linux-kernel - Re: [PATCH] mm: release the spinlock on zap_pte

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Date:   Wed, 31 Jul 2019 09:21:01 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Miguel de Dios <migueldedios@...gle.com>,
        Wei Wang <wvw@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH] mm: release the spinlock on zap_pte_range

On Wed 31-07-19 14:44:47, Minchan Kim wrote:
> On Tue, Jul 30, 2019 at 02:57:51PM +0200, Michal Hocko wrote:
> > [Cc Nick - the email thread starts http://lkml.kernel.org/r/20190729071037.241581-1-minchan@kernel.org
> >  A very brief summary is that mark_page_accessed seems to be quite
> >  expensive and the question is whether we still need it and why
> >  SetPageReferenced cannot be used instead. More below.]
> > 
> > On Tue 30-07-19 21:39:35, Minchan Kim wrote:
[...]
> > > commit bf3f3bc5e73
> > > Author: Nick Piggin <npiggin@...e.de>
> > > Date:   Tue Jan 6 14:38:55 2009 -0800
> > > 
> > >     mm: don't mark_page_accessed in fault path
> > > 
> > >     Doing a mark_page_accessed at fault-time, then doing SetPageReferenced at
> > >     unmap-time if the pte is young has a number of problems.
> > > 
> > >     mark_page_accessed is supposed to be roughly the equivalent of a young pte
> > >     for unmapped references. Unfortunately it doesn't come with any context:
> > >     after being called, reclaim doesn't know who or why the page was touched.
> > > 
> > >     So calling mark_page_accessed not only adds extra lru or PG_referenced
> > >     manipulations for pages that are already going to have pte_young ptes anyway,
> > >     but it also adds these references which are difficult to work with from the
> > >     context of vma specific references (eg. MADV_SEQUENTIAL pte_young may not
> > >     wish to contribute to the page being referenced).
> > > 
> > >     Then, simply doing SetPageReferenced when zapping a pte and finding it is
> > >     young, is not a really good solution either. SetPageReferenced does not
> > >     correctly promote the page to the active list for example. So after removing
> > >     mark_page_accessed from the fault path, several mmap()+touch+munmap() would
> > >     have a very different result from several read(2) calls for example, which
> > >     is not really desirable.
> > 
> > Well, I have to say that this is rather vague to me. Nick, could you be
> > more specific about which workloads do benefit from this change? Let's
> > say that the zapped pte is the only referenced one and then reclaim
> > finds the page on inactive list. We would go and reclaim it. But does
> > that matter so much? Hot pages would be referenced from multiple ptes
> > very likely, no?
> 
> As Nick mentioned in the description, without mark_page_accessed in
> zapping part, repeated mmap + touch + munmap never acticated the page
> while several read(2) calls easily promote it.

And is this really a problem? If we refault the same page then the
refaults detection should catch it no? In other words is the above still
a problem these days?

-- 
Michal Hocko
SUSE Labs