linux-kernel - Re: [PATCH] mm/swap: Use page flags to determine LRU list in __activate

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Message-Id: <d01827c0-8858-5688-dc16-1e2f597ec55c@linux.vnet.ibm.com>
Date:   Mon, 23 Oct 2017 08:52:34 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, shli@...nel.org
Subject: Re: [PATCH] mm/swap: Use page flags to determine LRU list in
 __activate_page()

On 10/19/2017 09:03 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?

Not really. Not only it removes couple of lines of code but it also
makes it look more logical from function flow point of view as well.

> 
> 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.

Why not ? I mean, that is the essence of the function page_lru() which
should get us the exact LRU list the page should be on and hence we
should not hand craft these manually.