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

[John David Anglin]

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@xxxxxxxx



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