Re: [PATCH] mm: vmscan: fix not scanning anonymous pages when detecting file refaults

From: Minchan Kim <minchan@kernel.org>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kuo-Hsin Yang <vovoy@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>, Sonny Rao <sonnyrao@chromium.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm: vmscan: fix not scanning anonymous pages when detecting file refaults
Date: Sat, 29 Jun 2019 08:34:13 +0900	[thread overview]
Message-ID: <20190628233413.GA245333@google.com> (raw)
In-Reply-To: <20190628142252.GA17212@cmpxchg.org>

On Fri, Jun 28, 2019 at 10:22:52AM -0400, Johannes Weiner wrote:
> Hi Minchan,
> 
> On Fri, Jun 28, 2019 at 03:51:38PM +0900, Minchan Kim wrote:
> > On Thu, Jun 27, 2019 at 02:41:23PM -0400, Johannes Weiner wrote:
> > > On Wed, Jun 19, 2019 at 04:08:35PM +0800, Kuo-Hsin Yang wrote:
> > > > Fixes: 2a2e48854d70 ("mm: vmscan: fix IO/refault regression in cache workingset transition")
> > > > Signed-off-by: Kuo-Hsin Yang <vovoy@chromium.org>
> > > 
> > > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > > 
> > > Your change makes sense - we should indeed not force cache trimming
> > > only while the page cache is experiencing refaults.
> > > 
> > > I can't say I fully understand the changelog, though. The problem of
> > 
> > I guess the point of the patch is "actual_reclaim" paramter made divergency
> > to balance file vs. anon LRU in get_scan_count. Thus, it ends up scanning
> > file LRU active/inactive list at file thrashing state.
> 
> Look at the patch again. The parameter was only added to retain
> existing behavior. We *always* did file-only reclaim while thrashing -
> all the way back to the two commits I mentioned below.

Yeah, I know it that we did force file relcaim if we have enough file LRU.
What I confused from the description was "actual_reclaim" part.
Thanks for the pointing out, Johannes. I confirmed it kept the old
behavior in get_scan_count.

> 
> > So, Fixes: 2a2e48854d70 ("mm: vmscan: fix IO/refault regression in cache workingset transition")
> > would make sense to me since it introduces the parameter.
> 
> What is the observable behavior problem that this patch introduced?
> 
> > > forcing cache trimming while there is enough page cache is older than
> > > the commit you refer to. It could be argued that this commit is
> > > incomplete - it could have added refault detection not just to
> > > inactive:active file balancing, but also the file:anon balancing; but
> > > it didn't *cause* this problem.
> > > 
> > > Shouldn't this be
> > > 
> > > Fixes: e9868505987a ("mm,vmscan: only evict file pages when we have plenty")
> > > Fixes: 7c5bd705d8f9 ("mm: memcg: only evict file pages when we have plenty")
> > 
> > That would affect, too but it would be trouble to have stable backport
> > since we don't have refault machinery in there.
> 
> Hm? The problematic behavior is that we force-scan file while file is
> thrashing. We can obviously only solve this in kernels that can
> actually detect thrashing.

What I meant is I thought it's -stable material but in there, we don't have
refault machinery in v3.8.
I agree this patch fixes above two commits you mentioned so we should use it.