Re: Xen pv_ops domU :: BUG() in remove_from_page_cache()

[Jeremy Fitzhardinge <jeremy@goop.org>]

Mark McLoughlin wrote:
> Hi,
>
> 2.6.29-rc3 x86_64 guest on x86_64 RHEL5.3 host:
>
>   https://bugzilla.redhat.com/484295
>
> kernel BUG at mm/filemap.c:123!                    
> invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC      
>   

This will fix it.  I hope.

    J

Subject: x86: don't apply __supported_pte_mask to non-present ptes

__supported_pte_mask contains the set of flags we support on the
current hardware.  We also use bits in the pte for things like
logically present ptes with no permissions, and swap entries for
swapped out pages.  We should only apply __supported_pte_mask to
present ptes, because otherwise we may destroy other information being
stored in the ptes.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
 arch/x86/include/asm/pgtable.h  |   26 ++++++++++++++++++++------
 arch/x86/include/asm/xen/page.h |    2 +-
 2 files changed, 21 insertions(+), 7 deletions(-)

===================================================================
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -316,16 +316,30 @@
 
 extern pteval_t __supported_pte_mask;
 
+/*
+ * Mask out unsupported bits in a present pgprot.  Non-present pgprots
+ * can use those bits for other purposes, so leave them be.
+ */
+static inline pgprotval_t massage_pgprot(pgprot_t pgprot)
+{
+	pgprotval_t protval = pgprot_val(pgprot);
+
+	if (protval & _PAGE_PRESENT)
+		protval &= __supported_pte_mask;
+
+	return protval;
+}
+
 static inline pte_t pfn_pte(unsigned long page_nr, pgprot_t pgprot)
 {
-	return __pte((((phys_addr_t)page_nr << PAGE_SHIFT) |
-		      pgprot_val(pgprot)) & __supported_pte_mask);
+	return __pte(((phys_addr_t)page_nr << PAGE_SHIFT) |
+		     massage_pgprot(pgprot));
 }
 
 static inline pmd_t pfn_pmd(unsigned long page_nr, pgprot_t pgprot)
 {
-	return __pmd((((phys_addr_t)page_nr << PAGE_SHIFT) |
-		      pgprot_val(pgprot)) & __supported_pte_mask);
+	return __pmd(((phys_addr_t)page_nr << PAGE_SHIFT) |
+		     massage_pgprot(pgprot));
 }
 
 static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
@@ -337,7 +351,7 @@
 	 * the newprot (if present):
 	 */
 	val &= _PAGE_CHG_MASK;
-	val |= pgprot_val(newprot) & (~_PAGE_CHG_MASK) & __supported_pte_mask;
+	val |= massage_pgprot(newprot) & ~_PAGE_CHG_MASK;
 
 	return __pte(val);
 }
@@ -353,7 +367,7 @@
 
 #define pte_pgprot(x) __pgprot(pte_flags(x) & PTE_FLAGS_MASK)
 
-#define canon_pgprot(p) __pgprot(pgprot_val(p) & __supported_pte_mask)
+#define canon_pgprot(p) __pgprot(massage_pgprot(p))
 
 static inline int is_new_memtype_allowed(unsigned long flags,
 						unsigned long new_flags)
===================================================================
--- a/arch/x86/include/asm/xen/page.h
+++ b/arch/x86/include/asm/xen/page.h
@@ -137,7 +137,7 @@
 	pte_t pte;
 
 	pte.pte = ((phys_addr_t)page_nr << PAGE_SHIFT) |
-		(pgprot_val(pgprot) & __supported_pte_mask);
+			massage_pgprot(pgprot);
 
 	return pte;
 }