linux-kernel - Re: [PATCH] mm/sparsemem: pfn_to_page is not valid yet on SPARSEMEM

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Date:   Thu, 6 Feb 2020 14:28:53 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Wei Yang <richardw.yang@...ux.intel.com>,
        akpm@...ux-foundation.org, osalvador@...e.de,
        dan.j.williams@...el.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org, bhe@...hat.com
Subject: Re: [PATCH] mm/sparsemem: pfn_to_page is not valid yet on SPARSEMEM

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@...ux.intel.com>
> CC: Dan Williams <dan.j.williams@...el.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.

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