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?
> 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?
---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@xxxxxxx>
---
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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