ARM: 8579/1: mm: Fix definition of pmd_mknotpresent

Currently pmd_mknotpresent will use a zero entry to respresent an
invalidated pmd.

Unfortunately this definition clashes with pmd_none, thus it is
possible for a race condition to occur if zap_pmd_range sees pmd_none
whilst __split_huge_pmd_locked is running too with pmdp_invalidate
just called.

This patch fixes the race condition by modifying pmd_mknotpresent to
create non-zero faulting entries (as is done in other architectures),
removing the ambiguity with pmd_none.

[[email protected]: using L_PMD_SECT_VALID instead of PMD_TYPE_SECT]

Fixes: 8d96250 ("ARM: mm: Transparent huge page support for LPAE systems.")
Cc: <[email protected]> # 3.11+
Reported-by: Kirill A. Shutemov <[email protected]>
Acked-by: Will Deacon <[email protected]>
Cc: Russell King <[email protected]>
Signed-off-by: Steve Capper <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>
Signed-off-by: Russell King <[email protected]>

Author: stevecapperarm

arch/arm/include/asm/pgtable-3level.h

@@ -250,10 +250,10 @@ PMD_BIT_FUNC(mkyoung,   |= PMD_SECT_AF);
 #define pfn_pmd(pfn,prot)	(__pmd(((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot)))
 #define mk_pmd(page,prot)	pfn_pmd(page_to_pfn(page),prot)
 
-/* represent a notpresent pmd by zero, this is used by pmdp_invalidate */
+/* represent a notpresent pmd by faulting entry, this is used by pmdp_invalidate */
 static inline pmd_t pmd_mknotpresent(pmd_t pmd)
 {
-	return __pmd(0);
+	return __pmd(pmd_val(pmd) & ~L_PMD_SECT_VALID);
 }
 
 static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)