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

4 files changed, 58 insertions, 7 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1d14cc6b79a..c9866b0b77d 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 6fe6767b712..e58f03b206c 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 */
diff --git a/fs/dcache.c b/fs/dcache.c
index b60ddc41d78..b80531c9177 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -141,18 +141,29 @@ int proc_nr_dentry(ctl_table *table, int write, void __user *buffer,
  * Compare 2 name strings, return 0 if they match, otherwise non-zero.
  * The strings are both count bytes long, and count is non-zero.
  */
+#ifdef CONFIG_DCACHE_WORD_ACCESS
+
+#include <asm/word-at-a-time.h>
+/*
+ * NOTE! 'cs' and 'scount' come from a dentry, so it has a
+ * aligned allocation for this particular component. We don't
+ * strictly need the load_unaligned_zeropad() safety, but it
+ * doesn't hurt either.
+ *
+ * In contrast, 'ct' and 'tcount' can be from a pathname, and do
+ * need the careful unaligned handling.
+ */
 static inline int dentry_cmp(const unsigned char *cs, size_t scount,
 				const unsigned char *ct, size_t tcount)
 {
-#ifdef CONFIG_DCACHE_WORD_ACCESS
 	unsigned long a,b,mask;
 
 	if (unlikely(scount != tcount))
 		return 1;
 
 	for (;;) {
-		a = *(unsigned long *)cs;
-		b = *(unsigned long *)ct;
+		a = load_unaligned_zeropad(cs);
+		b = load_unaligned_zeropad(ct);
 		if (tcount < sizeof(unsigned long))
 			break;
 		if (unlikely(a != b))
@@ -165,7 +176,13 @@ static inline int dentry_cmp(const unsigned char *cs, size_t scount,
 	}
 	mask = ~(~0ul << tcount*8);
 	return unlikely(!!((a ^ b) & mask));
+}
+
 #else
+
+static inline int dentry_cmp(const unsigned char *cs, size_t scount,
+				const unsigned char *ct, size_t tcount)
+{
 	if (scount != tcount)
 		return 1;
 
@@ -177,9 +194,10 @@ static inline int dentry_cmp(const unsigned char *cs, size_t scount,
 		tcount--;
 	} while (tcount);
 	return 0;
-#endif
 }
 
+#endif
+
 static void __d_free(struct rcu_head *head)
 {
 	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
diff --git a/fs/namei.c b/fs/namei.c
index 0062dd17eb5..c42791914f8 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1429,7 +1429,7 @@ unsigned int full_name_hash(const unsigned char *name, unsigned int len)
 	unsigned long hash = 0;
 
 	for (;;) {
-		a = *(unsigned long *)name;
+		a = load_unaligned_zeropad(name);
 		if (len < sizeof(unsigned long))
 			break;
 		hash += a;
@@ -1459,7 +1459,7 @@ static inline unsigned long hash_name(const char *name, unsigned int *hashp)
 	do {
 		hash = (hash + a) * 9;
 		len += sizeof(unsigned long);
-		a = *(unsigned long *)(name+len);
+		a = load_unaligned_zeropad(name+len);
 		/* Do we have any NUL or '/' bytes in this word? */
 		mask = has_zero(a) | has_zero(a ^ REPEAT_BYTE('/'));
 	} while (!mask);