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Date:   Tue, 13 Aug 2019 11:36:59 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Michal Hocko <mhocko@kernel.org>, khlebnikov@yandex-team.ru
Cc:     linux-kernel@vger.kernel.org, Minchan Kim <minchan@kernel.org>,
        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, Robin Murphy <robin.murphy@arm.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 v5 2/6] mm/page_idle: Add support for handling swapped
 PG_Idle pages
Message-ID: <20190813153659.GD14622@google.com>
References: <20190807171559.182301-1-joel@joelfernandes.org>
 <20190807171559.182301-2-joel@joelfernandes.org>
 <20190813150450.GN17933@dhcp22.suse.cz>
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On Tue, Aug 13, 2019 at 05:04:50PM +0200, Michal Hocko wrote:
> On Wed 07-08-19 13:15:55, Joel Fernandes (Google) wrote:
> > Idle page tracking currently does not work well in the following
> > scenario:
> >  1. mark page-A idle which was present at that time.
> >  2. run workload
> >  3. page-A is not touched by workload
> >  4. *sudden* memory pressure happen so finally page A is finally swapped out
> >  5. now see the page A - it appears as if it was accessed (pte unmapped
> >     so idle bit not set in output) - but it's incorrect.
> > 
> > To fix this, we store the idle information into a new idle bit of the
> > swap PTE during swapping of anonymous pages.
> >
> > Also in the future, madvise extensions will allow a system process
> > manager (like Android's ActivityManager) to swap pages out of a process
> > that it knows will be cold. To an external process like a heap profiler
> > that is doing idle tracking on another process, this procedure will
> > interfere with the idle page tracking similar to the above steps.
> 
> This could be solved by checking the !present/swapped out pages
> right? Whoever decided to put the page out to the swap just made it
> idle effectively.  So the monitor can make some educated guess for
> tracking. If that is fundamentally not possible then please describe
> why.

But the monitoring process (profiler) does not have control over the 'whoever
made it effectively idle' process.

As you said it will be a guess, it will not be accurate.

I am curious what is your concern with using a bit in the swap PTE?

(Adding Konstantin as well since we may be interested in this, since we also
suggested this idea).

thanks,

 - Joel