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

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Date:	Mon, 9 Jul 2012 14:25:23 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-mm@...ck.org, akpm@...ux-foundation.org, dhillf@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: hugetlb: flush dcache before returning zeroed huge
 page to userspace

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

>  
>  		if (vma->vm_flags & VM_MAYSHARE) {
> -- 
> 1.7.4.1
> 
> --
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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