Re: [PATCH v4 1/5] mm/page_idle: Add per-pid idle page tracking using virtual indexing

From: Michal Hocko <mhocko@kernel.org>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.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>,
	joelaf@google.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 v4 1/5] mm/page_idle: Add per-pid idle page tracking using virtual indexing
Date: Tue, 6 Aug 2019 10:56:05 +0200	[thread overview]
Message-ID: <20190806085605.GL11812@dhcp22.suse.cz> (raw)
In-Reply-To: <20190805170451.26009-1-joel@joelfernandes.org>

On Mon 05-08-19 13:04:47, Joel Fernandes (Google) wrote:
> The page_idle tracking feature currently requires looking up the pagemap
> for a process followed by interacting with /sys/kernel/mm/page_idle.
> Looking up PFN from pagemap in Android devices is not supported by
> unprivileged process and requires SYS_ADMIN and gives 0 for the PFN.
> 
> This patch adds support to directly interact with page_idle tracking at
> the PID level by introducing a /proc/<pid>/page_idle file.  It follows
> the exact same semantics as the global /sys/kernel/mm/page_idle, but now
> looking up PFN through pagemap is not needed since the interface uses
> virtual frame numbers, and at the same time also does not require
> SYS_ADMIN.
> 
> 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.

As already mentioned in one of the previous versions. The interface
seems sane and the usecase as well. So I do not really have high level
objections.

From a quick look at the patch I would just object to pulling swap idle
tracking into this patch because it makes the review harder and it is
essentially a dead code until a later patch. I am also not sure whether
that is really necessary and it really begs for an explicit
justification.

I will try to go through the patch more carefully later as time allows.

> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
-- 
Michal Hocko
SUSE Labs