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Subject: Re: [Patch v2] mm/sparsemem: get address to page struct instead of
 address to pfn
From:   David Hildenbrand <david@redhat.com>
To:     Wei Yang <richardw.yang@linux.intel.com>,
        akpm@linux-foundation.org, osalvador@suse.de,
        dan.j.williams@intel.com
Cc:     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>
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Organization: Red Hat GmbH
Message-ID: <0beb353c-0192-874f-a2c9-f6c575716e25@redhat.com>
Date:   Mon, 10 Feb 2020 10:13:16 +0100
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On 10.02.20 10:00, 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 the following would be needed as well with Wei's fix:

@@ -876,15 +880,13 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
         * Poison uninitialized struct pages in order to catch invalid flags
         * combinations.
         */
-       page_init_poison(pfn_to_page(start_pfn), sizeof(struct page) * nr_pages);
+       page_init_poison(memmap + start_pfn - SECTION_ALIGN_DOWN(start_pfn),
+                        sizeof(struct page) * nr_pages);
 
        ms = __nr_to_section(section_nr);

Or we can simply move the poisoning behind sparse_init_one_section().

-- 
Thanks,

David / dhildenb