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

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Date:	Mon, 18 Nov 2013 11:54:43 +0100
From:	Simon Baatz <gmbnomis@...il.com>
To:	Helge Deller <deller@....de>
Cc:	Benjamin LaHaise <bcrl@...ck.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	linux-aio@...ck.org, linux-kernel@...r.kernel.org,
	linux-parisc@...r.kernel.org
Subject: Re: [PATCH] aio: fix D-cache aliasing issues

Hi Helge,

On Sun, Nov 17, 2013 at 10:23:08PM +0100, Helge Deller wrote:
> On 11/16/2013 09:09 PM, Benjamin LaHaise wrote:
> > On Sat, Nov 16, 2013 at 09:07:18PM +0100, Simon Baatz wrote:
> >> 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.
> > 
> > Helge -- are you going to resubmit a version of this patch that makes the 
> > recommended change?
> 
> Sure, I'll do. May need some time for testing the various machine types though.
> Maybe in the end you can drop my patch since we might be able to fix it in the 
> parisc arch code.

Could you provide me with the test case(s) you are running to
reproduce the problem?  I could test this on aliasing D-cache on ARM
as well. Since kmap/kunmap do not flush in general on ARM, we
might need the explicit flushes here.


- Simon
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