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Date:   Thu, 6 Feb 2020 21:50:16 +0800
From:   Baoquan He <bhe@redhat.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
Subject: Re: [PATCH] mm/sparsemem: pfn_to_page is not valid yet on SPARSEMEM
Message-ID: <20200206135016.GA25537@MiWiFi-R3L-srv>
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 <6d9e36cb-ee4a-00c8-447b-9b75a0262c3a@redhat.com>
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On 02/06/20 at 02:28pm, David Hildenbrand wrote:
> On 06.02.20 13:53, Wei Yang wrote:
> > When we use SPARSEMEM instead of SPARSEMEM_VMEMMAP, pfn_to_page()
> > doesn't work before sparse_init_one_section() is called. This leads to a
> > crash when hotplug memory.
> > 
> > We should use memmap as it did.
> > 
> > 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 5a8599041a2a..2efb24ff8f96 100644
> > --- a/mm/sparse.c
> > +++ b/mm/sparse.c
> > @@ -882,7 +882,7 @@ 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, sizeof(struct page) * nr_pages);
> 
> If you add sub-sections that don't fall onto the start of the section,
> 
> pfn_to_page(start_pfn) != memmap
> 
> and your patch would break that under SPARSEMEM_VMEMMAP if I am not wrong.

It returns the pfn_to_page(pfn) from __populate_section_memmap() and
assign to memmap in vmemmap case, how come it breaks anything. Correct
me if I was wrong.

> 
> Instead of memmap, there would have to be something like
> 
> memmap + (start_pfn - SECTION_ALIGN_DOWN(start_pfn))
> 
> If I am not wrong :)
> 
> -- 
> Thanks,
> 
> David / dhildenb