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From:   Daniel Colascione <dancol@google.com>
Date:   Mon, 3 Jun 2019 08:43:59 -0700
Message-ID: <CAKOZuetW1UsPP3fDm-zTBOiO_oWkkDwADu+Apy53abWNs-UcUA@mail.gmail.com>
Subject: Re: [RFCv2 1/6] mm: introduce MADV_COLD
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tim Murray <timmurray@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Sonny Rao <sonnyrao@google.com>,
        Brian Geffon <bgeffon@google.com>,
        Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        Christian Brauner <christian@brauner.io>, oleksandr@redhat.com,
        hdanton@sina.com
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On Mon, Jun 3, 2019 at 12:16 AM Michal Hocko <mhocko@kernel.org> wrote:
> On Fri 31-05-19 23:34:07, Minchan Kim wrote:
> > On Fri, May 31, 2019 at 04:03:32PM +0200, Michal Hocko wrote:
> > > On Fri 31-05-19 22:39:04, Minchan Kim wrote:
> > > > On Fri, May 31, 2019 at 10:47:52AM +0200, Michal Hocko wrote:
> > > > > On Fri 31-05-19 15:43:08, Minchan Kim wrote:
> > > > > > When a process expects no accesses to a certain memory range, it could
> > > > > > give a hint to kernel that the pages can be reclaimed when memory pressure
> > > > > > happens but data should be preserved for future use.  This could reduce
> > > > > > workingset eviction so it ends up increasing performance.
> > > > > >
> > > > > > This patch introduces the new MADV_COLD hint to madvise(2) syscall.
> > > > > > MADV_COLD can be used by a process to mark a memory range as not expected
> > > > > > to be used in the near future. The hint can help kernel in deciding which
> > > > > > pages to evict early during memory pressure.
> > > > > >
> > > > > > Internally, it works via deactivating pages from active list to inactive's
> > > > > > head if the page is private because inactive list could be full of
> > > > > > used-once pages which are first candidate for the reclaiming and that's a
> > > > > > reason why MADV_FREE move pages to head of inactive LRU list. Therefore,
> > > > > > if the memory pressure happens, they will be reclaimed earlier than other
> > > > > > active pages unless there is no access until the time.
> > > > >
> > > > > [I am intentionally not looking at the implementation because below
> > > > > points should be clear from the changelog - sorry about nagging ;)]
> > > > >
> > > > > What kind of pages can be deactivated? Anonymous/File backed.
> > > > > Private/shared? If shared, are there any restrictions?
> > > >
> > > > Both file and private pages could be deactived from each active LRU
> > > > to each inactive LRU if the page has one map_count. In other words,
> > > >
> > > >     if (page_mapcount(page) <= 1)
> > > >         deactivate_page(page);
> > >
> > > Why do we restrict to pages that are single mapped?
> >
> > Because page table in one of process shared the page would have access bit
> > so finally we couldn't reclaim the page. The more process it is shared,
> > the more fail to reclaim.
>
> So what? In other words why should it be restricted solely based on the
> map count. I can see a reason to restrict based on the access
> permissions because we do not want to simplify all sorts of side channel
> attacks but memory reclaim is capable of reclaiming shared pages and so
> far I haven't heard any sound argument why madvise should skip those.
> Again if there are any reasons, then document them in the changelog.

Whether to reclaim shared pages is a policy decision best left to
userland, IMHO.