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Subject: Re: [Patch v2] mm/sparsemem: get address to page struct instead of
 address to pfn
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     akpm@linux-foundation.org, osalvador@suse.de,
        dan.j.williams@intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Baoquan He <bhe@redhat.com>
References: <20200210005048.10437-1-richardw.yang@linux.intel.com>
 <90742479-cbb1-4ea9-c20c-53a1df34b806@redhat.com>
 <20200210231623.GC32495@richard>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <e36cd022-c1f1-a7e8-8888-5bf5b4cd993d@redhat.com>
Date:   Tue, 11 Feb 2020 15:01:35 +0100
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In-Reply-To: <20200210231623.GC32495@richard>
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On 11.02.20 00:16, Wei Yang wrote:
> On Mon, Feb 10, 2020 at 10:00:47AM +0100, David Hildenbrand wrote:
>> On 10.02.20 01:50, Wei Yang wrote:
>>> memmap should be the address to page struct instead of address to pfn.
>>>
>>
>> "mm/sparsemem: fix wrong address in ms->section_mem_map with sub-sections
>>
>> We want to store the address of the memmap, not the address of the first
>> pfn.
>>
>> E.g., we can have both (boot) system memory and devmem residing in a
>> single section. Once we hot-add the devmem part, the address stored in
>> ms->section_mem_map would be wrong, and kdump would not be able to
>> dump the right memory.
>> "
>>
>> ? See below
>>
>>> As mentioned by David, if system memory and devmem sit within a
>>> section, the mismatch address would lead kdump to dump unexpected
>>> memory.
>>>
>>> Since sub-section only works for SPARSEMEM_VMEMMAP, pfn_to_page() is
>>> valid to get the page struct address 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>
>>> CC: David Hildenbrand <david@redhat.com>
>>> CC: Baoquan He <bhe@redhat.com>
>>>
>>> ---
>>> v2:
>>>   * adjust comment to mention the mismatch data would affect kdump
>>>
>>> ---
>>>  mm/sparse.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index 586d85662978..4862ec2cfbc0 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -887,7 +887,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
>>>  
>>>  	/* Align memmap to section boundary in the subsection case */
>>>  	if (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));
>>
>> I think this whole code should be reworked.
>>
>> Callee returns a pointer. Caller: Nah, I know it better.
>>
>> Just nasty.
>>
>>
>> Can we do something like this instead:
>>
>>
>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index 200aef686722..c5091feef29e 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -266,5 +266,5 @@ struct page * __meminit
>> __populate_section_memmap(unsigned long pfn,
>>        if (vmemmap_populate(start, end, nid, altmap))
>>                return NULL;
>>
>> -       return pfn_to_page(pfn);
>> +       return pfn_to_page(SECTION_ALIGN_DOWN(pfn));
>> }
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index c184b69460b7..21902d7931e4 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -788,6 +788,10 @@ static void section_deactivate(unsigned long pfn,
>> unsigned long nr_pages,
>>                depopulate_section_memmap(pfn, nr_pages, altmap);
>> }
>>
>> +/*
>> + * Returns the memmap of the first pfn of the section (not of
>> + * sub-sections).
>> + */
>> static struct page * __meminit section_activate(int nid, unsigned long pfn,
>>                unsigned long nr_pages, struct vmem_altmap *altmap)
>> {
>> @@ -882,9 +886,6 @@ int __meminit sparse_add_section(int nid, unsigned
>> long start_pfn,
>>        set_section_nid(section_nr, nid);
>>        section_mark_present(ms);
>>
>> -       /* Align memmap to section boundary in the subsection case */
>> -       if (section_nr_to_pfn(section_nr) != start_pfn)
>> -               memmap = pfn_to_kaddr(section_nr_to_pfn(section_nr));
>>        sparse_init_one_section(ms, section_nr, memmap, ms->usage, 0);
>>
>>        return 0;
>>
>>
>> Untested, of course :)
> 
> I think you get some point. As you mentioned in the following reply, we need
> to adjust poisoning after this change.

We can just poison after setting up the section (IOW, move it further down).

> 
> This looks like a trade off between two options. I don't have a strong
> preference.

I clearly prefer if *section*_activate() returns the memmap of the
section. This code is just confusing. But I can send a cleanup on top if
you want to keep it like that for now.


-- 
Thanks,

David / dhildenb