Re: [PATCH] i386: PAE entries must have their low word cleared first

[Zachary Amsden <zach@vmware.com>]

Keir Fraser wrote:
> We cannot use pte_clear() unless we redefine it for PAE. Currently it 
> reduces to set_pte() which explicitly uses the wrong ordering (sets 
> high *then* low, because it's normally used to introduce a mapping).

Yes, that means pte_clear() has the same bug.  Why don't we redefine 
pte_clear for PAE?  Then the ifdef can go away.

> Also, I think wmb() should be used in PAE's set_pte(), rather than the 
> current smp_wmb(). They reduce to the same thing, but wmb() makes it 
> clear this is is not an issue specific to SMP systems. 

I agree with that.  The problem is not related to SMP at all.  I would 
propose this approach.
Proposed fix for ptep_get_and_clear_full PAE bug.  Pte_clear had the same
bug, so use the same fix for both.

Signed-off-by: Zachary Amsden <zach@vmware.com>

Index: linux-2.6.17-rc/include/asm-i386/pgtable.h
===================================================================
--- linux-2.6.17-rc.orig/include/asm-i386/pgtable.h	2006-04-25 15:41:06.000000000 -0700
+++ linux-2.6.17-rc/include/asm-i386/pgtable.h	2006-04-26 08:39:42.000000000 -0700
@@ -204,7 +204,6 @@ extern unsigned long long __PAGE_KERNEL,
 extern unsigned long pg0[];
 
 #define pte_present(x)	((x).pte_low & (_PAGE_PRESENT | _PAGE_PROTNONE))
-#define pte_clear(mm,addr,xp)	do { set_pte_at(mm, addr, xp, __pte(0)); } while (0)
 
 /* To avoid harmful races, pmd_none(x) should check only the lower when PAE */
 #define pmd_none(x)	(!(unsigned long)pmd_val(x))
@@ -268,7 +267,7 @@ static inline pte_t ptep_get_and_clear_f
 	pte_t pte;
 	if (full) {
 		pte = *ptep;
-		*ptep = __pte(0);
+		pte_clear(mm, addr, ptep);
 	} else {
 		pte = ptep_get_and_clear(mm, addr, ptep);
 	}
Index: linux-2.6.17-rc/include/asm-i386/pgtable-3level.h
===================================================================
--- linux-2.6.17-rc.orig/include/asm-i386/pgtable-3level.h	2006-04-25 15:41:06.000000000 -0700
+++ linux-2.6.17-rc/include/asm-i386/pgtable-3level.h	2006-04-26 08:38:57.000000000 -0700
@@ -85,6 +85,13 @@ static inline void pud_clear (pud_t * pu
 #define pmd_offset(pud, address) ((pmd_t *) pud_page(*(pud)) + \
 			pmd_index(address))
 
+static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
+{
+	ptep->pte_low = 0;
+	wmb();
+	ptep->pte_high = 0;
+}
+
 static inline pte_t ptep_get_and_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
 {
 	pte_t res;
Index: linux-2.6.17-rc/include/asm-i386/pgtable-2level.h
===================================================================
--- linux-2.6.17-rc.orig/include/asm-i386/pgtable-2level.h	2006-04-25 15:41:06.000000000 -0700
+++ linux-2.6.17-rc/include/asm-i386/pgtable-2level.h	2006-04-26 08:37:34.000000000 -0700
@@ -18,6 +18,8 @@
 #define set_pte_atomic(pteptr, pteval) set_pte(pteptr,pteval)
 #define set_pmd(pmdptr, pmdval) (*(pmdptr) = (pmdval))
 
+#define pte_clear(mm,addr,xp)	do { set_pte_at(mm, addr, xp, __pte(0)); } while (0)
+
 #define ptep_get_and_clear(mm,addr,xp)	__pte(xchg(&(xp)->pte_low, 0))
 #define pte_same(a, b)		((a).pte_low == (b).pte_low)
 #define pte_page(x)		pfn_to_page(pte_pfn(x))