linux-kernel - Re: [PATCH v12 00/31] Speculative page faults

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Date:   Fri, 26 Apr 2019 23:00:10 -0700
From:   Michel Lespinasse <walken@...gle.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, dave@...olabs.net,
        Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
        aneesh.kumar@...ux.ibm.com,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        mpe@...erman.id.au, Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        sergey.senozhatsky.work@...il.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Punit Agrawal <punitagrawal@...il.com>,
        vinayak menon <vinayakm.list@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        zhong jiang <zhongjiang@...wei.com>,
        Haiyan Song <haiyanx.song@...el.com>,
        Balbir Singh <bsingharora@...il.com>, sj38.park@...il.com,
        Mike Rapoport <rppt@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, haren@...ux.vnet.ibm.com,
        Nick Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v12 00/31] Speculative page faults

On Wed, Apr 24, 2019 at 08:01:20PM +0200, Laurent Dufour wrote:
> Le 22/04/2019 à 23:29, Michel Lespinasse a écrit :
> > Hi Laurent,
> > 
> > Thanks a lot for copying me on this patchset. It took me a few days to
> > go through it - I had not been following the previous iterations of
> > this series so I had to catch up. I will be sending comments for
> > individual commits, but before tat I would like to discuss the series
> > as a whole.
> 
> Hi Michel,
> 
> Thanks for reviewing this series.
> 
> > I think these changes are a big step in the right direction. My main
> > reservation about them is that they are additive - adding some complexity
> > for speculative page faults - and I wonder if it'd be possible, over the
> > long term, to replace the existing complexity we have in mmap_sem retry
> > mechanisms instead of adding to it. This is not something that should
> > block your progress, but I think it would be good, as we introduce spf,
> > to evaluate whether we could eventually get all the way to removing the
> > mmap_sem retry mechanism, or if we will actually have to keep both.
> 
> Until we get rid of the mmap_sem which seems to be a very long story, I
> can't see how we could get rid of the retry mechanism.

Short answer: I'd like spf to be extended to handle file vmas,
populating page tables, and the vm_normal_page thing, so that we
wouldn't have to fall back to the path that grabs (and possibly
has to drop) the read side mmap_sem.

Even doing the above, there are still cases spf can't solve - for
example, gup, or the occasional spf abort, or even the case of a large
mmap/munmap delaying a smaller one. I think replacing mmap_sem with a
reader/writer interval lock would be a very nice generic solution to
this problem, allowing false conflicts to proceed in parallel, while
synchronizing true conflicts which is exactly what we want. But I
don't think such a lock can be implemented efficiently enough to be
put on the page fault fast-path, so I think spf could be the solution
there - it would allow us to skip taking that interval lock on most
page faults. The other places where we use mmap_sem are not as critical
for performance (they normally operate on a larger region at a time)
so I think we could afford the interval lock in those places.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.