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

From: Michal Hocko <mhocko@kernel.org>
To: Joel Fernandes <joel@joelfernandes.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 16:57:48 +0200	[thread overview]
Message-ID: <20190813145748.GM17933@dhcp22.suse.cz> (raw)
In-Reply-To: <20190813144517.GE258732@google.com>

On Tue 13-08-19 10:45:17, Joel Fernandes wrote:
> 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?).

It would work as long as we keep page tables around after unmap. As they
are easily reconstructable this is a good candidate for reclaim as well.

> 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/

For some reason I've missed it. I will coment on that.

-- 
Michal Hocko
SUSE Labs