linux-kernel - Re: [PATCH v4 2/5] mm: Teach pfn_to_online_page() to consider subsection validity

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Message-ID: <20210120102400.GG9371@dhcp22.suse.cz>
Date:   Wed, 20 Jan 2021 11:24:00 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Qian Cai <cai@....pw>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/5] mm: Teach pfn_to_online_page() to consider
 subsection validity

On Wed 13-01-21 16:43:21, Dan Williams wrote:
> pfn_section_valid() determines pfn validity on subsection granularity
> where pfn_valid() may be limited to coarse section granularity.
> Explicitly validate subsections after pfn_valid() succeeds.

The changelog is not really clear on the underlying problem. It would
benefit from some clarification. What about something like this?
"
pfn_to_online_page is primarily used to filter out offline or fully
uninitialized pages. pfn_valid resp. online_section_nr have a coarse
per memory section granularity. If a section shared with a partially
offline memory (e.g. part of ZONE_DEVICE) then pfn_to_online_page would
lead to a false positive on some pfns. Fix this by adding
pfn_section_valid check which is subsection aware.
"

> 
> Fixes: b13bc35193d9 ("mm/hotplug: invalid PFNs from pfn_to_online_page()")
> Cc: Qian Cai <cai@....pw>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Reported-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

With that feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/memory_hotplug.c |   23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 55a69d4396e7..d0c81f7a3347 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -308,11 +308,26 @@ 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;
> +
> +	/*
> +	 * Save some code text when online_section() +
> +	 * pfn_section_valid() are sufficient.
> +	 */
> +	if (IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) && !pfn_valid(pfn))
> +		return NULL;
> +
> +	if (!pfn_section_valid(ms, pfn))
> +		return NULL;
>  
> -	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);
>  
> 

-- 
Michal Hocko
SUSE Labs