linux-kernel - Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge

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Message-Id: <F5E4E2C2-5F1C-42AC-8707-0D0B8C00D251@gmail.com>
Date:   Mon, 25 Oct 2021 09:29:09 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>, x86@...nel.org
Subject: Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd()



> On Oct 25, 2021, at 3:52 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Thu, Oct 21, 2021 at 05:21:09AM -0700, Nadav Amit wrote:
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index 448cd01eb3ec..18c3366f8f4d 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -1146,6 +1146,14 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
>> 	}
>> }
>> #endif
>> +
>> +#define __HAVE_ARCH_PMDP_INVALIDATE_AD
>> +static inline pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma,
>> +				       unsigned long address, pmd_t *pmdp)
>> +{
>> +	return pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));
> 
> Did this want to be something like:
> 
> 	pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));
> 	if (cpu_feature_enabled(X86_BUG_PTE_LEAK))
> 		flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> 	return old;
> 
> instead?

Yes. Of course. Where did my code go to? :(

> 
>> +}
>> +
>> /*
>>  * Page table pages are page-aligned.  The lower half of the top
>>  * level is used for userspace and the top half for the kernel.
> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index e5ea5f775d5c..435da011b1a2 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1795,10 +1795,11 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>> 	 * The race makes MADV_DONTNEED miss the huge pmd and don't clear it
>> 	 * which may break userspace.
>> 	 *
>> -	 * pmdp_invalidate() is required to make sure we don't miss
>> -	 * dirty/young flags set by hardware.
>> +	 * pmdp_invalidate_ad() is required to make sure we don't miss
>> +	 * dirty/young flags (which are also known as access/dirty) cannot be
>> +	 * further modifeid by the hardware.
> 
> "modified", I think is the more common spelling.

I tried to start a new trend. I will fix it.

> 
>> 	 */
>> -	entry = pmdp_invalidate(vma, addr, pmd);
>> +	entry = pmdp_invalidate_ad(vma, addr, pmd);
>> 
>> 	entry = pmd_modify(entry, newprot);
>> 	if (preserve_write)
>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>> index 4e640baf9794..b0ce6c7391bf 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -200,6 +200,14 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>> }
>> #endif
>> 
>> +#ifndef __HAVE_ARCH_PMDP_INVALIDATE_AD
> 
> /*
> * Does this deserve a comment to explain the intended difference vs
> * pmdp_invalidate() ?
> */

I will add a comment.