[PATCH] mm/page_alloc: fix the potential memory waste in page_frag_alloc_align

[wang wei]

First step, allocating a memory fragment with size 1KB bytes uses
page_frag_alloc_align.  It will allocate PAGE_FRAG_CACHE_MAX_SIZE
bytes by __page_frag_cache_refill, store the pointer at nc->va and
then return the last 1KB memory fragment which address is nc->va
+ PAGE_FRAG_CACHE_MAX_SIZE - 1KB. The remaining PAGE_FRAG_CACHE_MAX_SIZE
- 1KB bytes of memory can Meet future memory requests.

Second step, if the caller requests a memory fragment with size
more then PAGE_FRAG_CACHE_MAX_SIZE bytes,  page_frag_alloc_align,
it will also allocate PAGE_FRAG_CACHE_MAX_SIZE bytes by
__page_frag_cache_refill,  store the pointer at nc->va, and
return NULL.  this behavior makes the rest of
PAGE_FRAG_CACHE_MAX_SIZE - 1KB bytes memory at First step are
wasted(allocate from buddy system but not used).

So we should check the size of memory requests. If it beyound
PAGE_FRAG_CACHE_MAX_SIZE, we should return NULL directly.

Signed-off-by: wang wei <a929244872@xxxxxxx>
---
 mm/page_alloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8cf86d0c6aa8..3182c648e86e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4757,6 +4757,9 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
 	struct page *page;
 	int offset;
 
+	if（unlikely(fragsz > PAGE_FRAG_CACHE_MAX_SIZE)）
+		return NULL;
+
 	if (unlikely(!nc->va)) {
 refill:
 		page = __page_frag_cache_refill(nc, gfp_mask);
-- 
2.25.1