[PATCH v2 6/8] page_frag_cache: Use a mask instead of offset

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>

By combining 'va' and 'offset' into 'addr' and using a mask instead,
we can save a compare-and-branch in the fast-path of the allocator.
This removes 4 instructions on x86 (both 32 and 64 bit).

We can avoid storing the mask at all if we know that we're only allocating
a single page.  This shrinks page_frag_cache from 12 to 8 bytes on 32-bit
CONFIG_BASE_SMALL build.

Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
---
 include/linux/mm_types.h | 12 +++++++-----
 mm/page_alloc.c          | 40 +++++++++++++++-------------------------
 2 files changed, 22 insertions(+), 30 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0defff9e3c0e..ebe93edec752 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -225,12 +225,9 @@ struct page {
 #define PFC_MEMALLOC			(1U << 31)
 
 struct page_frag_cache {
-	void * va;
+	void *addr;
 #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
-	__u16 offset;
-	__u16 size;
-#else
-	__u32 offset;
+	unsigned int mask;
 #endif
 	/* we maintain a pagecount bias, so that we dont dirty cache line
 	 * containing page->_refcount every time we allocate a fragment.
@@ -239,6 +236,11 @@ struct page_frag_cache {
 };
 
 #define page_frag_cache_pfmemalloc(pfc)	((pfc)->pagecnt_bias & PFC_MEMALLOC)
+#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
+#define page_frag_cache_mask(pfc)	(pfc)->mask
+#else
+#define page_frag_cache_mask(pfc)	(~PAGE_MASK)
+#endif
 
 typedef unsigned long vm_flags_t;
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5a2e3e293079..d15a5348a8e4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4336,22 +4336,19 @@ EXPORT_SYMBOL(free_pages);
  * drivers to provide a backing region of memory for use as either an
  * sk_buff->head, or to be used in the "frags" portion of skb_shared_info.
  */
-static struct page *__page_frag_cache_refill(struct page_frag_cache *pfc,
+static void *__page_frag_cache_refill(struct page_frag_cache *pfc,
 					     gfp_t gfp_mask)
 {
 	unsigned int size = PAGE_SIZE;
 	struct page *page = NULL;
-	struct page *old = pfc->va ? virt_to_page(pfc->va) : NULL;
+	struct page *old = pfc->addr ? virt_to_head_page(pfc->addr) : NULL;
 	gfp_t gfp = gfp_mask;
 	unsigned int pagecnt_bias = pfc->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 = pfc->size;
-#endif
+		size = page_frag_cache_mask(pfc) + 1;
 		/* Page count is 0, we can safely set it */
 		set_page_count(page, size);
 		goto reset;
@@ -4364,27 +4361,24 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *pfc,
 				PAGE_FRAG_CACHE_MAX_ORDER);
 	if (page)
 		size = PAGE_FRAG_CACHE_MAX_SIZE;
-	pfc->size = size;
+	pfc->mask = size - 1;
 #endif
 	if (unlikely(!page))
 		page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
 	if (!page) {
-		pfc->va = NULL;
+		pfc->addr = NULL;
 		return NULL;
 	}
 
-	pfc->va = page_address(page);
-
 	/* 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 */
 	pfc->pagecnt_bias = size;
 	if (page_is_pfmemalloc(page))
 		pfc->pagecnt_bias |= PFC_MEMALLOC;
-	pfc->offset = size;
+	pfc->addr = page_address(page) + size;
 
-	return page;
+	return pfc->addr;
 }
 
 void __page_frag_cache_drain(struct page *page, unsigned int count)
@@ -4405,24 +4399,20 @@ EXPORT_SYMBOL(__page_frag_cache_drain);
 void *page_frag_alloc(struct page_frag_cache *pfc,
 		      unsigned int size, gfp_t gfp_mask)
 {
-	struct page *page;
-	int offset;
+	void *addr = pfc->addr;
+	unsigned int offset = (unsigned long)addr & page_frag_cache_mask(pfc);
 
-	if (unlikely(!pfc->va)) {
-refill:
-		page = __page_frag_cache_refill(pfc, gfp_mask);
-		if (!page)
+	if (unlikely(offset < size)) {
+		addr = __page_frag_cache_refill(pfc, gfp_mask);
+		if (!addr)
 			return NULL;
 	}
 
-	offset = pfc->offset - size;
-	if (unlikely(offset < 0))
-		goto refill;
-
+	addr -= size;
+	pfc->addr = addr;
 	pfc->pagecnt_bias--;
-	pfc->offset = offset;
 
-	return pfc->va + offset;
+	return addr;
 }
 EXPORT_SYMBOL(page_frag_alloc);
 
-- 
2.16.2