[net PATCH 1/2] mm: Use fixed constant in page_frag_alloc instead of size + 1

From: Alexander Duyck <alexander.duyck@gmail.com>
To: netdev@vger.kernel.org, davem@davemloft.net
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, jannh@google.com
Subject: [net PATCH 1/2] mm: Use fixed constant in page_frag_alloc instead of size + 1
Date: Fri, 15 Feb 2019 14:44:12 -0800	[thread overview]
Message-ID: <20190215224412.16881.89296.stgit@localhost.localdomain> (raw)
In-Reply-To: <20190215223741.16881.84864.stgit@localhost.localdomain>

From: Alexander Duyck <alexander.h.duyck@linux.intel.com>

This patch replaces the size + 1 value introduced with the recent fix for 1
byte allocs with a constant value.

The idea here is to reduce code overhead as the previous logic would have
to read size into a register, then increment it, and write it back to
whatever field was being used. By using a constant we can avoid those
memory reads and arithmetic operations in favor of just encoding the
maximum value into the operation itself.

Fixes: 2c2ade81741c ("mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs")
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
---
 mm/page_alloc.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ebb35e4d0d90..37ed14ad0b59 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4857,11 +4857,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
 		/* 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);
+		page_ref_add(page, PAGE_FRAG_CACHE_MAX_SIZE);
 
 		/* reset page count bias and offset to start of new frag */
 		nc->pfmemalloc = page_is_pfmemalloc(page);
-		nc->pagecnt_bias = size + 1;
+		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
 		nc->offset = size;
 	}
 
@@ -4877,10 +4877,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
 		size = nc->size;
 #endif
 		/* OK, page count is 0, we can safely set it */
-		set_page_count(page, size + 1);
+		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
 
 		/* reset page count bias and offset to start of new frag */
-		nc->pagecnt_bias = size + 1;
+		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
 		offset = size - fragsz;
 	}
 

