linux-kernel - Re: [PATCH 1/6] arm64: pgtable: Fix pte

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Message-ID: <20201120160321.GB3377168@google.com>
Date:   Fri, 20 Nov 2020 08:03:21 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Catalin Marinas <catalin.marinas@....com>,
        Yu Zhao <yuzhao@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/6] arm64: pgtable: Fix pte_accessible()

On Fri, Nov 20, 2020 at 02:35:52PM +0000, Will Deacon wrote:
> pte_accessible() is used by ptep_clear_flush() to figure out whether TLB
> invalidation is necessary when unmapping pages for reclaim. Although our
> implementation is correct according to the architecture, returning true
> only for valid, young ptes in the absence of racing page-table
> modifications, this is in fact flawed due to lazy invalidation of old
> ptes in ptep_clear_flush_young() where we elide the expensive DSB
> instruction for completing the TLB invalidation.
> 
> Rather than penalise the aging path, adjust pte_accessible() to return
> true for any valid pte, even if the access flag is cleared.
> 
> Cc: <stable@...r.kernel.org>
> Fixes: 76c714be0e5e ("arm64: pgtable: implement pte_accessible()")
> Reported-by: Yu Zhao <yuzhao@...gle.com>
> Signed-off-by: Will Deacon <will@...nel.org>
Reviewed-by: Minchan Kim <minchan@...nel.org>