Re: [PATCH net-next v9 03/13] mm: page_frag: use initial zero offset for page_frag_alloc_align()

On 2024/7/2 7:27, Alexander H Duyck wrote:

On Tue, 2024-06-25 at 21:52 +0800, Yunsheng Lin wrote:

quoted

We are above to use page_frag_alloc_*() API to not just

"about to use", not "above to use"

Ack.

quoted

allocate memory for skb->data, but also use them to do
the memory allocation for skb frag too. Currently the
implementation of page_frag in mm subsystem is running
the offset as a countdown rather than count-up value,
there may have several advantages to that as mentioned
in [1], but it may have some disadvantages, for example,
it may disable skb frag coaleasing and more correct cache
prefetching

...

quoted

diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
index 88f567ef0e29..da244851b8a4 100644
--- a/mm/page_frag_cache.c
+++ b/mm/page_frag_cache.c

@@ -72,10 +72,6 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
 		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.
 		 */

@@ -84,11 +80,16 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
 		/* reset page count bias and offset to start of new frag */
 		nc->pfmemalloc = page_is_pfmemalloc(page);
 		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
-		nc->offset = size;
+		nc->offset = 0;
 	}
 
-	offset = nc->offset - fragsz;
-	if (unlikely(offset < 0)) {
+#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
+	/* if size can vary use size else just use PAGE_SIZE */
+	size = nc->size;
+#endif
+
+	offset = __ALIGN_KERNEL_MASK(nc->offset, ~align_mask);
+	if (unlikely(offset + fragsz > size)) {

The fragsz check below could be moved to here.

quoted

 		page = virt_to_page(nc->va);
 
 		if (!page_ref_sub_and_test(page, nc->pagecnt_bias))

@@ -99,17 +100,13 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
 			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, PAGE_FRAG_CACHE_MAX_SIZE + 1);
 
 		/* reset page count bias and offset to start of new frag */
 		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
-		offset = size - fragsz;
-		if (unlikely(offset < 0)) {
+		offset = 0;
+		if (unlikely(fragsz > PAGE_SIZE)) {

Since we aren't taking advantage of the flag that is left after the
subtraction we might just want to look at moving this piece up to just
after the offset + fragsz check. That should prevent us from trying to
refill if we have a request that is larger than a single page. In
addition we could probably just drop the 3 PAGE_SIZE checks above as
they would be redundant.

I am not sure I understand the 'drop the 3 PAGE_SIZE checks' part and
the 'redundant' part, where is the '3 PAGE_SIZE checks'? And why they
are redundant?

quoted

 			/*
 			 * The caller is trying to allocate a fragment
 			 * with fragsz > PAGE_SIZE but the cache isn't big

@@ -124,8 +121,7 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
 	}
 
 	nc->pagecnt_bias--;
-	offset &= align_mask;
-	nc->offset = offset;
+	nc->offset = offset + fragsz;
 
 	return nc->va + offset;
 }

.