Make sure that highmem pages are not added to symlink page cache

inode_nohighmem() is sufficient to make sure that page_get_link()
won't try to allocate a highmem page.  Moreover, it is sufficient
to make sure that page_symlink/__page_symlink won't do the same
thing.  However, any filesystem that manually preseeds the symlink's
page cache upon symlink(2) needs to make sure that the page it
inserts there won't be a highmem one.

Fortunately, only nfs and shmem have run afoul of that...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/porting

@@ -508,7 +508,11 @@ in your dentry operations instead.
 [mandatory]
 	any symlink that might use page_follow_link_light/page_put_link() must
 	have inode_nohighmem(inode) called before anything might start playing with
-	its pagecache.
+	its pagecache.  No highmem pages should end up in the pagecache of such
+	symlinks.  That includes any preseeding that might be done during symlink
+	creation.  __page_symlink() will honour the mapping gfp flags, so once
+	you've done inode_nohighmem() it's safe to use, but if you allocate and
+	insert the page manually, make sure to use the right gfp flags.
 --
 [mandatory]
 	->follow_link() is replaced with ->get_link(); same API, except that

fs/nfs/dir.c

@@ -1894,15 +1894,14 @@ int nfs_symlink(struct inode *dir, struct dentry *dentry, const char *symname)
 	attr.ia_mode = S_IFLNK | S_IRWXUGO;
 	attr.ia_valid = ATTR_MODE;
 
-	page = alloc_page(GFP_HIGHUSER);
+	page = alloc_page(GFP_USER);
 	if (!page)
 		return -ENOMEM;
 
-	kaddr = kmap_atomic(page);
+	kaddr = page_address(page);
 	memcpy(kaddr, symname, pathlen);
 	if (pathlen < PAGE_SIZE)
 		memset(kaddr + pathlen, 0, PAGE_SIZE - pathlen);
-	kunmap_atomic(kaddr);
 
 	trace_nfs_symlink_enter(dir, dentry);
 	error = NFS_PROTO(dir)->symlink(dir, dentry, page, pathlen, &attr);

mm/shmem.c

@@ -2469,14 +2469,14 @@ static int shmem_symlink(struct inode *dir, struct dentry *dentry, const char *s
 		inode->i_op = &shmem_short_symlink_operations;
 		inode->i_link = info->symlink;
 	} else {
+		inode_nohighmem(inode);
 		error = shmem_getpage(inode, 0, &page, SGP_WRITE, NULL);
 		if (error) {
 			iput(inode);
 			return error;
 		}
 		inode->i_mapping->a_ops = &shmem_aops;
 		inode->i_op = &shmem_symlink_inode_operations;
-		inode_nohighmem(inode);
 		memcpy(page_address(page), symname, len);
 		SetPageUptodate(page);
 		set_page_dirty(page);