Re: [PATCH v4 3/5] [RFC] arm64: Add support for idle bit in swap PTE

From: Joel Fernandes <joel@joelfernandes.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>,
	linux-kernel@vger.kernel.org, Robin Murphy <robin.murphy@arm.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Borislav Petkov <bp@alien8.de>,
	Brendan Gregg <bgregg@netflix.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christian Hansen <chansen3@cisco.com>,
	dancol@google.com, fmayer@google.com,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	kernel-team@android.com, linux-api@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, Mike Rapoport <rppt@linux.ibm.com>,
	namhyung@google.com, paulmck@linux.ibm.com,
	Roman Gushchin <guro@fb.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	surenb@google.com, Thomas Gleixner <tglx@linutronix.de>,
	tkjos@google.com, Vladimir Davydov <vdavydov.dev@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH v4 3/5] [RFC] arm64: Add support for idle bit in swap PTE
Date: Tue, 6 Aug 2019 07:26:06 -0400	[thread overview]
Message-ID: <20190806112606.GC117316@google.com> (raw)
In-Reply-To: <20190806111452.GW11812@dhcp22.suse.cz>

On Tue, Aug 06, 2019 at 01:14:52PM +0200, Michal Hocko wrote:
> On Tue 06-08-19 20:07:37, Minchan Kim wrote:
> > On Tue, Aug 06, 2019 at 12:47:55PM +0200, Michal Hocko wrote:
> > > On Tue 06-08-19 06:36:27, Joel Fernandes wrote:
> > > > On Tue, Aug 06, 2019 at 10:42:03AM +0200, Michal Hocko wrote:
> > > > > On Mon 05-08-19 13:04:49, Joel Fernandes (Google) wrote:
> > > > > > This bit will be used by idle page tracking code to correctly identify
> > > > > > if a page that was swapped out was idle before it got swapped out.
> > > > > > Without this PTE bit, we lose information about if a page is idle or not
> > > > > > since the page frame gets unmapped.
> > > > > 
> > > > > And why do we need that? Why cannot we simply assume all swapped out
> > > > > pages to be idle? They were certainly idle enough to be reclaimed,
> > > > > right? Or what does idle actualy mean here?
> > > > 
> > > > Yes, but other than swapping, in Android a page can be forced to be swapped
> > > > out as well using the new hints that Minchan is adding?
> > > 
> > > Yes and that is effectivelly making them idle, no?
> > 
> > 1. mark page-A idle which was present at that time.
> > 2. run workload
> > 3. page-A is touched several times
> > 4. *sudden* memory pressure happen so finally page A is finally swapped out
> > 5. now see the page A idle - but it's incorrect.
> 
> Could you expand on what you mean by idle exactly? Why pageout doesn't
> really qualify as "mark-idle and reclaim"? Also could you describe a
> usecase where the swapout distinction really matters and it would lead
> to incorrect behavior?

Michal,
Did you read this post ? :
https://lore.kernel.org/lkml/20190806104715.GC218260@google.com/T/#m4ece68ceaf6e54d4d29e974f5f4c1080e733f6c1

Just wanted to be sure you did not miss it.

thanks,

 - Joel