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Date:   Fri, 7 Feb 2020 12:14:53 +0000
From:   Wei Yang <richard.weiyang@gmail.com>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Baoquan He <bhe@redhat.com>,
        Wei Yang <richardw.yang@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH 2/3] mm/sparsemem: get physical address to page struct
 instead of virtual address to pfn
Message-ID: <20200207121453.pgi4axyvx6peqgeo@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
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On Thu, Feb 06, 2020 at 07:21:49PM -0800, Dan Williams wrote:
>On Thu, Feb 6, 2020 at 7:10 PM Baoquan He <bhe@redhat.com> wrote:
>>
>> Hi Dan,
>>
>> On 02/06/20 at 06:19pm, Dan Williams wrote:
>> > On Thu, Feb 6, 2020 at 3:17 PM Wei Yang <richardw.yang@linux.intel.com> wrote:
>> > > diff --git a/mm/sparse.c b/mm/sparse.c
>> > > index b5da121bdd6e..56816f653588 100644
>> > > --- a/mm/sparse.c
>> > > +++ b/mm/sparse.c
>> > > @@ -888,7 +888,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
>> > >         /* Align memmap to section boundary in the subsection case */
>> > >         if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
>> > >                 section_nr_to_pfn(section_nr) != start_pfn)
>> > > -               memmap = pfn_to_kaddr(section_nr_to_pfn(section_nr));
>> > > +               memmap = pfn_to_page(section_nr_to_pfn(section_nr));
>> >
>> > Yes, this looks obviously correct. This might be tripping up
>> > makedumpfile. Do you see any practical effects of this bug? The kernel
>> > mostly avoids ->section_mem_map in the vmemmap case and in the
>> > !vmemmap case section_nr_to_pfn(section_nr) should always equal
>> > start_pfn.
>>
>> The practical effects is that the memmap for the first unaligned section will be lost
>> when destroy namespace to hot remove it. Because we encode the ->section_mem_map
>> into mem_section, and get memmap from the related mem_section to free it in
>> section_deactivate(). In fact in vmemmap, we don't need to encode the ->section_mem_map
>> with memmap.
>
>Right, but can you actually trigger that in the SPARSEMEM_VMEMMAP=n case?
>
>> By the way, sub-section support is only valid in vmemmap case, right?
>
>Yes.

Just one question from curiosity. Why we don't want sub-section for !vmemmap
case? Because it will wast memory for memmap?

>
>> Seems yes from code, but I don't find any document to prove it.
>
>check_pfn_span() enforces this requirement.

-- 
Wei Yang
Help you, Help me