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Date:   Wed, 12 Feb 2020 10:28:08 +0800
From:   Wei Yang <richardw.yang@linux.intel.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Wei Yang <richardw.yang@linux.intel.com>,
        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>
Subject: Re: [Patch v2] mm/sparsemem: get address to page struct instead of
 address to pfn
Message-ID: <20200212022808.GB7855@richard>
Reply-To: Wei Yang <richardw.yang@linux.intel.com>
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 <20200210231623.GC32495@richard>
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On Tue, Feb 11, 2020 at 03:01:35PM +0100, David Hildenbrand wrote:
>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.
>

Sure, a cleanup patch may help audience get more understanding about the
change.

>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me