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Date:   Fri, 7 Feb 2020 11:26:32 +0000
From:   Wei Yang <richard.weiyang@gmail.com>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     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>,
        Baoquan He <bhe@redhat.com>,
        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: <20200207112632.5inklkwyiewhrd75@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
References: <20200206231629.14151-1-richardw.yang@linux.intel.com>
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 <CAPcyv4h7dKE85EQ9jR1akXnT6PcG2M2g7YCCLqse=kKieP1H9w@mail.gmail.com>
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On Thu, Feb 06, 2020 at 06:19:46PM -0800, Dan Williams wrote:
>On Thu, Feb 6, 2020 at 3:17 PM Wei Yang <richardw.yang@linux.intel.com> wrote:
>>
>> memmap should be the physical address to page struct instead of virtual
>> address to pfn.
>>
>> Since we call this only for SPARSEMEM_VMEMMAP, pfn_to_page() is valid at
>> this point.
>>
>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> CC: Dan Williams <dan.j.williams@intel.com>
>> ---
>>  mm/sparse.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> 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.

I took another look into the code. Looks there is no practical effect after
this. Because in the vmemmap case, we don't need ->section_mem_map to retrieve
the real memmap.

But leave a inconsistent data in section_mem_map is not a good practice.

-- 
Wei Yang
Help you, Help me