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

[Yasuaki Ishimatsu]

(2014/12/10 9:14), James Custer wrote:
> 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.

Why don't you set CONFIG_HOLES_IN_ZONE? This BUG is occrred by hole in zone.
CONFIG_HOLE_IN_ZONE is propered for the system.

I think your patch fixes the BUG. But even if fixing the BUG, other issues
will be occurred by hole in zone.

Thanks,
Yasuaki Ishimatsu

> 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@xxxxxxxxxxxxxx]
> Sent: Tuesday, December 09, 2014 6:01 PM
> To: James Custer; linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; kamezawa.hiroyu@xxxxxxxxxxxxxx
> 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@xxxxxxx>
> > ---
> >    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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