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

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Message-ID: <20120712112645.GG2816@mudshark.cambridge.arm.com>
Date:	Thu, 12 Jul 2012 12:26:45 +0100
From:	Will Deacon <will.deacon@....com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hillf Danton <dhillf@...il.com>,
	"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, Jul 12, 2012 at 12:16:59PM +0100, Michal Hocko wrote:
> On Wed 11-07-12 18:48:02, Will Deacon wrote:
> > Just to confirm, the following quick hack at least results in the correct
> > flushing for me (on ARM):
> > 
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index e198831..7a7c9d3 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1141,6 +1141,7 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
> >         }
> >  
> >         set_page_private(page, (unsigned long)spool);
> > +       clear_bit(PG_arch_1, &page->flags);
> >  
> >         vma_commit_reservation(h, vma, addr);
> >  
> > 
> > 
> > The question is whether we should tidy that up for the core code or get
> > architectures to clear the bit in arch_make_huge_pte (which also seems to
> > work).
> 
> This should go into arch specific code IMO. Even the page flag name
> suggests this shouldn't be in the base code.

Well, the comment in linux/page-flags.h does state that:

 * PG_arch_1 is an architecture specific page state bit.  The generic code
 * guarantees that this bit is cleared for a page when it first is entered into
 * the page cache.

so it's not completely clear cut that the architecture should be responsible
for clearing this bit when allocating pages from the hugepage pool.

Will
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