linux-kernel - Re: [PATCH v2 2/5] mm: Teach pfn_to_online_page() to consider subsection validity

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Date:   Tue, 12 Jan 2021 10:53:17 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org
Cc:     Qian Cai <cai@....pw>, Michal Hocko <mhocko@...e.com>,
        vishal.l.verma@...el.com, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2 2/5] mm: Teach pfn_to_online_page() to consider
 subsection validity

On 12.01.21 10:34, Dan Williams wrote:
> pfn_section_valid() determines pfn validity on subsection granularity.
> 
> pfn_valid_within() internally uses pfn_section_valid(), but gates it
> with early_section() to preserve the traditional behavior of pfn_valid()
> before subsection support was added.
> 
> pfn_to_online_page() wants the explicit precision that pfn_valid() does
> not offer, so use pfn_section_valid() directly. Since
> pfn_to_online_page() already open codes the validity of the section
> number vs NR_MEM_SECTIONS, there's not much value to using
> pfn_valid_within(), just use pfn_section_valid(). This loses the
> valid_section() check that pfn_valid_within() was performing, but that
> was already redundant with the online check.
> 
> Fixes: b13bc35193d9 ("mm/hotplug: invalid PFNs from pfn_to_online_page()")
> Cc: Qian Cai <cai@....pw>
> Cc: Michal Hocko <mhocko@...e.com>
> Reported-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/memory_hotplug.c |   16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 55a69d4396e7..a845b3979bc0 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -308,11 +308,19 @@ static int check_hotplug_memory_addressable(unsigned long pfn,
>  struct page *pfn_to_online_page(unsigned long pfn)
>  {
>  	unsigned long nr = pfn_to_section_nr(pfn);
> +	struct mem_section *ms;
> +
> +	if (nr >= NR_MEM_SECTIONS)
> +		return NULL;
> +
> +	ms = __nr_to_section(nr);
> +	if (!online_section(ms))
> +		return NULL;
> +
> +	if (!pfn_section_valid(ms, pfn))
> +		return NULL;

That's not sufficient for alternative implementations of pfn_valid().

You still need some kind of pfn_valid(pfn) for alternative versions of
pfn_valid(). Consider arm64 memory holes in the memmap. See their
current (yet to be fixed/reworked) pfn_valid() implementation.
(pfn_valid_within() is implicitly active on arm64)

Actually, I think we should add something like the following, to make
this clearer (pfn_valid_within() is confusing)

#ifdef CONFIG_HAVE_ARCH_PFN_VALID
	/* We might have to check for holes inside the memmap. */
	if (!pfn_valid())
		return NULL;
#endif

>  
> -	if (nr < NR_MEM_SECTIONS && online_section_nr(nr) &&
> -	    pfn_valid_within(pfn))
> -		return pfn_to_page(pfn);
> -	return NULL;
> +	return pfn_to_page(pfn);
>  }
>  EXPORT_SYMBOL_GPL(pfn_to_online_page);
>  
> 


-- 
Thanks,

David / dhildenb