Re: [patch 4/6] mm: fix invalid use of pfn_valid_within in test_pages_in_a_zone

[Vlastimil Babka <vbabka@suse.cz>]

On 12/16/2014 12:03 AM, akpm@linux-foundation.org wrote:
> From: James Custer <jcuster@sgi.com>
> Subject: mm: fix invalid use of pfn_valid_within in test_pages_in_a_zone
> 
> 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.

It would still be useful to show the BUG, and provide /proc/zoneinfo for us to
see where are the corner cases.

> 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>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Russ Anderson <rja@sgi.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/memory_hotplug.c |   11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff -puN mm/memory_hotplug.c~mm-fix-invalid-use-of-pfn_valid_within-in-test_pages_in_a_zone mm/memory_hotplug.c
> --- a/mm/memory_hotplug.c~mm-fix-invalid-use-of-pfn_valid_within-in-test_pages_in_a_zone
> +++ a/mm/memory_hotplug.c
> @@ -1331,7 +1331,7 @@ int is_mem_section_removable(unsigned lo
>  }
>  
>  /*
> - * 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 s
>  	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 (i == MAX_ORDER_NR_PAGES)
>  			continue;
>  		page = pfn_to_page(pfn + i);
> _
> 
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