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

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On
> Behalf Of Oscar Salvador
> Sent: Tuesday, November 24, 2020 11:25 PM
> To: Muchun Song <songmuchun@bytedance.com>
> Cc: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de;
> mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com;
> dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org;
> viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org;
> mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com;
> rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com;
> jroedel@suse.de; almasrymina@google.com; rientjes@google.com;
> willy@infradead.org; mhocko@suse.com; Song Bao Hua (Barry Song)
> <song.bao.hua@hisilicon.com>; duanxiongchun@bytedance.com;
> linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> linux-fsdevel@vger.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