mm: add new 'read_cache_page_gfp()' helper function

It's a simplified 'read_cache_page()' which takes a page allocation
flag, so that different paths can control how aggressive the memory
allocations are that populate a address space.

In particular, the intel GPU object mapping code wants to be able to do
a certain amount of own internal memory management by automatically
shrinking the address space when memory starts getting tight.  This
allows it to dynamically use different memory allocation policies on a
per-allocation basis, rather than depend on the (static) address space
gfp policy.

The actual new function is a one-liner, but re-organizing the helper
functions to the point where you can do this with a single line of code
is what most of the patch is all about.
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagemap.h

@@ -253,6 +253,8 @@ extern struct page * read_cache_page_async(struct address_space *mapping,
 extern struct page * read_cache_page(struct address_space *mapping,
 				pgoff_t index, filler_t *filler,
 				void *data);
+extern struct page * read_cache_page_gfp(struct address_space *mapping,
+				pgoff_t index, gfp_t gfp_mask);
 extern int read_cache_pages(struct address_space *mapping,
 		struct list_head *pages, filler_t *filler, void *data);
 

mm/filemap.c

@@ -1634,14 +1634,15 @@ EXPORT_SYMBOL(generic_file_readonly_mmap);
 static struct page *__read_cache_page(struct address_space *mapping,
 				pgoff_t index,
 				int (*filler)(void *,struct page*),
-				void *data)
+				void *data,
+				gfp_t gfp)
 {
 	struct page *page;
 	int err;
 repeat:
 	page = find_get_page(mapping, index);
 	if (!page) {
-		page = page_cache_alloc_cold(mapping);
+		page = __page_cache_alloc(gfp | __GFP_COLD);
 		if (!page)
 			return ERR_PTR(-ENOMEM);
 		err = add_to_page_cache_lru(page, mapping, index, GFP_KERNEL);
@@ -1661,31 +1662,18 @@ static struct page *__read_cache_page(struct address_space *mapping,
 	return page;
 }
 
-/**
- * read_cache_page_async - read into page cache, fill it if needed
- * @mapping:	the page's address_space
- * @index:	the page index
- * @filler:	function to perform the read
- * @data:	destination for read data
- *
- * Same as read_cache_page, but don't wait for page to become unlocked
- * after submitting it to the filler.
- *
- * Read into the page cache. If a page already exists, and PageUptodate() is
- * not set, try to fill the page but don't wait for it to become unlocked.
- *
- * If the page does not get brought uptodate, return -EIO.
- */
-struct page *read_cache_page_async(struct address_space *mapping,
+static struct page *do_read_cache_page(struct address_space *mapping,
 				pgoff_t index,
 				int (*filler)(void *,struct page*),
-				void *data)
+				void *data,
+				gfp_t gfp)
+
 {
 	struct page *page;
 	int err;
 
 retry:
-	page = __read_cache_page(mapping, index, filler, data);
+	page = __read_cache_page(mapping, index, filler, data, gfp);
 	if (IS_ERR(page))
 		return page;
 	if (PageUptodate(page))
@@ -1710,38 +1698,86 @@ struct page *read_cache_page_async(struct address_space *mapping,
 	mark_page_accessed(page);
 	return page;
 }
-EXPORT_SYMBOL(read_cache_page_async);
 
 /**
- * read_cache_page - read into page cache, fill it if needed
+ * read_cache_page_async - read into page cache, fill it if needed
  * @mapping:	the page's address_space
  * @index:	the page index
  * @filler:	function to perform the read
  * @data:	destination for read data
  *
+ * Same as read_cache_page, but don't wait for page to become unlocked
+ * after submitting it to the filler.
+ *
  * Read into the page cache. If a page already exists, and PageUptodate() is
- * not set, try to fill the page then wait for it to become unlocked.
+ * not set, try to fill the page but don't wait for it to become unlocked.
  *
  * If the page does not get brought uptodate, return -EIO.
  */
-struct page *read_cache_page(struct address_space *mapping,
+struct page *read_cache_page_async(struct address_space *mapping,
 				pgoff_t index,
 				int (*filler)(void *,struct page*),
 				void *data)
 {
-	struct page *page;
+	return do_read_cache_page(mapping, index, filler, data, mapping_gfp_mask(mapping));
+}
+EXPORT_SYMBOL(read_cache_page_async);
 
-	page = read_cache_page_async(mapping, index, filler, data);
-	if (IS_ERR(page))
-		goto out;
+static struct page *wait_on_page_read(struct page *page)
+{
+	if (!IS_ERR(page)) {
 		wait_on_page_locked(page);
 		if (!PageUptodate(page)) {
 			page_cache_release(page);
 			page = ERR_PTR(-EIO);
 		}
- out:
+	}
 	return page;
 }
+
+/**
+ * read_cache_page_gfp - read into page cache, using specified page allocation flags.
+ * @mapping:	the page's address_space
+ * @index:	the page index
+ * @gfp:	the page allocator flags to use if allocating
+ *
+ * This is the same as "read_mapping_page(mapping, index, NULL)", but with
+ * any new page allocations done using the specified allocation flags. Note
+ * that the Radix tree operations will still use GFP_KERNEL, so you can't
+ * expect to do this atomically or anything like that - but you can pass in
+ * other page requirements.
+ *
+ * If the page does not get brought uptodate, return -EIO.
+ */
+struct page *read_cache_page_gfp(struct address_space *mapping,
+				pgoff_t index,
+				gfp_t gfp)
+{
+	filler_t *filler = (filler_t *)mapping->a_ops->readpage;
+
+	return wait_on_page_read(do_read_cache_page(mapping, index, filler, NULL, gfp));
+}
+EXPORT_SYMBOL(read_cache_page_gfp);
+
+/**
+ * read_cache_page - read into page cache, fill it if needed
+ * @mapping:	the page's address_space
+ * @index:	the page index
+ * @filler:	function to perform the read
+ * @data:	destination for read data
+ *
+ * Read into the page cache. If a page already exists, and PageUptodate() is
+ * not set, try to fill the page then wait for it to become unlocked.
+ *
+ * If the page does not get brought uptodate, return -EIO.
+ */
+struct page *read_cache_page(struct address_space *mapping,
+				pgoff_t index,
+				int (*filler)(void *,struct page*),
+				void *data)
+{
+	return wait_on_page_read(read_cache_page_async(mapping, index, filler, data));
+}
 EXPORT_SYMBOL(read_cache_page);
 
 /*