From: Minchan Kim <minchan@kernel.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
Miguel de Dios <migueldedios@google.com>,
Wei Wang <wvw@google.com>, Johannes Weiner <hannes@cmpxchg.org>,
Mel Gorman <mgorman@techsingularity.net>,
Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH] mm: release the spinlock on zap_pte_range
Date: Tue, 6 Aug 2019 19:55:09 +0900 [thread overview]
Message-ID: <20190806105509.GA94582@google.com> (raw)
In-Reply-To: <20190731072101.GX9330@dhcp22.suse.cz>
On Wed, Jul 31, 2019 at 09:21:01AM +0200, Michal Hocko wrote:
> 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@suse.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?
I admit we have been not fair for them because read(2) syscall pages are
easily promoted regardless of zap timing unlike mmap-based pages.
However, if we remove the mark_page_accessed in the zap_pte_range, it
would make them more unfair in that read(2)-accessed pages are easily
promoted while mmap-based page should go through refault to be promoted.
I also want to remove the costly overhead from the hot path but couldn't
come up with nice solution.
next prev parent reply other threads:[~2019-08-06 10:55 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-29 7:10 [PATCH] mm: release the spinlock on zap_pte_range Minchan Kim
2019-07-29 7:45 ` Michal Hocko
2019-07-29 8:20 ` Minchan Kim
2019-07-29 8:35 ` Michal Hocko
2019-07-30 12:11 ` Minchan Kim
2019-07-30 12:32 ` Michal Hocko
2019-07-30 12:39 ` Minchan Kim
2019-07-30 12:57 ` Michal Hocko
2019-07-31 5:44 ` Minchan Kim
2019-07-31 7:21 ` Michal Hocko
2019-08-06 10:55 ` Minchan Kim [this message]
2019-08-09 12:43 ` [RFC PATCH] mm: drop mark_page_access from the unmap path Michal Hocko
2019-08-09 17:57 ` Mel Gorman
2019-08-09 18:34 ` Johannes Weiner
2019-08-12 8:09 ` Michal Hocko
2019-08-12 15:07 ` Johannes Weiner
2019-08-13 10:51 ` Michal Hocko
2019-08-26 12:06 ` Michal Hocko
2019-08-27 16:00 ` Johannes Weiner
2019-08-27 18:41 ` Michal Hocko
2019-07-30 19:42 ` [PATCH] mm: release the spinlock on zap_pte_range Andrew Morton
2019-07-31 6:14 ` Minchan Kim
2019-08-06 7:05 ` [mm] 755d6edc1a: will-it-scale.per_process_ops -4.1% regression kernel test robot
[not found] ` <20190806080415.GG11812@dhcp22.suse.cz>
2019-08-06 11:00 ` Minchan Kim
2019-08-06 11:11 ` Michal Hocko
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