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

[Jaegeuk Kim <jaegeuk@kernel.org>]

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@kernel.org>
> > ---
> > 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.

> 
> - Eric