mm: extend prefault helpers to fault in more than PAGE_SIZE

drm/i915 wants to read/write more than one page in its fastpath
and hence needs to prefault more than PAGE_SIZE bytes.

Add new functions in filemap.h to make that possible.

Also kill a copy&pasted spurious space in both functions while at it.

v2: As suggested by Andrew Morton, add a multipage parameter to both
functions to avoid the additional branch for the pagemap.c hotpath.
My gcc 4.6 here seems to dtrt and indeed reap these branches where not
needed.

v3: Becaus I couldn't find a way around adding a uaddr += PAGE_SIZE to
the filemap.c hotpaths (that the compiler couldn't remove again),
let's go with separate new functions for the multipage use-case.

v4: Adjust comment to CodingStlye and fix spelling.

Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 62 insertions, 2 deletions

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index cfaaa6949b8b..c93a9a9bcd35 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -426,7 +426,7 @@ static inline int fault_in_pages_writeable(char __user *uaddr, int size)
                  */
                 if (((unsigned long)uaddr & PAGE_MASK) !=
                                 ((unsigned long)end & PAGE_MASK))
-                        ret = __put_user(0, end);
+                        ret = __put_user(0, end);
         }
         return ret;
 }
@@ -445,13 +445,73 @@ static inline int fault_in_pages_readable(const char __user *uaddr, int size)
 
                 if (((unsigned long)uaddr & PAGE_MASK) !=
                                 ((unsigned long)end & PAGE_MASK)) {
-                        ret = __get_user(c, end);
+                        ret = __get_user(c, end);
                         (void)c;
                 }
         }
         return ret;
 }
 
+/*
+ * Multipage variants of the above prefault helpers, useful if more than
+ * PAGE_SIZE of data needs to be prefaulted. These are separate from the above
+ * functions (which only handle up to PAGE_SIZE) to avoid clobbering the
+ * filemap.c hotpaths.
+ */
+static inline int fault_in_multipages_writeable(char __user *uaddr, int size)
+{
+        int ret;
+        const char __user *end = uaddr + size - 1;
+
+        if (unlikely(size == 0))
+                return 0;
+
+        /*
+         * Writing zeroes into userspace here is OK, because we know that if
+         * the zero gets there, we'll be overwriting it.
+         */
+        while (uaddr <= end) {
+                ret = __put_user(0, uaddr);
+                if (ret != 0)
+                        return ret;
+                uaddr += PAGE_SIZE;
+        }
+
+        /* Check whether the range spilled into the next page. */
+        if (((unsigned long)uaddr & PAGE_MASK) ==
+                        ((unsigned long)end & PAGE_MASK))
+                ret = __put_user(0, end);
+
+        return ret;
+}
+
+static inline int fault_in_multipages_readable(const char __user *uaddr,
+                                               int size)
+{
+        volatile char c;
+        int ret;
+        const char __user *end = uaddr + size - 1;
+
+        if (unlikely(size == 0))
+                return 0;
+
+        while (uaddr <= end) {
+                ret = __get_user(c, uaddr);
+                if (ret != 0)
+                        return ret;
+                uaddr += PAGE_SIZE;
+        }
+
+        /* Check whether the range spilled into the next page. */
+        if (((unsigned long)uaddr & PAGE_MASK) ==
+                        ((unsigned long)end & PAGE_MASK)) {
+                ret = __get_user(c, end);
+                (void)c;
+        }
+
+        return ret;
+}
+
 int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
                                 pgoff_t index, gfp_t gfp_mask);
 int add_to_page_cache_lru(struct page *page, struct address_space *mapping,