linux-kernel - Re: [PATCH V2 2/2] powerpc/thp: Serialize pmd clear against a linux page table walk.

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Message-Id: <20150508152149.7fd52bb4b7c2a0911c33be00@linux-foundation.org>
Date:	Fri, 8 May 2015 15:21:49 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	mpe@...erman.id.au, paulus@...ba.org, benh@...nel.crashing.org,
	kirill.shutemov@...ux.intel.com, aarcange@...hat.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH V2 2/2] powerpc/thp: Serialize pmd clear against a linux
 page table walk.

On Thu,  7 May 2015 12:53:28 +0530 "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:

> Serialize against find_linux_pte_or_hugepte which does lock-less
> lookup in page tables with local interrupts disabled. For huge pages
> it casts pmd_t to pte_t. Since format of pte_t is different from
> pmd_t we want to prevent transit from pmd pointing to page table
> to pmd pointing to huge page (and back) while interrupts are disabled.
> We clear pmd to possibly replace it with page table pointer in
> different code paths. So make sure we wait for the parallel
> find_linux_pte_or_hugepage to finish.

I'm not seeing here any description of the problem which is being
fixed.  Does the patch make the machine faster?  Does the machine
crash?
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