Re: [RFC PATCH 5/9] ARM64: mm: Add support for flushing huge pages.

[Will Deacon <will.deacon@arm.com>]

Hi Steve,

On Tue, Apr 30, 2013 at 05:30:44PM +0100, Steve Capper wrote:
> The code to flush the dcache of a dirty page, __flush_dcache_page,
> will only flush the head of a HugeTLB/THP page.
> 
> This patch adjusts __flush_dcache_page such that the order of the
> compound page is used to determine the size of area to flush.
> 
> Signed-off-by: Steve Capper <steve.capper@linaro.org>
> ---
>  arch/arm64/mm/flush.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
> index 88611c3..71c182d 100644
> --- a/arch/arm64/mm/flush.c
> +++ b/arch/arm64/mm/flush.c
> @@ -72,7 +72,8 @@ void copy_to_user_page(struct vm_area_struct *vma, struct page *page,
>  
>  void __flush_dcache_page(struct page *page)
>  {
> -	__flush_dcache_area(page_address(page), PAGE_SIZE);
> +	size_t page_size = PAGE_SIZE << compound_order(page);
> +	__flush_dcache_area(page_address(page), page_size);
>  }

This penalises flush_dcache_page, while it might only be required for
__sync_icache_dcache (called when installing executable ptes).

Now, the job of flush_dcache_page is to deal with D-side aliases between
concurrent user and kernel mappings. On arm64, D-side aliasing is not a
problem so in theory flush_dcache_page could be a nop (well, a clear_bit,
but close enough).

The reason that flush_dcache_page *isn't* currently a nop, is because we
have harvard caches, so if we end up in a situation where the kernel is
writing executable text to a page which is also mapped into userspace (i.e.
there won't be a subsequent call to set_pte), then the user will execute
junk if we don't flush/invalidate things here.

This might be overkill. I can't think of a scenario where the above is true,
but I'm no mm expert, so it would be great if somebody else could chime in
to confirm/rubbish out suspicions...

Cheers,

Will

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