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

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Date:	Fri, 24 Aug 2012 15:01:54 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	"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>
Subject: Re: [PATCH v2] mm: hugetlb: add arch hook for clearing page flags
 before entering pool

On Thu, 23 Aug 2012 18:36:02 +0100
Will Deacon <will.deacon@....com> wrote:

> 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.

You could have used __weak here quite neatly, but whatever.

> Next step: start posting the ARM code!

I suggest you keep this patch in whichever tree holds that arm code.  If
I see this patch turn up in linux-next then I'll just drop my copy,
expecting that this patch will be merged alongside the ARM changes.

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