From: Eric Biggers <ebiggers@kernel.org>
To: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: linux-kernel@vger.kernel.org,
linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com
Subject: Re: [PATCH v2] f2fs: avoid readahead race condition
Date: Mon, 29 Jun 2020 11:25:02 -0700 [thread overview]
Message-ID: <20200629182502.GF20492@sol.localdomain> (raw)
In-Reply-To: <20200629182414.GA1117827@google.com>
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@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.
>
Okay, then it should use READ_ONCE() and WRITE_ONCE().
- Eric
next prev parent reply other threads:[~2020-06-29 21:30 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-24 1:21 [PATCH] f2fs: avoid readahead race condition Jaegeuk Kim
2020-06-28 2:35 ` [f2fs-dev] " Chao Yu
2020-06-29 15:03 ` [PATCH v2] " Jaegeuk Kim
2020-06-29 16:09 ` [f2fs-dev] " Eric Biggers
2020-06-29 18:24 ` Jaegeuk Kim
2020-06-29 18:25 ` Eric Biggers [this message]
2020-06-29 20:27 ` [f2fs-dev] [PATCH v3] " Jaegeuk Kim
2020-06-30 20:43 ` Nathan Chancellor
2020-06-30 20:56 ` Jaegeuk Kim
2020-07-01 1:59 ` Chao Yu
2020-07-01 7:47 ` Chao Yu
2020-07-01 16:14 ` Jaegeuk Kim
2020-07-03 1:34 ` Chao Yu
2020-07-01 16:14 ` [f2fs-dev] [PATCH v4] " Jaegeuk Kim
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