[PATCH v2 2/8] page_frag_cache: Move slowpath code from page_frag_alloc

[Matthew Wilcox <willy@infradead.org>]

From: Matthew Wilcox <mawilcox@microsoft.com>

Put all the unlikely code in __page_frag_cache_refill to make the
fastpath code more obvious.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
---
 mm/page_alloc.c | 70 ++++++++++++++++++++++++++++-----------------------------
 1 file changed, 34 insertions(+), 36 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 61366f23e8c8..6d2c106f4e5d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4339,20 +4339,50 @@ EXPORT_SYMBOL(free_pages);
 static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
 					     gfp_t gfp_mask)
 {
+	unsigned int size = PAGE_SIZE;
 	struct page *page = NULL;
+	struct page *old = nc->va ? virt_to_page(nc->va) : NULL;
 	gfp_t gfp = gfp_mask;
+	unsigned int pagecnt_bias = nc->pagecnt_bias & ~PFC_MEMALLOC;
+
+	/* If all allocations have been freed, we can reuse this page */
+	if (old && page_ref_sub_and_test(old, pagecnt_bias)) {
+		page = old;
+#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
+		/* if size can vary use size else just use PAGE_SIZE */
+		size = nc->size;
+#endif
+		/* Page count is 0, we can safely set it */
+		set_page_count(page, size);
+		goto reset;
+	}
 
 #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
 	gfp_mask |= __GFP_COMP | __GFP_NOWARN | __GFP_NORETRY |
 		    __GFP_NOMEMALLOC;
 	page = alloc_pages_node(NUMA_NO_NODE, gfp_mask,
 				PAGE_FRAG_CACHE_MAX_ORDER);
-	nc->size = page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE;
+	if (page)
+		size = PAGE_FRAG_CACHE_MAX_SIZE;
+	nc->size = size;
 #endif
 	if (unlikely(!page))
 		page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
+	if (!page) {
+		nc->va = NULL;
+		return NULL;
+	}
+
+	nc->va = page_address(page);
 
-	nc->va = page ? page_address(page) : NULL;
+	/* Using atomic_set() would break get_page_unless_zero() users. */
+	page_ref_add(page, size - 1);
+reset:
+	/* reset page count bias and offset to start of new frag */
+	nc->pagecnt_bias = size;
+	if (page_is_pfmemalloc(page))
+		nc->pagecnt_bias |= PFC_MEMALLOC;
+	nc->offset = size;
 
 	return page;
 }
@@ -4375,7 +4405,6 @@ EXPORT_SYMBOL(__page_frag_cache_drain);
 void *page_frag_alloc(struct page_frag_cache *nc,
 		      unsigned int fragsz, gfp_t gfp_mask)
 {
-	unsigned int size = PAGE_SIZE;
 	struct page *page;
 	int offset;
 
@@ -4384,42 +4413,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
 		page = __page_frag_cache_refill(nc, gfp_mask);
 		if (!page)
 			return NULL;
-
-#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
-		/* if size can vary use size else just use PAGE_SIZE */
-		size = nc->size;
-#endif
-		/* Even if we own the page, we do not use atomic_set().
-		 * This would break get_page_unless_zero() users.
-		 */
-		page_ref_add(page, size - 1);
-
-		/* reset page count bias and offset to start of new frag */
-		nc->pagecnt_bias = size;
-		if (page_is_pfmemalloc(page))
-			nc->pagecnt_bias |= PFC_MEMALLOC;
-		nc->offset = size;
 	}
 
 	offset = nc->offset - fragsz;
-	if (unlikely(offset < 0)) {
-		unsigned int pagecnt_bias = nc->pagecnt_bias & ~PFC_MEMALLOC;
-		page = virt_to_page(nc->va);
-
-		if (!page_ref_sub_and_test(page, pagecnt_bias))
-			goto refill;
-
-#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
-		/* if size can vary use size else just use PAGE_SIZE */
-		size = nc->size;
-#endif
-		/* OK, page count is 0, we can safely set it */
-		set_page_count(page, size);
-
-		/* reset page count bias and offset to start of new frag */
-		nc->pagecnt_bias = size | (nc->pagecnt_bias - pagecnt_bias);
-		offset = size - fragsz;
-	}
+	if (unlikely(offset < 0))
+		goto refill;
 
 	nc->pagecnt_bias--;
 	nc->offset = offset;
-- 
2.16.2