24:14AM -0700, Jaegeuk Kim wrote:
> On 06/29, Eric Biggers wrote:
> > On Mon, Jun 29, 2020 at 08:03:23AM -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>
> > > ---
> > > 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, 20 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > > index d6094b9f3916..9b69a159cc6c 100644
> > > --- a/fs/f2fs/data.c
> > > +++ b/fs/f2fs/data.c
> > > @@ -2403,6 +2403,7 @@ int f2fs_mpage_readpages(struct address_space *mapping,
> > >  #endif
> > >  	unsigned max_nr_pages = nr_pages;
> > >  	int ret = 0;
> > > +	bool drop_ra = false;
> > >  
> > >  	map.m_pblk = 0;
> > >  	map.m_lblk = 0;
> > > @@ -2413,13 +2414,25 @@ int f2fs_mpage_readpages(struct address_space *mapping,
> > >  	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 (pages && is_readahead) {
> > > +		page = list_last_entry(pages, struct page, lru);
> > > +		if (F2FS_I(inode)->ra_offset == page_index(page))
> > > +			drop_ra = true;
> > > +		else
> > > +			F2FS_I(inode)->ra_offset = page_index(page);
> > > +	}
> > 
> > This is a data race because ra_offset can be read/written by different threads
> > concurrently.
> > 
> > It either needs locking, or READ_ONCE() and WRITE_ONCE() if races are okay.
> 
> I just wanted to keep zero overhead, since it doesn't matter either cases of
> skipping readahead or not.
> 

Okay, then it should use READ_ONCE() and WRITE_ONCE().

- Eric