[PATCH] mips,mm: Reinstate move_pte optimization

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

CC to mips folks.

> If something like this or your replacment does go forward,
> then I think that test is better inside the "if (!page->mapping)"
> below.  Admittedly that adds even more mm-dependence here (relying
> on a zero page to have NULL page->mapping); but isn't page_to_pfn()
> one of those functions which is trivial on many configs but expensive
> on some?  Better call it only in the rare case that it's needed.
> 
> Though wouldn't it be even better not to use is_zero_pfn() at all?
> That was convenient in mm/memory.c because it had the pfn or pte right
> at hand, but here a traditional (page == ZERO_PAGE(address)) would be
> more efficient.
> 
> Which would save having to move is_zero_pfn() from mm/memory.c
> to include/linux/mm.h - I'd prefer to keep it private if we can.
> But for completeness, this would involve resurrecting the 2.6.19
> MIPS move_pte(), which makes sure mremap() move doesn't interfere
> with our assumptions.  Something like
> 
> #define __HAVE_ARCH_MOVE_PTE
> pte_t move_pte(pte_t pte, pgprot_t prot, unsigned long old_addr,
>                                          unsigned long new_addr)
> {
> 	if (pte_present(pte) && is_zero_pfn(pte_pfn(pte)))
> 		pte = mk_pte(ZERO_PAGE(new_addr), prot);
> 	return pte;
> }
> 
> in arch/mips/include/asm/pgtable.h.

I agree with resurrecting mips move_pte. At least your patch
passed my cross compile test :)

Ralf, can you please review following patch?


======================================================
Subject: [PATCH] mips,mm: Reinstate move_pte optimization
From: Hugh Dickins <hugh.dickins@tiscali.co.uk>

About three years ago, we removed mips specific move_pte by commit
701dfbc1cb (mm: mremap correct rmap accounting). because it is only
small optimization and it has bug.

However new zero-page thing doesn't have such problem and behavior
consistency of mremap have worth a bit.

This patch reinstate it.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: Nick Piggin <npiggin@suse.de>
Cc: Carsten Otte <cotte@de.ibm.com>
---
 arch/mips/include/asm/pgtable.h |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/arch/mips/include/asm/pgtable.h b/arch/mips/include/asm/pgtable.h
index 1854336..6ad2f73 100644
--- a/arch/mips/include/asm/pgtable.h
+++ b/arch/mips/include/asm/pgtable.h
@@ -387,6 +387,14 @@ static inline int io_remap_pfn_range(struct vm_area_struct *vma,
 		remap_pfn_range(vma, vaddr, pfn, size, prot)
 #endif
 
+#define __HAVE_ARCH_MOVE_PTE
+pte_t move_pte(pte_t pte, pgprot_t prot, unsigned long old_addr, unsigned long new_addr)
+{
+	if (pte_present(pte) && is_zero_pfn(pte_pfn(pte)))
+		pte = mk_pte(ZERO_PAGE(new_addr), prot);
+	return pte;
+}
+
 #include <asm-generic/pgtable.h>
 
 /*
-- 
1.6.5.2