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

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Message-ID: <BLU0-SMTP37F8F388A1DE7927BB765997FA0@phx.gbl>
Date:	Sat, 16 Nov 2013 17:49:33 -0500
From:	John David Anglin <dave.anglin@...l.net>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	Simon Baatz <gmbnomis@...il.com>, 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 16-Nov-13, at 5:37 PM, James Bottomley wrote:

> On Sat, 2013-11-16 at 17:32 -0500, John David Anglin wrote:
>> On 16-Nov-13, at 5:06 PM, James Bottomley wrote:
>>
>>> On Sat, 2013-11-16 at 21:07 +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.
>>>
>>> On Parisc, kmap/kunmap is currently a nop.  However, if we ever
>>> implemented highmem, we would also need to flush before we unmap,
>>> which
>>> is why the flush needs to go before the kunmap.
>>
>> Not quite.  On PA8800/PA8900, we currently do a flush in kunmap.
>>
>> I'm fairly certain from discussion with Helge that he saw this bug on
>> a C8000
>> with PA8800 processor.  In that case, adding a call to
>> flush_kernel_dcache_page()
>> just duplicates the call in kunmap().
>
> Yes, if that's true it won't be the solution.

The calls may be needed for the two cases where the page is changed,  
but I don't
know how to avoid double flush on PA8800/PA8900 without changing the  
primatives.

Dave
--
John David Anglin	dave.anglin@...l.net



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