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Date:   Tue, 13 Aug 2019 16:14:32 +0200
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,
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        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,
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        Robin Murphy <robin.murphy@arm.com>,
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        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
Message-ID: <20190813141432.GL17933@dhcp22.suse.cz>
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On Tue 13-08-19 09:51:52, Joel Fernandes wrote:
> On Tue, Aug 13, 2019 at 11:14:30AM +0200, Michal Hocko wrote:
> > On Mon 12-08-19 10:56:20, Joel Fernandes wrote:
> > > On Thu, Aug 08, 2019 at 10:00:44AM +0200, Michal Hocko wrote:
> > > > On Wed 07-08-19 17:31:05, Joel Fernandes wrote:
> > > > > On Wed, Aug 07, 2019 at 01:58:40PM -0700, Andrew Morton wrote:
> > > > > > On Wed, 7 Aug 2019 16:45:30 -0400 Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > > > 
> > > > > > > On Wed, Aug 07, 2019 at 01:04:02PM -0700, Andrew Morton wrote:
> > > > > > > > On Wed,  7 Aug 2019 13:15:54 -0400 "Joel Fernandes (Google)" <joel@joelfernandes.org> wrote:
> > > > > > > > 
> > > > > > > > > In Android, we are using this for the heap profiler (heapprofd) which
> > > > > > > > > profiles and pin points code paths which allocates and leaves memory
> > > > > > > > > idle for long periods of time. This method solves the security issue
> > > > > > > > > with userspace learning the PFN, and while at it is also shown to yield
> > > > > > > > > better results than the pagemap lookup, the theory being that the window
> > > > > > > > > where the address space can change is reduced by eliminating the
> > > > > > > > > intermediate pagemap look up stage. In virtual address indexing, the
> > > > > > > > > process's mmap_sem is held for the duration of the access.
> > > > > > > > 
> > > > > > > > So is heapprofd a developer-only thing?  Is heapprofd included in
> > > > > > > > end-user android loads?  If not then, again, wouldn't it be better to
> > > > > > > > make the feature Kconfigurable so that Android developers can enable it
> > > > > > > > during development then disable it for production kernels?
> > > > > > > 
> > > > > > > Almost all of this code is already configurable with
> > > > > > > CONFIG_IDLE_PAGE_TRACKING. If you disable it, then all of this code gets
> > > > > > > disabled.
> > > > > > > 
> > > > > > > Or are you referring to something else that needs to be made configurable?
> > > > > > 
> > > > > > Yes - the 300+ lines of code which this patchset adds!
> > > > > > 
> > > > > > The impacted people will be those who use the existing
> > > > > > idle-page-tracking feature but who will not use the new feature.  I
> > > > > > guess we can assume this set is small...
> > > > > 
> > > > > Yes, I think this set should be small. The code size increase of page_idle.o
> > > > > is from ~1KB to ~2KB. Most of the extra space is consumed by
> > > > > page_idle_proc_generic() function which this patch adds. I don't think adding
> > > > > another CONFIG option to disable this while keeping existing
> > > > > CONFIG_IDLE_PAGE_TRACKING enabled, is worthwhile but I am open to the
> > > > > addition of such an option if anyone feels strongly about it. I believe that
> > > > > once this patch is merged, most like this new interface being added is what
> > > > > will be used more than the old interface (for some of the usecases) so it
> > > > > makes sense to keep it alive with CONFIG_IDLE_PAGE_TRACKING.
> > > > 
> > > > I would tend to agree with Joel here. The functionality falls into an
> > > > existing IDLE_PAGE_TRACKING config option quite nicely. If there really
> > > > are users who want to save some space and this is standing in the way
> > > > then they can easily add a new config option with some justification so
> > > > the savings are clear. Without that an additional config simply adds to
> > > > the already existing configurability complexity and balkanization.
> > > 
> > > Michal, Andrew, Minchan,
> > > 
> > > Would you have any other review comments on the v5 series? This is just a new
> > > interface that does not disrupt existing users of the older page-idle
> > > tracking, so as such it is a safe change (as in, doesn't change existing
> > > functionality except for the draining bug fix).
> > 
> > I hope to find some more time to finish the review but let me point out
> > that "it's new it is regression safe" is not really a great argument for
> > a new user visible API.
> 
> Actually, I think you misunderstood me and took it out of context. I never
> intended to say "it is regression safe". I meant to say it is "low risk", as
> in that in all likelihood should not be hurting *existing users* of the *old
> interface*. Also as you know, it has been tested.

Yeah, misreading on my end.

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

-- 
Michal Hocko
SUSE Labs