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

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Message-ID: <961889b3-ef08-2ee9-e3a1-6aba003f47c1@arm.com>
Date:   Wed, 4 Sep 2019 10:07:47 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Jia He <justin.he@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.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] mm: fix double page fault on arm64 if PTE_AF is cleared



On 09/04/2019 08:49 AM, Anshuman Khandual wrote:
>  		/*
>  		 * This really shouldn't fail, because the page is there
>  		 * in the page tables. But it might just be unreadable,
>  		 * in which case we just give up and fill the result with
> -		 * zeroes.
> +		 * zeroes. If PTE_AF is cleared on arm64, it might
> +		 * cause double page fault here. so makes pte young here
>  		 */
> +		if (!pte_young(vmf->orig_pte)) {
> +			entry = pte_mkyoung(vmf->orig_pte);
> +			if (ptep_set_access_flags(vmf->vma, vmf->address,
> +				vmf->pte, entry, vmf->flags & FAULT_FLAG_WRITE))
> +				update_mmu_cache(vmf->vma, vmf->address,
> +						vmf->pte);
> +		}

This looks correct where it updates the pte entry with PTE_AF which
will prevent a subsequent page fault. But I think what we really need
here is to make sure 'uaddr' is mapped correctly at vma->pte. Probably
a generic function arch_map_pte() when defined for arm64 should check
CPU version and ensure continuance of PTE_AF if required. The comment
above also need to be updated saying not only the page should be there
in the page table, it needs to mapped appropriately as well.