linux-kernel - Re: [PATCH v2 RESEND update 1/2] mm/page_alloc: free order-0 pages through PCP in page_frag

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Date:   Wed, 21 Nov 2018 19:15:11 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org,
        Paweł Staszewski <pstaszewski@...are.pl>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Yoel Caspersen <yoel@...knet.dk>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Ian Kumlien <ian.kumlien@...il.com>
Subject: Re: [PATCH v2 RESEND update 1/2] mm/page_alloc: free order-0 pages
 through PCP in page_frag_free()

On Tue, 20 Nov 2018 09:45:44 +0800 Aaron Lu <aaron.lu@...el.com> wrote:

> page_frag_free() calls __free_pages_ok() to free the page back to
> Buddy. This is OK for high order page, but for order-0 pages, it
> misses the optimization opportunity of using Per-Cpu-Pages and can
> cause zone lock contention when called frequently.
> 

Looks nice to me.  Let's tell our readers why we're doing this.

--- a/mm/page_alloc.c~mm-page_alloc-free-order-0-pages-through-pcp-in-page_frag_free-fix
+++ a/mm/page_alloc.c
@@ -4684,7 +4684,7 @@ void page_frag_free(void *addr)
 	if (unlikely(put_page_testzero(page))) {
 		unsigned int order = compound_order(page);
 
-		if (order == 0)
+		if (order == 0)		/* Via pcp? */
 			free_unref_page(page);
 		else
 			__free_pages_ok(page, order);
_