linux-kernel - Re: [PATCH 2/4] MIPS: Flush highmem pages in __flush_dcache

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Date:	Tue, 29 Mar 2016 10:35:43 +0200
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Paul Burton <paul.burton@...tec.com>
Cc:	linux-mips@...ux-mips.org, Lars Persson <lars.persson@...s.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 2/4] MIPS: Flush highmem pages in __flush_dcache_page

On Tue, Mar 01, 2016 at 02:37:57AM +0000, Paul Burton wrote:

> When flush_dcache_page is called on an executable page, that page is
> about to be provided to userland & we can presume that the icache
> contains no valid entries for its address range. However if the icache
> does not fill from the dcache then we cannot presume that the pages
> content has been written back as far as the memories that the dcache
> will fill from (ie. L2 or further out).
> 
> This was being done for lowmem pages, but not for highmem which can lead
> to icache corruption. Fix this by mapping highmem pages & flushing their
> content from the dcache in __flush_dcache_page before providing the page
> to userland, just as is done for lowmem pages.
> 
> Signed-off-by: Paul Burton <paul.burton@...tec.com>
> Cc: Lars Persson <lars.persson@...s.com>
> ---
> 
>  arch/mips/mm/cache.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/mips/mm/cache.c b/arch/mips/mm/cache.c
> index 3f159ca..5a67d8c 100644
> --- a/arch/mips/mm/cache.c
> +++ b/arch/mips/mm/cache.c
> @@ -16,6 +16,7 @@
>  #include <linux/mm.h>
>  
>  #include <asm/cacheflush.h>
> +#include <asm/highmem.h>
>  #include <asm/processor.h>
>  #include <asm/cpu.h>
>  #include <asm/cpu-features.h>
> @@ -83,8 +84,6 @@ void __flush_dcache_page(struct page *page)
>  	struct address_space *mapping = page_mapping(page);
>  	unsigned long addr;
>  
> -	if (PageHighMem(page))
> -		return;
>  	if (mapping && !mapping_mapped(mapping)) {
>  		SetPageDcacheDirty(page);
>  		return;
> @@ -95,8 +94,15 @@ void __flush_dcache_page(struct page *page)
>  	 * case is for exec env/arg pages and those are %99 certainly going to
>  	 * get faulted into the tlb (and thus flushed) anyways.
>  	 */
> -	addr = (unsigned long) page_address(page);
> +	if (PageHighMem(page))
> +		addr = (unsigned long)kmap_atomic(page);
> +	else
> +		addr = (unsigned long)page_address(page);
> +
>  	flush_data_cache_page(addr);
> +
> +	if (PageHighMem(page))
> +		__kunmap_atomic((void *)addr);
>  }
>  
>  EXPORT_SYMBOL(__flush_dcache_page);

I don't see how this should work with cache aliases.  If the page is unmapped
kmap_atomic will pick a deterministic address only under some circumstances,
kmap won't.  As the result the wrong cache way will be flushed out, I think.

  Ralf