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

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Message-ID: <20190423104707.GK25106@dhcp22.suse.cz>
Date:   Tue, 23 Apr 2019 12:47:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Michel Lespinasse <walken@...gle.com>
Cc:     Laurent Dufour <ldufour@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.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 Mon 22-04-19 14:29:16, Michel Lespinasse wrote:
[...]
> I want to add a note about mmap_sem. In the past there has been
> discussions about replacing it with an interval lock, but these never
> went anywhere because, mostly, of the fact that such mechanisms were
> too expensive to use in the page fault path. I think adding the spf
> mechanism would invite us to revisit this issue - interval locks may
> be a great way to avoid blocking between unrelated mmap_sem writers
> (for example, do not delay stack creation for new threads while a
> large mmap or munmap may be going on), and probably also to handle
> mmap_sem readers that can't easily use the spf mechanism (for example,
> gup callers which make use of the returned vmas). But again that is a
> separate topic to explore which doesn't have to get resolved before
> spf goes in.

Well, I believe we should _really_ re-evaluate the range locking sooner
rather than later. Why? Because it looks like the most straightforward
approach to the mmap_sem contention for most usecases I have heard of
(mostly a mm{unm}ap, mremap standing in the way of page faults).
On a plus side it also makes us think about the current mmap (ab)users
which should lead to an overall code improvements and maintainability.

SPF sounds like a good idea but it is a really big and intrusive surgery
to the #PF path. And more importantly without any real world usecase
numbers which would justify this. That being said I am not opposed to
this change I just think it is a large hammer while we haven't seen
attempts to tackle problems in a simpler way.

-- 
Michal Hocko
SUSE Labs