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Date:   Thu, 6 Feb 2020 22:07:03 +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
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On 02/06/20 at 02:55pm, David Hildenbrand wrote:
> On 06.02.20 14:50, Baoquan He wrote:
> > 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.
> 
> I'm sorry, I can't follow :) Can you elaborate?
> 
> Was your comment targeted at why the old code cannot be broken or why
> this patch cannot be broken?

Sorry for the confusion :-) the latter. I mean the returned memmap has been
at the pfn_to_page(start_pfn) in SPARSEMEM_VMEMMAP case.