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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: Balbir Singh <bsingharora@gmail.com>, benh@kernel.crashing.org,
 paulus@samba.org, mpe@ellerman.id.au, akpm@linux-foundation.org,
 Mel Gorman <mgorman@techsingularity.net>,
 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP
 update
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References: <1454980831-16631-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com>
 <1455504278.16012.18.camel@gmail.com> <87lh6mfv2j.fsf@linux.vnet.ibm.com>
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Date: Mon, 15 Feb 2016 16:31:59 +0530
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Balbir Singh <bsingharora@gmail.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.
>>=20
>> http://article.gmane.org/gmane.linux.ports.ppc.embedded/81105
> Can be racy as a CPU moves from non-idle to idle
>
> In
>
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0pmd_hugepage_update(vma->vm_mm, address, =
pmdp, ~0UL, 0);
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0/*
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0* This ensures that generic code th=
at rely on IRQ disabling
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0* to prevent a parallel THP split w=
ork as expected.
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0*/
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0kick_all_cpus_sync();
>
> pmdp_invalidate()->pmd_hugepage_update() can still run in parallel with=
=C2=A0
> 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