Re: [PATCH 2/3] mm/sparsemem: get physical address to page struct instead of virtual address to pfn

From: Wei Yang <richardw.yang@linux.intel.com>
To: David Hildenbrand <david@redhat.com>
Cc: Baoquan He <bhe@redhat.com>, Wei Yang <richard.weiyang@gmail.com>,
	Dan Williams <dan.j.williams@intel.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>
Subject: Re: [PATCH 2/3] mm/sparsemem: get physical address to page struct instead of virtual address to pfn
Date: Mon, 10 Feb 2020 08:36:39 +0800	[thread overview]
Message-ID: <20200210003639.GD7326@richard> (raw)
In-Reply-To: <A25CC0EC-73A0-426D-93A0-DD9DDC43800F@redhat.com>

On Sun, Feb 09, 2020 at 03:14:28PM +0100, David Hildenbrand wrote:
>
>
>> Am 09.02.2020 um 14:50 schrieb Baoquan He <bhe@redhat.com>:
>> 
>> On 02/07/20 at 11:26am, Wei Yang wrote:
>>>> 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.
>> 
>> Yeah, it does has no pratical effect. I tried to create sub-section
>> alighed namespace, then trigger crash, makedumpfile isn't impacted.
>> Because pmem memory is only added, but not onlined. We don't report it
>> to kdump, makedumpfile will ignore it.
>> 
>> I think it's worth fixing it to encode a correct memmap address. We
>> don't know if in the future this will break anything.
>
>We can have system memory and devmem overlap within a section (which is still buggy and to be fixed in other regard - e.g., pfn_to_online_page() does not work correctly).
>
>E.g., 64 mb of (boot) system memory in a section. Then you can hot-add devmem that spans the remaining 64 mb of that section.
>
>So some of that memory will be kdumped - and should be fixed if broken.
>
>Cheers

Thanks for the explanation, I will add this in the changelog.

>
>
>> 
>> Thanks
>> Baoquan

-- 
Wei Yang
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