btrfs: pass __GFP_WRITE for buffered write page allocations

Tell the page allocator that pages allocated for a buffered write are
expected to become dirty soon.

Signed-off-by: Johannes Weiner <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Wu Fengguang <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Chris Mason <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/btrfs/file.c

@@ -1081,7 +1081,7 @@ static noinline int prepare_pages(struct btrfs_root *root, struct file *file,
 again:
 	for (i = 0; i < num_pages; i++) {
 		pages[i] = find_or_create_page(inode->i_mapping, index + i,
-					       mask);
+					       mask | __GFP_WRITE);
 		if (!pages[i]) {
 			faili = i - 1;
 			err = -ENOMEM;