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Date:   Tue, 11 Feb 2020 07:16:23 +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: <20200210231623.GC32495@richard>
Reply-To: Wei Yang <richardw.yang@linux.intel.com>
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 <90742479-cbb1-4ea9-c20c-53a1df34b806@redhat.com>
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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.

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

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

-- 
Wei Yang
Help you, Help me