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Date: Wed, 21 Nov 2018 19:15:11 -0800
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2 RESEND update 1/2] mm/page_alloc: free order-0 pages
 through PCP in page_frag_free()
Message-Id: <20181121191511.658e0d41504e146edd88af53@linux-foundation.org>
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References: <20181119134834.17765-1-aaron.lu@intel.com>
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	<20181120014544.GB10657@intel.com>
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To: Aaron Lu <aaron.lu@intel.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, =?UTF-8?Q?Pawe=C5=82?= Staszewski <pstaszewski@itcare.pl>, Jesper Dangaard Brouer <brouer@redhat.com>, Eric Dumazet <eric.dumazet@gmail.com>, Tariq Toukan <tariqt@mellanox.com>, Ilias Apalodimas <ilias.apalodimas@linaro.org>, Yoel Caspersen <yoel@kviknet.dk>, Mel Gorman <mgorman@techsingularity.net>, Saeed Mahameed <saeedm@mellanox.com>, Michal Hocko <mhocko@suse.com>, Vlastimil Babka <vbabka@suse.cz>, Dave Hansen <dave.hansen@linux.intel.com>, Alexander Duyck <alexander.h.duyck@linux.intel.com>, Ian Kumlien <ian.kumlien@gmail.com>

On Tue, 20 Nov 2018 09:45:44 +0800 Aaron Lu <aaron.lu@intel.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);
_