linux-kernel - Re: [External] Re: [PATCH v19 7/8] mm: hugetlb: add a kernel parameter hugetlb_free

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Message-ID: <CAMZfGtXAgcJQp59AVuieqLT+1Qb3RGQmFK-SGNZH-T6K83Y=HQ@mail.gmail.com>
Date:   Fri, 19 Mar 2021 20:15:58 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.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 ML <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>,
        Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.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 v19 7/8] mm: hugetlb: add a kernel
 parameter hugetlb_free_vmemmap

On Fri, Mar 19, 2021 at 4:59 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Mon, Mar 15, 2021 at 05:20:14PM +0800, Muchun Song wrote:
> > --- a/arch/x86/mm/init_64.c
> > +++ b/arch/x86/mm/init_64.c
> > @@ -34,6 +34,7 @@
> >  #include <linux/gfp.h>
> >  #include <linux/kcore.h>
> >  #include <linux/bootmem_info.h>
> > +#include <linux/hugetlb.h>
> >
> >  #include <asm/processor.h>
> >  #include <asm/bios_ebda.h>
> > @@ -1557,7 +1558,8 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> >  {
> >       int err;
> >
> > -     if (end - start < PAGES_PER_SECTION * sizeof(struct page))
> > +     if ((is_hugetlb_free_vmemmap_enabled()  && !altmap) ||
> > +         end - start < PAGES_PER_SECTION * sizeof(struct page))
> >               err = vmemmap_populate_basepages(start, end, node, NULL);
> >       else if (boot_cpu_has(X86_FEATURE_PSE))
> >               err = vmemmap_populate_hugepages(start, end, node, altmap);
>
> I've been thinking about this some more.
>
> Assume you opt-in the hugetlb-vmemmap feature, and assume you pass a valid altmap
> to vmemmap_populate.
> This will lead to use populating the vmemmap array with hugepages.

Right.

>
> What if then, a HugeTLB gets allocated and falls within that memory range (backed
> by hugetpages)?

I am not sure whether we can allocate the HugeTLB pages from there.
Will only device memory pass a valid altmap parameter to
vmemmap_populate()? If yes, can we allocate HugeTLB pages from
device memory? Sorry, I am not an expert on this.


> AFAIK, this will get us in trouble as currently the code can only operate on memory
> backed by PAGE_SIZE pages, right?
>
> I cannot remember, but I do not think nothing prevents that from happening?
> Am I missing anything?

Maybe David H is more familiar with this.

Hi David,

Do you have some suggestions on this?

Thanks.


>
> --
> Oscar Salvador
> SUSE L3