RE: [PATCH] mm: fix invalid use of pfn_valid_within in test_pages_in_a_zone

[James Custer <jcuster@sgi.com>]

It is exactly the same if CONFIG_HOLES_IN_NODE is set, but if CONFIG_HOLES_IN_NODE is not set, then pfn_valid_within is always 1.

From: https://lkml.org/lkml/2007/3/21/272

"Generally we work under the assumption that memory the mem_map
array is contigious and valid out to MAX_ORDER_NR_PAGES block
of pages, ie. that if we have validated any page within this
MAX_ORDER_NR_PAGES block we need not check any other.  This is not
true when CONFIG_HOLES_IN_ZONE is set and we must check each and
every reference we make from a pfn.

Add a pfn_valid_within() helper which should be used when scanning
pages within a MAX_ORDER_NR_PAGES block when we have already
checked the validility of the block normally with pfn_valid().
This can then be optimised away when we do not have holes within
a MAX_ORDER_NR_PAGES block of pages."

So, since we're iterating over a pageblock there must be a valid pfn to be able to use pfn_valid_within (which makes sense since if CONFIG_HOLES_IN_NODE is not set, it is always 1).

I'm just going off of the documentation there and what makes sense to me based off that documentation. Does that explanation help?

Regards,
James Custer
________________________________________
From: Yasuaki Ishimatsu [isimatu.yasuaki@jp.fujitsu.com]
Sent: Tuesday, December 09, 2014 6:01 PM
To: James Custer; linux-kernel@vger.kernel.org; linux-mm@kvack.org; akpm@linux-foundation.org; kamezawa.hiroyu@jp.fujitsu.com
Cc: Russ Anderson; Derek Fults
Subject: Re: [PATCH] mm: fix invalid use of pfn_valid_within in test_pages_in_a_zone

(2014/12/10 4:34), James Custer wrote:
> Offlining memory by 'echo 0 > /sys/devices/system/memory/memory#/online'
> or reading valid_zones 'cat /sys/devices/system/memory/memory#/valid_zones'

> causes BUG: unable to handle kernel paging request due to invalid use of
> pfn_valid_within. This is due to a bug in test_pages_in_a_zone.

The information is not enough to understand what happened on your system.
Could you show full BUG messages?

>
> In order to use pfn_valid_within within a MAX_ORDER_NR_PAGES block of pages,
> a valid pfn within the block must first be found. There only needs to be
> one valid pfn found in test_pages_in_a_zone in the first place. So the
> fix is to replace pfn_valid_within with pfn_valid such that the first
> valid pfn within the pageblock is found (if it exists). This works
> independently of CONFIG_HOLES_IN_ZONE.
>
> Signed-off-by: James Custer <jcuster@sgi.com>
> ---
>   mm/memory_hotplug.c | 11 ++++++-----
>   1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 1bf4807..304c187 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1331,7 +1331,7 @@ int is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
>   }
>
>   /*
> - * Confirm all pages in a range [start, end) is belongs to the same zone.
> + * Confirm all pages in a range [start, end) belong to the same zone.
>    */
>   int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
>   {
> @@ -1342,10 +1342,11 @@ int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn)
>       for (pfn = start_pfn;
>            pfn < end_pfn;
>            pfn += MAX_ORDER_NR_PAGES) {

> -             i = 0;
> -             /* This is just a CONFIG_HOLES_IN_ZONE check.*/
> -             while ((i < MAX_ORDER_NR_PAGES) && !pfn_valid_within(pfn + i))
> -                     i++;
> +             /* Find the first valid pfn in this pageblock */
> +             for (i = 0; i < MAX_ORDER_NR_PAGES; i++) {
> +                     if (pfn_valid(pfn + i))
> +                             break;
> +             }

If CONFIG_HOLES_IN_NODE is set, there is no difference. Am I making a mistake?

Thanks,
Yasuaki Ishimatsu


>               if (i == MAX_ORDER_NR_PAGES)
>                       continue;
>               page = pfn_to_page(pfn + i);
>



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