[PATCH] mm: shmem: use proper gfp flags for shmem_writepage()

[Yang Shi]

The shmem_writepage() uses GFP_ATOMIC to allocate swap cache.
GFP_ATOMIC used to mean __GFP_HIGH, but now it means __GFP_HIGH |
__GFP_ATOMIC | __GFP_KSWAPD_RECLAIM.  However, shmem_writepage() should
write out to swap only in response to memory pressure, so
__GFP_KSWAPD_RECLAIM looks useless since the caller may be kswapd itself
or in direct reclaim already.

In addition, XArray node allocations from PF_MEMALLOC contexts could
completely exhaust the page allocator, __GFP_NOMEMALLOC stops emergency
reserves from being allocated.

Here just copy the gfp flags used by add_to_swap().

Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
 mm/shmem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 220be9f..9691dec 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1369,7 +1369,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	if (list_empty(&info->swaplist))
 		list_add(&info->swaplist, &shmem_swaplist);
 
-	if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) {
+	if (add_to_swap_cache(page, swap,
+			__GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN) == 0) {
 		spin_lock_irq(&info->lock);
 		shmem_recalc_inode(inode);
 		info->swapped++;
-- 
1.8.3.1