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

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Date:   Tue, 23 Apr 2019 14:48:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Michel Lespinasse <walken@...gle.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, dave@...olabs.net,
        Jan Kara <jack@...e.cz>, 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 Tue, Apr 23, 2019 at 05:41:48AM -0700, Matthew Wilcox wrote:
> On Tue, Apr 23, 2019 at 12:47:07PM +0200, Michal Hocko wrote:
> > 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.
> 
> Dave Chinner recently did evaluate the range lock for solving a problem
> in XFS and didn't like what he saw:
> 
> https://lore.kernel.org/linux-fsdevel/20190418031013.GX29573@dread.disaster.area/T/#md981b32c12a2557a2dd0f79ad41d6c8df1f6f27c
> 
> I think scaling the lock needs to be tied to the actual data structure
> and not have a second tree on-the-side to fake-scale the locking.

Right, which is how I ended up using the split PT locks. They already
provide fine(r) grained locking.