linux-kernel - Re: [PATCH v2] mm: fix double page fault on arm64 if PTE

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Message-ID: <20190910090845.GD14442@C02TF0J2HF1T.local>
Date:   Tue, 10 Sep 2019 10:08:45 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jia He <justin.he@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Airlie <airlied@...hat.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Souptick Joarder <jrdr.linux@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: fix double page fault on arm64 if PTE_AF is
 cleared

On Mon, Sep 09, 2019 at 02:27:12PM -0700, Matthew Wilcox wrote:
> On Fri, Sep 06, 2019 at 09:57:47PM +0800, Jia He wrote:
> > +		if (!pte_young(vmf->orig_pte)) {
> > +			entry = pte_mkyoung(vmf->orig_pte);
> > +			if (ptep_set_access_flags(vmf->vma, vmf->address,
> > +				vmf->pte, entry, 0))
> > +				update_mmu_cache(vmf->vma, vmf->address,
> > +						vmf->pte);
> > +		}
> > +
> 
> Oh, btw, why call update_mmu_cache() here?  All you've done is changed
> the 'accessed' bit.  What is any architecture supposed to do in response
> to this?

For arm64 and x86 that's a no-op but an architecture with software TLBs
may preload them to avoid a subsequent fault on access after the pte was
made young.

-- 
Catalin