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

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Message-ID: <20190423093851.GJ11158@hirez.programming.kicks-ass.net>
Date:   Tue, 23 Apr 2019 11:38:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michel Lespinasse <walken@...gle.com>
Cc:     Laurent Dufour <ldufour@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.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, Apr 22, 2019 at 02:29:16PM -0700, Michel Lespinasse wrote:
> The proposed spf mechanism only handles anon vmas. Is there a
> fundamental reason why it couldn't handle mapped files too ?
> My understanding is that the mechanism of verifying the vma after
> taking back the ptl at the end of the fault would work there too ?
> The file has to stay referenced during the fault, but holding the vma's
> refcount could be made to cover that ? the vm_file refcount would have
> to be released in __free_vma() instead of remove_vma; I'm not quite sure
> if that has more implications than I realize ?

IIRC (and I really don't remember all that much) the trickiest bit was
vs unmount. Since files can stay open past the 'expected' duration,
umount could be delayed.

But yes, I think I had a version that did all that just 'fine'. Like
mentioned, I didn't keep the refcount because it sucked just as hard as
the mmap_sem contention, but the SRCU callback did the fput() just fine
(esp. now that we have delayed_fput).