[PATCH] Use kmap_atomic() for generic_file_read()

This patch allows the kernel to hold atomic kmaps in file_read_actor().

We try to fault in the page, then take an atomic kmap.  If the atomic
copy_to_user() then faults, drop a printk and fall back to kmap().

diff --git a/mm/filemap.c b/mm/filemap.c
index 771676ff57a9a690077ff4dc331eeed6ff9d3509..95bb3f67eca6fe19d6ce2174e3fa3d7d475d6fb9 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1036,7 +1036,37 @@ no_cached_page:
        UPDATE_ATIME(inode);
 }
 
-int file_read_actor(read_descriptor_t * desc, struct page *page, unsigned long offset, unsigned long size)
+/*
+ * Fault a userspace page into pagetables.  Return non-zero on a fault.
+ *
+ * FIXME: this assumes that two userspace pages are always sufficient.  That's
+ * not true if PAGE_CACHE_SIZE > PAGE_SIZE.
+ */
+static inline int fault_in_pages_writeable(char *uaddr, int size)
+{
+       int ret;
+
+       /*
+        * Writing zeroes into userspace here is OK, because we know that if
+        * the zero gets there, we'll be overwriting it.
+        */
+       ret = __put_user(0, uaddr);
+       if (ret == 0) {
+               char *end = uaddr + size - 1;
+
+               /*
+                * If the page was already mapped, this will get a cache miss
+                * for sure, so try to avoid doing it.
+                */
+               if (((unsigned long)uaddr & PAGE_MASK) !=
+                               ((unsigned long)end & PAGE_MASK))
+                       ret = __put_user(0, end);
+       }
+       return ret;
+}
+
+int file_read_actor(read_descriptor_t *desc, struct page *page,
+                       unsigned long offset, unsigned long size)
 {
        char *kaddr;
        unsigned long left, count = desc->count;
@@ -1044,14 +1074,28 @@ int file_read_actor(read_descriptor_t * desc, struct page *page, unsigned long o
        if (size > count)
                size = count;
 
+       /*
+        * Faults on the destination of a read are common, so do it before
+        * taking the kmap.
+        */
+       if (!fault_in_pages_writeable(desc->buf, size)) {
+               kaddr = kmap_atomic(page, KM_USER0);
+               left = __copy_to_user(desc->buf, kaddr + offset, size);
+               kunmap_atomic(kaddr, KM_USER0);
+               if (left == 0)
+                       goto success;
+       }
+
+       /* Do it the slow way */
        kaddr = kmap(page);
        left = __copy_to_user(desc->buf, kaddr + offset, size);
        kunmap(page);
-       
+
        if (left) {
                size -= left;
                desc->error = -EFAULT;
        }
+success:
        desc->count = count - size;
        desc->written += size;
        desc->buf += size;