linux-kernel - Re: [External] Re: [PATCH v2 04/19] mm/hugetlb: Introduce nr_free_vmemmap

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Message-ID: <CAMZfGtWr94M0_MgBYzMX=OmHx+1s9VWCGLGzVMU38Cp-bmBk5g@mail.gmail.com>
Date:   Thu, 29 Oct 2020 21:41:46 +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>,
        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 v2 04/19] mm/hugetlb: Introduce
 nr_free_vmemmap_pages in the struct hstate

On Thu, Oct 29, 2020 at 9:26 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Mon, Oct 26, 2020 at 10:50:59PM +0800, Muchun Song wrote:
> > If the size of hugetlb page is 2MB, we need 512 struct page structures
> > (8 pages) to be associated with it. As far as I know, we only use the
> > first 4 struct page structures.
>
> As Mike pointed out, better describe what those "4" mean.

Yeah, thanks.

>
> > For tail pages, the value of compound_dtor is the same. So we can reuse
>
> I might be missing something, but HUGETLB_PAGE_DTOR is only set on the
> first tail, right?

Sorry for confusion. Here I mean the `compound_head` is the same.

>
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +#define RESERVE_VMEMMAP_NR   2U
>
> Although you can get that from the changelog, maybe a brief comment explaining
> why RESERVE_VMEMMAP_NR == 2.
> > +
> > +static inline unsigned int nr_free_vmemmap(struct hstate *h)
> > +{
> > +     return h->nr_free_vmemmap_pages;
> > +}
>
> Better add this in the patch that is used?

OK, I will do it. thanks.

>
> > +     if (vmemmap_pages > RESERVE_VMEMMAP_NR)
> > +             h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
> > +     else
> > +             h->nr_free_vmemmap_pages = 0;
>
> Can we really have an scenario where we end up with vmemmap_pages < RESERVE_VMEMMAP_NR?

I think that this is impossible. On the safe side, I do this comparison.
Do you think we should remove this comparison? Is that right?

>
> > +
> > +     pr_info("HugeTLB: can free %d vmemmap pages for %s\n",
> > +             h->nr_free_vmemmap_pages, h->name);
>
> I do not think this is useful unless debugging situations, so I would either
> scratch that or make it pr_debug.

Thanks for your suggestions.

>
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun