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

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Date:	Thu, 12 Jul 2012 13:16:59 +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>,
	"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 Wed 11-07-12 18:48:02, Will Deacon wrote:
> On Tue, Jul 10, 2012 at 11:42:34AM +0100, Will Deacon wrote:
> > On Tue, Jul 10, 2012 at 10:45:13AM +0100, Will Deacon wrote:
> > > On Tue, Jul 10, 2012 at 12:57:14AM +0100, Hugh Dickins wrote:
> > > > If I start to grep the architectures for non-empty flush_dcache_page(),
> > > > I soon find things in arch/arm such as v4_mc_copy_user_highpage() doing
> > > > if (!test_and_set_bit(PG_dcache_clean,)) __flush_dcache_page() - where
> > > > the naming suggests that I'm right, it's the architecture's responsibility
> > > > to arrange whatever flushing is needed in its copy and clear page functions.
> 
> [...]
> 
> > Ok, so this is exactly the problem. The hugetlb allocator uses its own
> > pool of huge pages, so free_huge_page followed by a later alloc_huge_page
> > will give you something where the page flags of the compound head do not
> > guarantee that PG_arch_1 is clear.
> 
> 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.

> 
> Will

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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