Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 4 Jun 2019 13:26:51 +0900
From:   Minchan Kim <minchan@kernel.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, 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>,
        Daniel Colascione <dancol@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Sonny Rao <sonnyrao@google.com>,
        Brian Geffon <bgeffon@google.com>, jannh@google.com,
        oleg@redhat.com, christian@brauner.io, oleksandr@redhat.com,
        hdanton@sina.com
Subject: Re: [RFCv2 1/6] mm: introduce MADV_COLD
Message-ID: <20190604042651.GC43390@google.com>
References: <20190531064313.193437-1-minchan@kernel.org>
 <20190531064313.193437-2-minchan@kernel.org>
 <20190531084752.GI6896@dhcp22.suse.cz>
 <20190531133904.GC195463@google.com>
 <20190531140332.GT6896@dhcp22.suse.cz>
 <20190531143407.GB216592@google.com>
 <20190603071607.GB4531@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190603071607.GB4531@dhcp22.suse.cz>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Jun 03, 2019 at 09:16:07AM +0200, Michal Hocko 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.

I will go with removing the part so that defer to decision to the VM reclaim
based on the review.

>  
> [...]
> 
> > > Please document this, if this is really a desirable semantic because
> > > then you have the same set of problems as we've had with the early
> > > MADV_FREE implementation mentioned above.
> > 
> > IIRC, the problem of MADV_FREE was that we couldn't discard freeable
> > pages because VM never scan anonymous LRU with swapless system.
> > However, it's not the our case because we should reclaim them, not
> > discarding.
> 
> Right. But there is still the page cache reclaim. Is it expected that
> an explicitly cold memory doesn't get reclaimed because we have a
> sufficient amount of page cache (a very common case) and we never age
> anonymous memory because of that?

If there are lots of used-once pages in file-LRU, I think there is no
need to reclaim anonymous pages because it needs bigger overhead due to
IO. It has been true for a long time in current VM policy.

Reclaim preference model based on hints is as following based on cost:

MADV_DONTNEED >> MADV_PAGEOUT > used-once pages > MADV_FREE >= MADV_COLD

It is desirable for the new hints to be placed in the reclaiming preference
order such that a) they don't overlap functionally with existing hints and
b) we have a balanced ordering of disruptive and non-disruptive hints.