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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YEnbBPviwU6N2RzK@dhcp22.suse.cz>
Date:   Thu, 11 Mar 2021 09:55:32 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     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>,
        Oscar Salvador <osalvador@...e.de>,
        "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 11-03-21 15:33:20, Muchun Song wrote:
> On Wed, Mar 10, 2021 at 11:41 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Mon 08-03-21 18:28:07, Muchun Song wrote:
> > > When the "struct page size" crosses page boundaries we cannot
> > > make use of this feature. Let free_vmemmap_pages_per_hpage()
> > > return zero if that is the case, most of the functions can be
> > > optimized away.
> >
> > I am confused. Don't you check for this in early_hugetlb_free_vmemmap_param already?
> 
> Right.
> 
> > Why do we need any runtime checks?
> 
> If the size of the struct page is not power of 2, compiler can think
> is_hugetlb_free_vmemmap_enabled() always return false. So
> the code snippet of this user can be optimized away.
> 
> E.g.
> 
> if (is_hugetlb_free_vmemmap_enabled())
>         /* do something */
> 
> The compiler can drop "/* do something */" directly, because
> it knows is_hugetlb_free_vmemmap_enabled() always returns
> false.

OK, so this is a micro-optimization to generate a better code?
Is this measurable to warrant more code?
-- 
Michal Hocko
SUSE Labs