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From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org
Cc: kernel-team@android.com,
	Catalin Marinas <catalin.marinas@arm.com>,
	Yu Zhao <yuzhao@google.com>, Minchan Kim <minchan@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
	stable@vger.kernel.org
Subject: Re: [PATCH 1/6] arm64: pgtable: Fix pte_accessible()
Date: Tue, 24 Nov 2020 15:32:18 +0530	[thread overview]
Message-ID: <6eb6dead-4c76-d14a-dcc7-0d1411337dc6@arm.com> (raw)
In-Reply-To: <20201120143557.6715-2-will@kernel.org>


On 11/20/20 8:05 PM, 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

Just curious, a PTE mapping would go into the TLB only if it is an
young one with PTE_AF bit set per the architecture ?

> 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.

IOW, an old PTE might have missed the required TLB invalidation via
ptep_clear_flush_young() because it's done in lazy mode. Hence just
include old valid PTEs in pte_accessible() so that TLB invalidation
could be done in ptep_clear_flush() path instead. May be TLB flush
could be done for every PTE, irrespective of its PTE_AF bit in
ptep_clear_flush_young().

> 
> Rather than penalise the aging path, adjust pte_accessible() to return
> true for any valid pte, even if the access flag is cleared.

But will not this cause more (possibly not required) TLB invalidation
in normal unmapping paths ? The cover letter mentions that this patch
fixes a real world crash. Should not the crash also be described here
in the commit message as this patch is marked for stable and has a
"Fixes: " tag.

> 
> Cc: <stable@vger.kernel.org>
> Fixes: 76c714be0e5e ("arm64: pgtable: implement pte_accessible()")
> Reported-by: Yu Zhao <yuzhao@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/include/asm/pgtable.h | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 4ff12a7adcfd..1bdf51f01e73 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -115,8 +115,6 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
>  #define pte_valid(pte)		(!!(pte_val(pte) & PTE_VALID))
>  #define pte_valid_not_user(pte) \
>  	((pte_val(pte) & (PTE_VALID | PTE_USER)) == PTE_VALID)
> -#define pte_valid_young(pte) \
> -	((pte_val(pte) & (PTE_VALID | PTE_AF)) == (PTE_VALID | PTE_AF))
>  #define pte_valid_user(pte) \
>  	((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER))
>  
> @@ -126,7 +124,7 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
>   * remapped as PROT_NONE but are yet to be flushed from the TLB.
>   */
>  #define pte_accessible(mm, pte)	\
> -	(mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte))
> +	(mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))
>  
>  /*
>   * p??_access_permitted() is true for valid user mappings (subject to the
> 

  parent reply	other threads:[~2020-11-24 10:02 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20201120143557.6715-1-will@kernel.org>
2020-11-20 14:35 ` Will Deacon
2020-11-20 16:03   ` Minchan Kim
2020-11-20 19:53   ` Yu Zhao
2020-11-23 13:27   ` Catalin Marinas
2020-11-24 10:02   ` Anshuman Khandual [this message]
2020-11-20 14:35 ` [PATCH 2/6] arm64: pgtable: Ensure dirty bit is preserved across pte_wrprotect() Will Deacon
2020-11-20 17:09   ` Minchan Kim
2020-11-23 14:31     ` Catalin Marinas
2020-11-23 14:22   ` Catalin Marinas

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