linux-kernel - Re: [PATCH] aio: fix D-cache aliasing issues

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Date:	Sat, 16 Nov 2013 21:07:18 +0100
From:	Simon Baatz <gmbnomis@...il.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Helge Deller <deller@....de>, Benjamin LaHaise <bcrl@...ck.org>,
	linux-aio@...ck.org, linux-kernel@...r.kernel.org,
	linux-parisc@...r.kernel.org
Subject: Re: [PATCH] aio: fix D-cache aliasing issues

On Fri, Nov 15, 2013 at 02:42:05PM -0800, James Bottomley wrote:
> On Fri, 2013-11-15 at 23:05 +0100, Helge Deller wrote:
> > When a user page mapping is released via kunmap*() functions, the D-cache needs
> > to be flushed via flush_dcache_page() to avoid D-cache aliasing issues.
> > 
> > This patch fixes aio on the parisc platform (and probably others).
> 
> This should be flush_kernel_dcache_page().  flush_dcache_page() is for
> full coherency but for unmap, we know the page was coherent going in and
> may have been modified by the kernel, so only the kernel view needs to
> be sync'd.  Technically, by the kernel API, the flush should be done
> *before* unmapping.  This would have mattered on parisc until we did
> flush via tmpalias which means we no-longer care if the mapping for the
> flush exists or not because we always recreate it via the tmpalias
> pages.

On ARM, flush_kernel_dcache_page() actually assumes that the page is
mapped.  It avoids double flushing of highmem pages by not flushing
in those cases where kunmap_atomic() already takes care of flushing.


- Simon
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