Re: [PATCH v5 1/6] mm/page_idle: Add per-pid idle page tracking using virtual index

From: Joel Fernandes <joel@joelfernandes.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org,
	Alexey Dobriyan <adobriyan@gmail.com>,
	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>,
	minchan@kernel.org, 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 1/6] mm/page_idle: Add per-pid idle page tracking using virtual index
Date: Tue, 13 Aug 2019 10:45:17 -0400	[thread overview]
Message-ID: <20190813144517.GE258732@google.com> (raw)
In-Reply-To: <20190813141432.GL17933@dhcp22.suse.cz>

On Tue, Aug 13, 2019 at 04:14:32PM +0200, Michal Hocko wrote:
[snip] 
> > > If the API is flawed then this is likely going
> > > to kick us later and will be hard to fix. I am still not convinced about
> > > the swap part of the thing TBH.
> > 
> > Ok, then let us discuss it. As I mentioned before, without this we lose the
> > access information due to MADVISE or swapping. Minchan and Konstantin both
> > suggested it that's why I also added it (other than me also realizing that it
> > is neeed).
> 
> I have described my concerns about the general idle bit behavior after
> unmapping pointing to discrepancy with !anon pages. And I believe those
> haven't been addressed yet.

You are referring to this post right?
https://lkml.org/lkml/2019/8/6/637

Specifically your question was:
How are you going to handle situation when the page is unmapped  and refaulted again (e.g. a normal reclaim of a pagecache)?

Currently I don't know how to implement that. Would it work if I stored the
page-idle bit information in the pte of the file page (after the page is
unmapped by reclaim?).

Also, this could be a future extension - the Android heap profiler does not
need it right now. I know that's not a good argument but it is useful to say
that it doesn't affect a real world usecase.. the swap issue on the other
hand, is a real usecase. Since the profiler should not get affected by
swapping or MADVISE_COLD hints.

> Besides that I am still not seeing any
> description of the usecase that would suffer from the lack of the
> functionality in changelogs.

You are talking about the swap usecase? The usecase is well layed out in v5
2/6. Did you see it? https://lore.kernel.org/patchwork/patch/1112283/

thanks,

 - Joel