linux-kernel - Re: [PATCH] Flush dcache before writing into page to avoid alias

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Date:	Mon, 25 Jan 2010 21:33:08 +0800
From:	anfei <anfei.zhou@...il.com>
To:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux@....linux.org.uk, Jamie Lokier <jamie@...reable.org>
Subject: Re: [PATCH] Flush dcache before writing into page to avoid alias

Hi Andrew,

On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote:
> The cache alias problem will happen if the changes of user shared mapping
> is not flushed before copying, then user and kernel mapping may be mapped
> into two different cache line, it is impossible to guarantee the coherence
> after iov_iter_copy_from_user_atomic.  So the right steps should be:
> 	flush_dcache_page(page);
> 	kmap_atomic(page);
> 	write to page;
> 	kunmap_atomic(page);
> 	flush_dcache_page(page);
> More precisely, we might create two new APIs flush_dcache_user_page and
> flush_dcache_kern_page to replace the two flush_dcache_page accordingly.
> 
> Here is a snippet tested on omap2430 with VIPT cache, and I think it is
> not ARM-specific:
> 	int val = 0x11111111;
> 	fd = open("abc", O_RDWR);
> 	addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
> 	*(addr+0) = 0x44444444;
> 	tmp = *(addr+0);
> 	*(addr+1) = 0x77777777;
> 	write(fd, &val, sizeof(int));
> 	close(fd);
> The results are not always 0x11111111 0x77777777 at the beginning as expected.
> 
Is this a real bug or not necessary to support?

Thanks,
Anfei.

> Signed-off-by: Anfei <anfei.zhou@...il.com>
> ---
>  fs/fuse/file.c |    3 +++
>  mm/filemap.c   |    3 +++
>  2 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index c18913a..a9f5e13 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -828,6 +828,9 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
>  		if (!page)
>  			break;
> 
> +		if (mapping_writably_mapped(mapping))
> +			flush_dcache_page(page);
> +
>  		pagefault_disable();
>  		tmp = iov_iter_copy_from_user_atomic(page, ii, offset, bytes);
>  		pagefault_enable();
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 96ac6b0..07056fb 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2196,6 +2196,9 @@ again:
>  		if (unlikely(status))
>  			break;
> 
> +		if (mapping_writably_mapped(mapping))
> +			flush_dcache_page(page);
> +
>  		pagefault_disable();
>  		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
>  		pagefault_enable();
> -- 
> 1.6.3.1
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