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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Wed, 21 Apr 2021 11:41:24 +0800
Message-ID: <CAMZfGtXmDhkCWateAR0q_EgRPDmGh_=D-6UuhMd+Si6=TDvghQ@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v20 8/9] mm: memory_hotplug: disable
 memmap_on_memory when hugetlb_free_vmemmap enabled
To:     Oscar Salvador <osalvador@suse.de>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, bp@alien8.de,
        X86 ML <x86@kernel.org>, hpa@zytor.com,
        dave.hansen@linux.intel.com, luto@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>, paulmck@kernel.org,
        pawan.kumar.gupta@linux.intel.com,
        Randy Dunlap <rdunlap@infradead.org>, oneukum@suse.com,
        anshuman.khandual@arm.com, jroedel@suse.de,
        Mina Almasry <almasrymina@google.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
        David Hildenbrand <david@redhat.com>,
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        <naoya.horiguchi@nec.com>,
        Joao Martins <joao.m.martins@oracle.com>,
        Xiongchun duan <duanxiongchun@bytedance.com>,
        fam.zheng@bytedance.com, linux-doc@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
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On Tue, Apr 20, 2021 at 6:35 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Apr 15, 2021 at 04:40:04PM +0800, Muchun Song wrote:
> >  bool mhp_supports_memmap_on_memory(unsigned long size)
> >  {
> > +     bool supported;
> >       unsigned long nr_vmemmap_pages = size / PAGE_SIZE;
> >       unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page);
> >       unsigned long remaining_size = size - vmemmap_size;
> > @@ -1011,11 +1012,18 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
> >        *       altmap as an alternative source of memory, and we do not exactly
> >        *       populate a single PMD.
> >        */
> > -     return memmap_on_memory &&
> > -            IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
> > -            size == memory_block_size_bytes() &&
> > -            IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
> > -            IS_ALIGNED(remaining_size, pageblock_nr_pages << PAGE_SHIFT);
> > +     supported = memmap_on_memory &&
> > +                 IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
> > +                 size == memory_block_size_bytes() &&
> > +                 IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
> > +                 IS_ALIGNED(remaining_size, pageblock_nr_pages << PAGE_SHIFT);
> > +
> > +     if (supported && is_hugetlb_free_vmemmap_enabled()) {
> > +             pr_info("Cannot enable memory_hotplug.memmap_on_memory, it is not compatible with hugetlb_free_vmemmap\n");
> > +             supported = false;
> > +     }
>
> I would not print anything and rather have
>
> return memmap_on_memory &&
>        !is_hugetlb_free_vmemmap_enabled &&
>        IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
>        size == memory_block_size_bytes() &&
>        IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
>        IS_ALIGNED(remaining_size, pageblock_nr_pages << PAGE_SHIFT);

OK. Will do.

>
> Documentation/admin-guide/kernel-parameters.txt already provides an
> explanation on memory_hotplug.memmap_on_memory parameter that states
> that the feature cannot be enabled when using hugetlb-vmemmap
> optimization.
>
> Users can always check whether the feature is enabled via
> /sys/modules/memory_hotplug/parameters/memmap_on_memory.

If memory_hotplug.memmap_on_memory is enabled

    $ cat /sys/module/memory_hotplug/parameters/memmap_on_memory
    $ Y

If memory_hotplug.memmap_on_memory is disabled

    $ cat /sys/module/memory_hotplug/parameters/memmap_on_memory
    $ N

>
> Also, I did not check if it is, but if not, the fact about hugetlb-vmemmmap vs
> hotplug-vmemmap should also be called out in the hugetlb-vmemmap kernel
> parameter.

Make sense. I will update the doc.

Thanks.

>
> Thanks
>
> --
> Oscar Salvador
> SUSE L3