linux-kernel - Re: [External] Re: [PATCH v3 04/21] mm/hugetlb: Introduce nr_free_vmemmap

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Date:   Tue, 10 Nov 2020 10:42:00 +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>,
        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 v3 04/21] mm/hugetlb: Introduce
 nr_free_vmemmap_pages in the struct hstate

On Tue, Nov 10, 2020 at 12:48 AM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Sun, Nov 08, 2020 at 10:10:56PM +0800, Muchun Song wrote:
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +/*
> > + * There are 512 struct page structs(8 pages) associated with each 2MB
> > + * hugetlb page. For tail pages, the value of compound_dtor is the same.
> I gess you meant "For tail pages, the value of compound_head ...", right?

Yeah, Thanks.

>
> > + * So we can reuse first page of tail page structs. We map the virtual
> > + * addresses of the remaining 6 pages of tail page structs to the first
> > + * tail page struct, and then free these 6 pages. Therefore, we need to
> > + * reserve at least 2 pages as vmemmap areas.
> > + */
> > +#define RESERVE_VMEMMAP_NR   2U
> > +
> > +static void __init hugetlb_vmemmap_init(struct hstate *h)
> > +{
> > +     unsigned int order = huge_page_order(h);
> > +     unsigned int vmemmap_pages;
> > +
> > +     vmemmap_pages = ((1 << order) * sizeof(struct page)) >> PAGE_SHIFT;
> > +     /*
> > +      * The head page and the first tail page not free to buddy system,
>
> "The head page and the first tail page are not to be freed to..." better?

Yeah, sorry for my poor English :).

>
>
> > +      * the others page will map to the first tail page. So there are
> > +      * (@vmemmap_pages - RESERVE_VMEMMAP_NR) pages can be freed.
>                                                       ^^^
>                                                       that
>
> > +     else
> > +             h->nr_free_vmemmap_pages = 0;
>
> I would specify that this is not expected to happen.
> (At least I could not come up with a real scenario unless the system is
> corrupted)
> So, I would drop a brief comment pointing out that it is only a safety
> net.

I will add a comment to point out this.

>
>
> Unrelated to this patch but related in general, I am not sure about Mike but
> would it be cleaner to move all the vmemmap functions to hugetlb_vmemmap.c?
> hugetlb code is quite tricky, so I am not sure about stuffing more code
> in there.
>

I also think that you are right, moving all the vmemmap functions to
hugetlb_vmemmap.c may make the code cleaner.

Hi Mike, what's your opinion?

Thanks.

> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun