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:   Fri, 26 Apr 2019 23:00:10 -0700
From:   Michel Lespinasse <walken@google.com>
To:     Laurent Dufour <ldufour@linux.ibm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andi Kleen <ak@linux.intel.com>, dave@stgolabs.net,
        Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>,
        aneesh.kumar@linux.ibm.com,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        mpe@ellerman.id.au, Paul Mackerras <paulus@samba.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Will Deacon <will.deacon@arm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        sergey.senozhatsky.work@gmail.com,
        Andrea Arcangeli <aarcange@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        kemi.wang@intel.com, Daniel Jordan <daniel.m.jordan@oracle.com>,
        David Rientjes <rientjes@google.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Ganesh Mahendran <opensource.ganesh@gmail.com>,
        Minchan Kim <minchan@kernel.org>,
        Punit Agrawal <punitagrawal@gmail.com>,
        vinayak menon <vinayakm.list@gmail.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        zhong jiang <zhongjiang@huawei.com>,
        Haiyan Song <haiyanx.song@intel.com>,
        Balbir Singh <bsingharora@gmail.com>, sj38.park@gmail.com,
        Mike Rapoport <rppt@linux.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, haren@linux.vnet.ibm.com,
        Nick Piggin <npiggin@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org
Subject: Re: [PATCH v12 00/31] Speculative page faults
Message-ID: <20190427060010.GB174296@google.com>
References: <20190416134522.17540-1-ldufour@linux.ibm.com>
 <CANN689F1h9XoHPzr_FQY2WfN5bb2TTd6M3HLqoJ-DQuHkNbA7g@mail.gmail.com>
 <aadc7b03-9121-6800-341b-6f2849088a09@linux.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <aadc7b03-9121-6800-341b-6f2849088a09@linux.ibm.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Apr 24, 2019 at 08:01:20PM +0200, Laurent Dufour wrote:
> Le 22/04/2019 ? 23:29, Michel Lespinasse a ?crit?:
> > 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.