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

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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@xxxxxxxx



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