linux-kernel - Re: [PATCH v2] mm: hugetlb: add arch hook for clearing page flags before entering pool

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Message-ID: <20120823173602.GA3117@mudshark.cambridge.arm.com>
Date:	Thu, 23 Aug 2012 18:36:02 +0100
From:	Will Deacon <will.deacon@....com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] mm: hugetlb: add arch hook for clearing page flags
 before entering pool

On Thu, Aug 23, 2012 at 06:11:56PM +0100, Michal Hocko wrote:
> On Thu 23-08-12 17:37:13, Will Deacon wrote:
> > The core page allocator ensures that page flags are zeroed when freeing
> > pages via free_pages_check. A number of architectures (ARM, PPC, MIPS)
> > rely on this property to treat new pages as dirty with respect to the
> > data cache and perform the appropriate flushing before mapping the pages
> > into userspace.
> > 
> > This can lead to cache synchronisation problems when using hugepages,
> > since the allocator keeps its own pool of pages above the usual page
> > allocator and does not reset the page flags when freeing a page into
> > the pool.
> > 
> > This patch adds a new architecture hook, arch_clear_hugepage_flags, so
> > that architectures which rely on the page flags being in a particular
> > state for fresh allocations can adjust the flags accordingly when a
> > page is freed into the pool.
> > 
> > Cc: Michal Hocko <mhocko@...e.cz>
> > Signed-off-by: Will Deacon <will.deacon@....com>
> 
> Looks good to me
> Reviewed-by: Michal Hocko <mhocko@...e.cz>

Cheers Michal. Next step: start posting the ARM code!

Will
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