Merge branch 'fix-unmapped-word-at-a-time'

Jana Saout confirmed that this fixes the page faults he saw.

His problem was triggered by ocfs2 and autofs symlink lookups, where the
symlink allocation was at the end of a page.  But the deeper reason
seems to be the use of Xen-PV, which is what then causes him to have all
these unmapped pages, which is what then makes it a problem when the
unaligned word-at-a-time code fetches data past the end of a page.

* fix-unmapped-word-at-a-time:
  vfs: make word-at-a-time accesses handle a non-existing page

2 files changed, 34 insertions, 1 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1d14cc6b79ad..c9866b0b77d8 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -81,7 +81,7 @@ config X86
         select CLKEVT_I8253
         select ARCH_HAVE_NMI_SAFE_CMPXCHG
         select GENERIC_IOMAP
-        select DCACHE_WORD_ACCESS if !DEBUG_PAGEALLOC
+        select DCACHE_WORD_ACCESS
 
 config INSTRUCTION_DECODER
         def_bool (KPROBES || PERF_EVENTS)
diff --git a/arch/x86/include/asm/word-at-a-time.h b/arch/x86/include/asm/word-at-a-time.h
index 6fe6767b7124..e58f03b206c3 100644
--- a/arch/x86/include/asm/word-at-a-time.h
+++ b/arch/x86/include/asm/word-at-a-time.h
@@ -43,4 +43,37 @@ static inline unsigned long has_zero(unsigned long a)
         return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80);
 }
 
+/*
+ * Load an unaligned word from kernel space.
+ *
+ * In the (very unlikely) case of the word being a page-crosser
+ * and the next page not being mapped, take the exception and
+ * return zeroes in the non-existing part.
+ */
+static inline unsigned long load_unaligned_zeropad(const void *addr)
+{
+        unsigned long ret, dummy;
+
+        asm(
+                "1:\tmov %2,%0\n"
+                "2:\n"
+                ".section .fixup,\"ax\"\n"
+                "3:\t"
+                "lea %2,%1\n\t"
+                "and %3,%1\n\t"
+                "mov (%1),%0\n\t"
+                "leal %2,%%ecx\n\t"
+                "andl %4,%%ecx\n\t"
+                "shll $3,%%ecx\n\t"
+                "shr %%cl,%0\n\t"
+                "jmp 2b\n"
+                ".previous\n"
+                _ASM_EXTABLE(1b, 3b)
+                :"=&r" (ret),"=&c" (dummy)
+                :"m" (*(unsigned long *)addr),
+                 "i" (-sizeof(unsigned long)),
+                 "i" (sizeof(unsigned long)-1));
+        return ret;
+}
+
 #endif /* _ASM_WORD_AT_A_TIME_H */