Re: [BUG] 3.2.2 crash in isolate_migratepages

[Herbert van den Bergh <herbert.van.den.bergh@oracle.com>]

On 1/30/12 1:09 AM, Mel Gorman wrote:
> On Fri, Jan 27, 2012 at 01:43:07PM -0800, Herbert van den Bergh wrote:
>> 3.2.2 panics on a 16GB i686 blade:
>>
>> BUG: unable to handle kernel paging request at 01c00008
>> IP: [<c0522399>] isolate_migratepages+0x119/0x390
>> *pdpt = 000000002f7ce001 *pde = 0000000000000000
>>
>> The crash happens on this line in mm/compaction.c::isolate_migratepages:
>>
>>     328                 page = pfn_to_page(low_pfn);
>>
> This is not line 328 on kernel 3.2.2. Can you double check what version
> you are using?

That's right, I was using 3.1, but reproduced the problem on 3.2.2.  The
source code line numbers are from 3.1.  Sorry for the confusion.
>> This macro finds the struct page pointer for a given pfn.  These struct
>> page pointers are stored in sections of 131072 pages if
>> CONFIG_SPARSEMEM=y.  If an entire section has no memory pages, the page
>> structs are not allocated for this section.  On this particular machine,
>> there is no RAM mapped from 2GB - 4GB:
>>
>> # dmesg|grep usable
>>  BIOS-e820: 0000000000000000 - 000000000009f400 (usable)
>>  BIOS-e820: 0000000000100000 - 000000007fe4e000 (usable)
>>  BIOS-e820: 000000007fe56000 - 000000007fe57000 (usable)
>>  BIOS-e820: 0000000100000000 - 000000047ffff000 (usable)
>>
>> So there are no page structs for the sections between 2GB and 4GB.
>>
>> I believe this check was intended to catch page numbers that point to holes:
>>
>>     323                 if (!pfn_valid_within(low_pfn))
>>     324                         continue;
> Can you try the following patch please?
The following patch fixes the crash on this system.

Thanks,
Herbert.

>
> ---8<---
> mm: compaction: Check pfn_valid when entering a new MAX_ORDER_NR_PAGES block during isolation for migration
>
> When isolating for migration, migration starts at the start of a zone
> which is not necessarily pageblock aligned. Further, it stops isolating
> when COMPACT_CLUSTER_MAX pages are isolated so migrate_pfn is generally
> not aligned.
>
> The problem is that pfn_valid is only called on the first PFN being
> checked. Lets say we have a case like this
>
> H = MAX_ORDER_NR_PAGES boundary
> | = pageblock boundary
> m = cc->migrate_pfn
> f = cc->free_pfn
> o = memory hole
>
> H------|------H------|----m-Hoooooo|ooooooH-f----|------H
>
> The migrate_pfn is just below a memory hole and the free scanner is
> beyond the hole. When isolate_migratepages started, it scans from
> migrate_pfn to migrate_pfn+pageblock_nr_pages which is now in a memory
> hole. It checks pfn_valid() on the first PFN but then scans into the
> hole where there are not necessarily valid struct pages.
>
> This patch ensures that isolate_migratepages calls pfn_valid when
> necessary.
>
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  mm/compaction.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 899d956..edc1e26 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -313,6 +313,19 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
>  		} else if (!locked)
>  			spin_lock_irq(&zone->lru_lock);
>  
> +		/*
> +		 * migrate_pfn does not necessarily start aligned to a
> +		 * pageblock. Ensure that pfn_valid is called when moving
> +		 * into a new MAX_ORDER_NR_PAGES range in case of large
> +		 * memory holes within the zone
> +		 */
> +		if ((low_pfn & (MAX_ORDER_NR_PAGES - 1)) == 0) {
> +			if (!pfn_valid(low_pfn)) {
> +				low_pfn += MAX_ORDER_NR_PAGES - 1;
> +				continue;
> +			}
> +		}
> +
>  		if (!pfn_valid_within(low_pfn))
>  			continue;
>  		nr_scanned++;

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