linux-kernel - Re: [PATCH] mm,thp: khugepaged: call pte flush at the time of collapse

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Message-ID: <87fux1xifd.fsf@linux.vnet.ibm.com>
Date:	Wed, 10 Feb 2016 10:37:02 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm,thp: khugepaged: call pte flush at the time of collapse

Vineet Gupta <Vineet.Gupta1@...opsys.com> writes:

> This showed up on ARC when running LMBench bw_mem tests as
> Overlapping TLB Machine Check Exception triggered due to STLB entry
> (2M pages) overlapping some NTLB entry (regular 8K page).
>
> bw_mem 2m touches a large chunk of vaddr creating NTLB entries.
> In the interim khugepaged kicks in, collapsing the contiguous ptes into
> a single pmd. pmdp_collapse_flush()->flush_pmd_tlb_range() is called to
> flush out NTLB entries for the ptes. This for ARC (by design) can only
> shootdown STLB entries (for pmd). The stray NTLB entries cause the overlap
> with the subsequent STLB entry for collapsed page.
> So make pmdp_collapse_flush() call pte flush interface not pmd flush.
>
> Note that originally all thp flush call sites in generic code called
> flush_tlb_range() leaving it to architecture to implement the flush for
> pte and/or pmd. Commit 12ebc1581ad11454 changed this by calling a new
> opt-in API flush_pmd_tlb_range() which made the semantics more explicit
> but failed to distinguish the pte vs pmd flush in generic code, which is
> what this patch fixes.
>
> Note that ARC can fixed w/o touching the generic pmdp_collapse_flush()
> by defining a ARC version, but that defeats the purpose of generic
> version, plus sementically this is the right thing to do.
>
> Fixes STAR 9000961194: LMBench on AXS103 triggering duplicate TLB
> exceptions with super pages
>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: <stable@...r.kernel.org> #4.4
> Cc: <linux-snps-arc@...ts.infradead.org>
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-mm@...ck.org
> Fixes: 12ebc1581ad11454 ("mm,thp: introduce flush_pmd_tlb_range")
> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>

We do have reverse usage in migration code path, which I have as a patch
here.

https://github.com/kvaneesh/linux/commit/b8a78933fea93cb0b2978868e59a0a4b12eb92eb

> ---
>  mm/pgtable-generic.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index 7d3db0247983..1ba58213ad65 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -210,7 +210,9 @@ pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address,
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>  	VM_BUG_ON(pmd_trans_huge(*pmdp));
>  	pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
> -	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +
> +	/* collapse entails shooting down ptes not pmd */
> +	flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>  	return pmd;
>  }
>  #endif
> -- 
> 2.5.0

-aneesh