linux-kernel - Re: [External] Re: [PATCH v2 05/19] mm/hugetlb: Introduce pgtable allocation/freeing helpers

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Date:   Sat, 7 Nov 2020 00:43:12 +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 05/19] mm/hugetlb: Introduce pgtable
 allocation/freeing helpers

On Fri, Nov 6, 2020 at 5:46 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Fri, Nov 06, 2020 at 12:08:22AM +0800, Muchun Song wrote:
> > > I do not think you need this.
> > > We already have hugepages_supported().
> >
> > Maybe some architectures support hugepage, but the vmemmap do not
> > use the hugepage map. In  this case, we need it. But I am not sure if it
> > exists in the real world. At least, x86 can reuse hugepages_supported.
>
> Yes, but that is the point.
> IIUC, this patchset will enable HugeTLB vmemmap pages only for x86_64.
> Then, let us make the patchset specific to that architecture.
>
> If at some point this grows more users (powerpc, arm, ...), then we
> can add the missing code, but for now it makes sense to only include
> the bits to make this work on x86_64.
>
> And also according to this the changelog is a bit "misleading".
>
> "On some architectures, the vmemmap areas use huge page mapping.
> If we want to free the unused vmemmap pages, we have to split
> the huge pmd firstly. So we should pre-allocate pgtable to split
> huge pmd."

Thanks for your suggestions. I will rewrite the commit log.

>
> On x86_64, vmemmap is always PMD mapped if the machine has hugepages
> support and if we have 2MB contiguos pages and PMD aligned.
> e.g: I have seen cases where after the system has ran for a period
> of time hotplug operations were mapping the vmemmap representing
> the hot-added range on page base, because we could not find
> enough contiguos and aligned memory.
>
> Something that [1] tries to solve:
>
> [1] https://patchwork.kernel.org/project/linux-mm/cover/20201022125835.26396-1-osalvador@suse.de/
>
> But anyway, my point is that let us make it clear in the changelog that
> this is aimed for x86_64 at the moment.
> Saying "on some architures" might make think people that this is not
> x86_64 specific.
>
> >> > > +     vmemmap_pgtable_init(page);
> > >
> > > Maybe just open code this one?
> >
> > Sorry. I don't quite understand what it means. Could you explain?
>
> I meant doing
>
> page_huge_pte(page) = NULL
>
> But no strong feelings.
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun