linux-kernel - Re: [External] Re: [PATCH v8 10/12] mm/hugetlb: Introduce nr_free_vmemmap

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Date:   Thu, 10 Dec 2020 20:32:06 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>, 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>, 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>,
        Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.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>
Subject: Re: [External] Re: [PATCH v8 10/12] mm/hugetlb: Introduce
 nr_free_vmemmap_pages in the struct hstate

On Thu, Dec 10, 2020 at 7:40 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Thu, Dec 10, 2020 at 11:55:24AM +0800, Muchun Song wrote:
> > +void __init hugetlb_vmemmap_init(struct hstate *h)
> > +{
> > +     unsigned int nr_pages = pages_per_huge_page(h);
> > +     unsigned int vmemmap_pages;
> > +
> > +     /* We cannot optimize if a "struct page" crosses page boundaries. */
> > +     if (!is_power_of_2(sizeof(struct page)))
> > +             return;
> > +
> > +     if (!hugetlb_free_vmemmap_enabled)
> > +             return;
>
> I think it would make sense to squash the last patch and this one.
> As per the last patch, if "struct page" is not power of 2,
> early_hugetlb_free_vmemmap_param() does not set
> hugetlb_free_vmemmap_enabled, so the "!is_power_of_2" check from above
> would become useless here.
> We know that in order for hugetlb_free_vmemmap_enabled to become true,
> the is_power_of_2 must have succeed early on when calling the early_
> function.

Yeah, you are right. But if is_power_of_2 returns false. The compiler
can optimize this function to null. If we remove the check, it prevents
the compiler from optimizing the code of hugetlb_vmemmap_init().
So I think leaving it here makes sense. Right?

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun