fs: fuse: Workaround for CMA migration

The FUSE file system may hold references to pages for long
periods of time, preventing migration from occurring. If a CMA
page is used here, CMA allocations may fail. Work around this
by swapping out a CMA page for a non-CMA page when working with
the FUSE file system.

Change-Id: Id763ea833ee125c8732ae3759ec9e20d94aa8424
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>

fs/fuse/file.c

@@ -900,6 +900,41 @@ static int fuse_readpages_fill(void *_data, struct page *page)
 		return -EIO;
 	}
 
+#ifdef CONFIG_CMA
+	if (is_cma_pageblock(page)) {
+		struct page *oldpage = page, *newpage;
+		int err;
+
+		/* make sure that old page is not free in-between the calls */
+		page_cache_get(oldpage);
+
+		newpage = alloc_page(GFP_HIGHUSER);
+		if (!newpage) {
+			page_cache_release(oldpage);
+			return -ENOMEM;
+		}
+
+		err = replace_page_cache_page(oldpage, newpage, GFP_KERNEL);
+		if (err) {
+			__free_page(newpage);
+			page_cache_release(oldpage);
+			return err;
+		}
+
+		/*
+		 * Decrement the count on new page to make page cache the only
+		 * owner of it
+		 */
+		lock_page(newpage);
+		put_page(newpage);
+
+		/* finally release the old page and swap pointers */
+		unlock_page(oldpage);
+		page_cache_release(oldpage);
+		page = newpage;
+	}
+#endif
+
 	page_cache_get(page);
 	req->pages[req->num_pages] = page;
 	req->page_descs[req->num_pages].length = PAGE_SIZE;