linux-kernel - RE: [PATCH v6 07/16] x86/mm/64: Disable PMD page mapping of vmemmap

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Message-ID: <c938bb225ea84c06844b31023dad96c1@hisilicon.com>
Date:   Tue, 24 Nov 2020 10:31:12 +0000
From:   "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Muchun Song <songmuchun@...edance.com>
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Subject: RE: [PATCH v6 07/16] x86/mm/64: Disable PMD page mapping of vmemmap



> -----Original Message-----
> From: owner-linux-mm@...ck.org [mailto:owner-linux-mm@...ck.org] On
> Behalf Of Oscar Salvador
> Sent: Tuesday, November 24, 2020 11:25 PM
> To: Muchun Song <songmuchun@...edance.com>
> Cc: corbet@....net; mike.kravetz@...cle.com; tglx@...utronix.de;
> mingo@...hat.com; bp@...en8.de; x86@...nel.org; hpa@...or.com;
> dave.hansen@...ux.intel.com; luto@...nel.org; peterz@...radead.org;
> viro@...iv.linux.org.uk; akpm@...ux-foundation.org; paulmck@...nel.org;
> mchehab+huawei@...nel.org; pawan.kumar.gupta@...ux.intel.com;
> rdunlap@...radead.org; oneukum@...e.com; anshuman.khandual@....com;
> jroedel@...e.de; almasrymina@...gle.com; rientjes@...gle.com;
> willy@...radead.org; mhocko@...e.com; Song Bao Hua (Barry Song)
> <song.bao.hua@...ilicon.com>; duanxiongchun@...edance.com;
> linux-doc@...r.kernel.org; linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> linux-fsdevel@...r.kernel.org
> Subject: Re: [PATCH v6 07/16] x86/mm/64: Disable PMD page mapping of
> vmemmap
> 
> On Tue, Nov 24, 2020 at 05:52:50PM +0800, Muchun Song wrote:
> > If we enable the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, we can just
> > disbale PMD page mapping of vmemmap to simplify the code. In this
> > case, we do not need complex code doing vmemmap page table
> > manipulation. This is a way to simply the first version of this
> > patch series. In the future, we can add some code doing page table
> > manipulation.
> 
> IIRC, CONFIG_HUGETLB_PAGE_FREE_VMEMMAP was supposed to be enabled
> by default,
> right?
> And we would control whether we __really__ want to this by a boot option,
> which was disabled by default?
> 
> If you go for populating the memmap with basepages by checking
> CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, would not everybody, even the
> ones that
> did not enable this by the boot option be affected?
> 

I would believe we could only bypass the pmd mapping of vmemmap while
free_vmemmap is explicitly enabled.
pmd mapping shouldn't be disabled in default. Would a cmdline of enabling
vmemmap_free be used for the first patchset?

> --
> Oscar Salvador
> SUSE L3

Thanks
Barry