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

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Message-ID: <20120709141324.GK7315@mudshark.cambridge.arm.com>
Date:	Mon, 9 Jul 2012 15:13:24 +0100
From:	Will Deacon <will.deacon@....com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"dhillf@...il.com" <dhillf@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: hugetlb: flush dcache before returning zeroed huge
 page to userspace

On Mon, Jul 09, 2012 at 01:25:23PM +0100, Michal Hocko wrote:
> On Wed 04-07-12 15:32:56, Will Deacon wrote:
> > When allocating and returning clear huge pages to userspace as a
> > response to a fault, we may zero and return a mapping to a previously
> > dirtied physical region (for example, it may have been written by
> > a private mapping which was freed as a result of an ftruncate on the
> > backing file). On architectures with Harvard caches, this can lead to
> > I/D inconsistency since the zeroed view may not be visible to the
> > instruction stream.
> > 
> > This patch solves the problem by flushing the region after allocating
> > and clearing a new huge page. Note that PowerPC avoids this issue by
> > performing the flushing in their clear_user_page implementation to keep
> > the loader happy, however this is closely tied to the semantics of the
> > PG_arch_1 page flag which is architecture-specific.
> > 
> > Acked-by: Catalin Marinas <catalin.marinas@....com>
> > Signed-off-by: Will Deacon <will.deacon@....com>
> > ---
> >  mm/hugetlb.c |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index e198831..b83d026 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2646,6 +2646,7 @@ retry:
> >  			goto out;
> >  		}
> >  		clear_huge_page(page, address, pages_per_huge_page(h));
> > +		flush_dcache_page(page);
> >  		__SetPageUptodate(page);
> 
> Does this have to be explicit in the arch independent code?
> It seems that ia64 uses flush_dcache_page already in the clear_user_page

It would match what is done in similar situations by cow_user_page (mm/memory.c)
and shmem_writepage (mm/shmem.c). Other subsystems also have explicit page
flushing (DMA bounce, ksm) so I think this is the right place for it.

Will
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