linux-kernel - Re: [External] Re: [PATCH v20 8/9] mm: memory_hotplug: disable memmap_on_memory when hugetlb_free

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Date:   Wed, 21 Apr 2021 09:33:52 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.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,
        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>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        fam.zheng@...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 v20 8/9] mm: memory_hotplug: disable
 memmap_on_memory when hugetlb_free_vmemmap enabled

On Wed, Apr 21, 2021 at 11:41:24AM +0800, Muchun Song wrote:
> > 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.

Heh, I realized this is not completely true.
Users can check whether the feature is __enabled__ by checking the sys fs,
but although it is enabled, it might not be effective.

This might be due to a different number of reasons, vmemmap does not fully
span a PMD, the size we want to add spans more than a single memory block, etc.

That is what

"Note that even when enabled, there are a few cases where the feature is not
 effective."

is supposed to mean.

Anyway, I did not change my opionion on this.

Thanks

-- 
Oscar Salvador
SUSE L3