linux-kernel - Re: [External] Re: [PATCH v18 9/9] mm: hugetlb: optimize the code with the help of the compiler

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Message-ID: <20210311134531.GA24797@linux>
Date:   Thu, 11 Mar 2021 14:45:35 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [External] Re: [PATCH v18 9/9] mm: hugetlb: optimize the code
 with the help of the compiler

On Thu, Mar 11, 2021 at 01:16:37PM +0100, Michal Hocko wrote:
> On Thu 11-03-21 18:00:09, Muchun Song wrote:
> [...]
> > Sorry. I am confused why you disagree with this change.
> > It does not bring any disadvantages.
> 
> Because it is adding a code which is not really necessary and which will
> have to be maintained. Think of future changes which would need to grow
> more of these. Hugetlb code paths shouldn't really think about size of
> the struct page.

I have to confess that when I looked at the patch I found it nice in the way that
wipes out almost all clode dealing with vmemmap when sizeof(struct page) != power_of_2,
and I was convinced by the fact that only two places required the change.
So all in all it did not look like much churn, and not __that__ hard to maintain.

But I did not think in the case where this trick needs to be spread in more places
if the code changes over time.

So I agree that although it gets rid of a lot of code, it would seldomly pay off as
not many configuration out there are running on !power_of_2, and hugetlb is already
tricky enough.


-- 
Oscar Salvador
SUSE L3