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SubjectRe: [PATCH] aio: fix D-cache aliasing issues
Date
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(). Could be wrong though.

The problem is the coherence with userspace. That's why
flush_dcache_page()
is called in several places in this file. I agree with you that this
should be done
before.

>
> I've got to say on all of this, we've implemented the most inane set
> of
> primitives. It would be much easier if kmap/kunmap just took care of
> all of this. kmap should bring userspace into coherence (because why
> else would we be kmapping) and kunmap should force coherence on the
> kernel address ... and then no-one would need to worry when to and
> when
> not to flush.


I'm working on adding a flush to our kmap code to bring userspace into
coherence on PA8800/PA8900. It's ugly because it requires looping over
vma's as we do in in flush_dcache_page().

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





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