Re: [RFC 7/7] mm: madvise support MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER

From: Johannes Weiner <hannes@cmpxchg.org>
To: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>, Tim Murray <timmurray@google.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Daniel Colascione <dancol@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	Sonny Rao <sonnyrao@google.com>,
	Brian Geffon <bgeffon@google.com>,
	linux-api@vger.kernel.org
Subject: Re: [RFC 7/7] mm: madvise support MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER
Date: Tue, 21 May 2019 11:33:10 -0400	[thread overview]
Message-ID: <20190521153310.GA3218@cmpxchg.org> (raw)
In-Reply-To: <20190521025533.GH10039@google.com>

On Tue, May 21, 2019 at 11:55:33AM +0900, Minchan Kim wrote:
> On Mon, May 20, 2019 at 11:28:01AM +0200, Michal Hocko wrote:
> > [cc linux-api]
> > 
> > On Mon 20-05-19 12:52:54, Minchan Kim wrote:
> > > System could have much faster swap device like zRAM. In that case, swapping
> > > is extremely cheaper than file-IO on the low-end storage.
> > > In this configuration, userspace could handle different strategy for each
> > > kinds of vma. IOW, they want to reclaim anonymous pages by MADV_COLD
> > > while it keeps file-backed pages in inactive LRU by MADV_COOL because
> > > file IO is more expensive in this case so want to keep them in memory
> > > until memory pressure happens.
> > > 
> > > To support such strategy easier, this patch introduces
> > > MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER options in madvise(2) like
> > > that /proc/<pid>/clear_refs already has supported same filters.
> > > They are filters could be Ored with other existing hints using top two bits
> > > of (int behavior).
> > 
> > madvise operates on top of ranges and it is quite trivial to do the
> > filtering from the userspace so why do we need any additional filtering?
> > 
> > > Once either of them is set, the hint could affect only the interested vma
> > > either anonymous or file-backed.
> > > 
> > > With that, user could call a process_madvise syscall simply with a entire
> > > range(0x0 - 0xFFFFFFFFFFFFFFFF) but either of MADV_ANONYMOUS_FILTER and
> > > MADV_FILE_FILTER so there is no need to call the syscall range by range.
> > 
> > OK, so here is the reason you want that. The immediate question is why
> > cannot the monitor do the filtering from the userspace. Slightly more
> > work, all right, but less of an API to expose and that itself is a
> > strong argument against.
> 
> What I should do if we don't have such filter option is to enumerate all of
> vma via /proc/<pid>/maps and then parse every ranges and inode from string,
> which would be painful for 2000+ vmas.

Just out of curiosity, how do you get to 2000+ distinct memory regions
in the address space of a mobile app? I'm assuming these aren't files,
but rather anon objects with poor grouping. Is that from guard pages
between individual heap allocations or something?