linux-kernel - Re: [PATCH] mm: hugetlb: flush dcache before returning zeroed huge page to userspace

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Message-ID: <20120816180614.GB12578@dhcp22.suse.cz>
Date:	Thu, 16 Aug 2012 20:06:14 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Will Deacon <will.deacon@....com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hillf Danton <dhillf@...il.com>,
	Russell King <linux@....linux.org.uk>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: hugetlb: flush dcache before returning zeroed huge
 page to userspace

On Thu 16-08-12 18:34:59, Will Deacon wrote:
> On Thu, Aug 16, 2012 at 06:25:27PM +0100, Michal Hocko wrote:
> > On Thu 16-08-12 17:09:54, Will Deacon wrote:
> > > On Wed, Aug 08, 2012 at 05:26:07PM +0100, Michal Hocko wrote:
> > > > I guess the cleanest way is to hook into dequeue_huge_page_node and add
> > > > something like arch_clear_hugepage_flags.
> > > 
> > > I hooked into enqueue_huge_page instead, but how about something like this?:
> > 
> > Do you have any specific reason for that? enqueue_huge_page is called on
> > pages which potentially never get used so isn't that wasting a bit?
> > Not that it would be wrong I was just thinking why shouldn't we do it
> > when the page is actualy going to be used for sure.
> 
> I just did it that way to match the flag clearing for normal pages. I can
> move it into dequeue if you think it's worthwhile but in the worst case it
> just adds a clear_bit call, so I doubt it's measurable.

I do not have a strong opinion on that but the flags come in cleared when
they are freshly allocated (gather_surplus_pages) so then it would be
more appropriate in free_huge_page when enqueue_huge_page is called.

But this is just a nit.

> 
> Will

-- 
Michal Hocko
SUSE Labs
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