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

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Message-ID: <20190806112606.GC117316@google.com>
Date:   Tue, 6 Aug 2019 07:26:06 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org,
        Robin Murphy <robin.murphy@....com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Brendan Gregg <bgregg@...flix.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Christian Hansen <chansen3@...co.com>, dancol@...gle.com,
        fmayer@...gle.com, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>, kernel-team@...roid.com,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Mike Rapoport <rppt@...ux.ibm.com>, namhyung@...gle.com,
        paulmck@...ux.ibm.com, Roman Gushchin <guro@...com>,
        Stephen Rothwell <sfr@...b.auug.org.au>, surenb@...gle.com,
        Thomas Gleixner <tglx@...utronix.de>, tkjos@...gle.com,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v4 3/5] [RFC] arm64: Add support for idle bit in swap PTE

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