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Subject: Re: [PATCH] mm/swap: Use page flags to determine LRU list in
 __activate_page()
To: Michal Hocko <mhocko@kernel.org>,
        Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, shli@kernel.org
References: <20171019145657.11199-1-khandual@linux.vnet.ibm.com>
 <20171019153322.c4uqalws7l7fdzcx@dhcp22.suse.cz>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <23110557-b2db-9f4a-d072-ad58fd0c1931@suse.cz>
Date: Tue, 31 Oct 2017 13:45:39 +0100
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On 10/19/2017 05:33 PM, Michal Hocko wrote:
> On Thu 19-10-17 20:26:57, Anshuman Khandual wrote:
>> Its already assumed that the PageActive flag is clear on the input
>> page, hence page_lru(page) will pick the base LRU for the page. In
>> the same way page_lru(page) will pick active base LRU, once the
>> flag PageActive is set on the page. This change of LRU list should
>> happen implicitly through the page flags instead of being hard
>> coded.
> 
> The patch description tells what but it doesn't explain _why_? Does the
> resulting code is better, more optimized or is this a pure readability
> thing?
> 
> All I can see is that page_lru is more complex and a large part of it
> can be optimized away which has been done manually here. I suspect the
> compiler can deduce the same thing.

We shouldn't overestimate the compiler (or the objective conditions it
has) for optimizing stuff away:

After applying the patch:

./scripts/bloat-o-meter swap_before.o mm/swap.o
add/remove: 0/0 grow/shrink: 1/0 up/down: 160/0 (160)
function                                     old     new   delta
__activate_page                              708     868    +160
Total: Before=13538, After=13698, chg +1.18%

I don't think we want that, it's not exactly a cold code...