Re: [PATCH][RFC] mm: uncached vma support with writenotify

[Paul Mundt <lethal@linux-sh.org>]

On Sun, Jun 14, 2009 at 10:28:45PM +0900, Magnus Damm wrote:
> --- 0001/mm/mmap.c
> +++ work/mm/mmap.c	2009-06-11 21:43:16.000000000 +0900
> @@ -1209,8 +1209,20 @@ munmap_back:
>  	pgoff = vma->vm_pgoff;
>  	vm_flags = vma->vm_flags;
>  
> -	if (vma_wants_writenotify(vma))
> +	if (vma_wants_writenotify(vma)) {
> +		pgprot_t pprot = vma->vm_page_prot;
> +
> +		/* Can vma->vm_page_prot have changed??
> +		 *
> +		 * Answer: Yes, drivers may have changed it in their
> +		 *         f_op->mmap method.
> +		 *
> +		 * Ensures that vmas marked as uncached stay that way.
> +		 */
>  		vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);
> +		if (pgprot_val(pprot) == pgprot_val(pgprot_noncached(pprot)))
> +			vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +	}
>  
>  	vma_link(mm, vma, prev, rb_link, rb_parent);
>  	file = vma->vm_file;
> 
I guess the only real issue here is that we presently have no generic
interface in the kernel for setting a VMA uncached. pgprot_noncached()
is the closest approximation we have, but there are still architectures
that do not implement it.

Given that this comes up at least once a month, perhaps it makes sense to
see which platforms are still outstanding. At least cris, h8300,
m68knommu, s390, and xtensa all presently lack a definition for it. The
nommu cases are easily handled, but the rest still require some attention
from their architecture maintainers before we can really start treating
this as a generic interface.

Until then, you will have to do what every other user of
pgprot_noncached() code does in generic code:

	#ifdef pgprot_noncached
		vma->vm_page_prot = pgprot_noncached(...);
	#endif

OTOH, I guess we could just add something like:

	#define pgprot_noncached(x)	(x) 

which works fine for the nommu case, and which functionally is no
different from what happens right now anyways for the users that don't
wire it up sanely.

Arnd, what do you think about throwing this at asm-generic?

---

 include/asm-generic/pgtable.h |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index e410f60..e2bd73e 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -129,6 +129,10 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
 #define move_pte(pte, prot, old_addr, new_addr)	(pte)
 #endif
 
+#ifndef pgprot_noncached
+#define pgprot_noncached(prot)	(prot)
+#endif
+
 #ifndef pgprot_writecombine
 #define pgprot_writecombine pgprot_noncached
 #endif

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