Re: O_DIRECT patch for processors with VIPT cache for mainline kernel (specifically arm in our case)

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Nov 20, 2008 at 05:17:14PM +0000, Ralf Baechle wrote:
> On Thu, Nov 20, 2008 at 08:27:19AM -0700, Matthew Wilcox wrote:
> > I'm not quite sure why you need kmap_coherent().  If a page is mapped into
> > userspace, you can find what address it's mapped to from
> > page->mapping->i_mmap and page->index.  OTOH, that's potentially
> 
> Even if we know the userspace address of a page we do not necessarily have
> a usable mapping for kernel purposes.  The userspace mapping might be r/o
> when we need r/w or it might be in another process.  kmap_coherent takes
> the job of creating a r/w mapping on a suitable kernel virtual address
> that will avoid any aliases.

Ah, I didn't do a good enough job of explaining.  My question was why
you needed the 'address' argument to kmap_coherent (and thus had a
different prototype from kmap) instead of just implementing kmap() on
mips.

> The original reason for the introduction of kmap_coherent was avoiding
> a cache alias in when a multi-threaded process forks.  The issue has been
> debated on lkml in 2006 as part of my submission of a patchset under the
> subject of "Fix COW D-cache aliasing on fork".  The description is somewhat
> lengthy so I omit it here.
> 
> One of the ugly parts of kmap_coherent() is that it cannot be used safely
> if the page has been marked as dirty by flush_dcache_page(); the callers
> know about this and deal with it.

If you fix kmap() to only give you addresses that are coherent with
userspace, you no longer need flush_dcache_page().

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."