netdev - [PATCH net-next 1/4] mm: page_frag: Introduce page_frag_alloc

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Date:   Sat, 23 Jan 2021 19:59:00 +0800
From:   Kevin Hao <haokexin@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     netdev@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH net-next 1/4] mm: page_frag: Introduce page_frag_alloc_align()

In the current implementation of page_frag_alloc(), it doesn't have
any align guarantee for the returned buffer address. But for some
hardwares they do require the DMA buffer to be aligned correctly,
so we would have to use some workarounds like below if the buffers
allocated by the page_frag_alloc() are used by these hardwares for
DMA.
    buf = page_frag_alloc(really_needed_size + align);
    buf = PTR_ALIGN(buf, align);

These codes seems ugly and would waste a lot of memories if the buffers
are used in a network driver for the TX/RX. So introduce
page_frag_alloc_align() to make sure that an aligned buffer address is
returned.

Signed-off-by: Kevin Hao <haokexin@...il.com>
---
 include/linux/gfp.h |  3 +++
 mm/page_alloc.c     | 12 ++++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 6e479e9c48ce..e76e8618e9d7 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -583,6 +583,9 @@ extern void free_pages(unsigned long addr, unsigned int order);
 
 struct page_frag_cache;
 extern void __page_frag_cache_drain(struct page *page, unsigned int count);
+extern void *page_frag_alloc_align(struct page_frag_cache *nc,
+				   unsigned int fragsz, gfp_t gfp_mask,
+				   int align);
 extern void *page_frag_alloc(struct page_frag_cache *nc,
 			     unsigned int fragsz, gfp_t gfp_mask);
 extern void page_frag_free(void *addr);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 027f6481ba59..80f7c5f7d738 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5135,8 +5135,8 @@ void __page_frag_cache_drain(struct page *page, unsigned int count)
 }
 EXPORT_SYMBOL(__page_frag_cache_drain);
 
-void *page_frag_alloc(struct page_frag_cache *nc,
-		      unsigned int fragsz, gfp_t gfp_mask)
+void *page_frag_alloc_align(struct page_frag_cache *nc,
+		      unsigned int fragsz, gfp_t gfp_mask, int align)
 {
 	unsigned int size = PAGE_SIZE;
 	struct page *page;
@@ -5188,10 +5188,18 @@ void *page_frag_alloc(struct page_frag_cache *nc,
 	}
 
 	nc->pagecnt_bias--;
+	offset = align ? ALIGN_DOWN(offset, align) : offset;
 	nc->offset = offset;
 
 	return nc->va + offset;
 }
+EXPORT_SYMBOL(page_frag_alloc_align);
+
+void *page_frag_alloc(struct page_frag_cache *nc,
+		      unsigned int fragsz, gfp_t gfp_mask)
+{
+	return page_frag_alloc_align(nc, fragsz, gfp_mask, 0);
+}
 EXPORT_SYMBOL(page_frag_alloc);
 
 /*
-- 
2.29.2