Re: [patch 05/16] mm: flush dcache before writing into page to avoid alias

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2010-02-02 at 13:44 -0800, akpm@linux-foundation.org wrote:
> From: anfei zhou <anfei.zhou@gmail.com>
> 
> 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);

This is likely unnecessary if the page has come down a standard path ...
for fuse, it is possible it didn't go through __get_user_pages().

> 	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.

We already have one of those: flush_kernel_dcache_page().

James