[PATCH V3 2/2] ARM: Handle user space mapped pages in flush_kernel_dcache_page

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Oct 08, 2012 at 10:02:16PM +0200, Simon Baatz wrote:
> Hi Catalin,
> 
> On Mon, Oct 08, 2012 at 06:44:28PM +0100, Catalin Marinas wrote:
> > On Sun, Oct 07, 2012 at 12:29:12PM +0100, Simon Baatz wrote:
> > > Commit f8b63c1 made flush_kernel_dcache_page() a no-op assuming that
> > > the pages it needs to handle are kernel mapped only.  However, for
> > > example when doing direct I/O, pages with user space mappings may
> > > occur.
> > > 
> > > Thus, do lazy flushing like in flush_dcache_page() if there are no user
> > > space mappings.  Otherwise, flush the kernel cache lines directly.
> > 
> > Do you need to fix the VIPT non-aliasing case as well? Does
> > flush_kernel_dcache_page() need to handle I-cache?
> 
> Good question. My previous version of the patch did not handle it,
> but after our discussion on the arm64 case, I came to the conclusion
> that we probably need to handle it.
> 
> flush_dcache_page() and flush_kernel_dcache_page() are both used when
> a page was modified via its kernel mapping.  For example,
> crypto/scatterwalk.c uses flush_dcache_page() whereas
> lib/scatterlist.c uses flush_kernel_dcache_page().  

It's likely that this stuff is incredibly buggy - and where we suspect
there's problems (like using the wrong flush_dcache_page() vs
flush_kernel_dcache_page() call) that needs to be fixed.

I suspect crypto/scatterwalk.c was created long before
flush_kernel_dcache_page() came into existence, and it probably needs
to be fixed.

> Thus, the reasoning is that if flush_dcache_page() needs to handle
> I-cache in the case there is no hook later (already user-mapped page
> cache page) then flush_kernel_dcache_page() needs to do the same. 

This sounds like we're heading in the direction that flush_kernel_dcache_page()
and flush_dcache_page() end up being virtually identical.  This isn't
supposed to be - they _are_ supposed to be different.  Again, maybe
that's because the users have chosen the wrong function.

Remember that these functions only get used on so-called "minority"
architectures where the caches are a hinderance - to put that a
different way, most people don't care about these calls because x86
just works.

> With respect to clearing the flag in the VIPT non-aliasing case with
> anon pages: Declaring the pages dirty by default may already be
> sufficient in this case, at least that is what the current
> implementation assumes.

PG_arch_1 has no meaning what so ever for anon pages.  Don't even try
to make it have meaning there, it will cause lots of pain if you do.
The reason is that anon pages have no mapping, and so trying to do the
PG_arch_1 trick will result in most places deciding "there is no
userspace mapping we can skip that".