linux-kernel - Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

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Date:	Mon, 15 Feb 2016 16:31:59 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Balbir Singh <bsingharora@...il.com>, benh@...nel.crashing.org,
	paulus@...ba.org, mpe@...erman.id.au, akpm@...ux-foundation.org,
	Mel Gorman <mgorman@...hsingularity.net>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

Balbir Singh <bsingharora@...il.com> writes:

>> Now we can't depend for mm_cpumask, a parallel find_linux_pte_hugepte
>> can happen outside that. Now i had a variant for kick_all_cpus_sync that
>> ignored idle cpus. But then that needs more verification.
>> 
>> http://article.gmane.org/gmane.linux.ports.ppc.embedded/81105
> Can be racy as a CPU moves from non-idle to idle
>
> In
>
>> +     pmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0);
>> +     /*
>> +      * This ensures that generic code that rely on IRQ disabling
>> +      * to prevent a parallel THP split work as expected.
>> +      */
>> +     kick_all_cpus_sync();
>
> pmdp_invalidate()->pmd_hugepage_update() can still run in parallel with 
> find_linux_pte_or_hugepte() and race.. Am I missing something?
>

Yes. But then we make sure that the pte_t returned by
find_linux_pte_or_hugepte doesn't change to a regular pmd entry by using
that kick. Now callers of find_lnux_pte_or_hugepte will check for
_PAGE_PRESENT. So if it called before
pmd_hugepage_update(_PAGE_PRESENT), we wait for the caller to finish the
usage (via kick()). Or they bail out after finding _PAGE_PRESENT cleared.

-aneesh