vmemmap_verify() BUGs during memory hotplug (4.2-rc1 regression)

DISCUSSION David Vrabel 2 participants 3 messages Jul 27, 2015 – Jul 28, 2015 Memory management

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David Vrabel

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2015-07-27 15:32 UTC #0 lore raw

Mel,

As of commit 8a942fdea560d4ac0e9d9fabcd5201ad20e0c382 (mm: meminit: make __early_pfn_to_nid SMP-safe and introduce meminit_pfn_in_nid) vmemmap_verify() will BUG_ON() during memory hotplug because of its use of early_pfn_to_nid(). Previously, it would have reported bogus (or failed to report valid) warnings.

I believe this does not affect memory hotplug on most x86 systems because vmemmap_populate() would normally call vmemmap_populate_hugepages() which avoids calling vmemmap_verify() in the common case (no existing mappings covering the new area).

I'm triggering the early_pfn_to_nid() BUG_ON() with the Xen balloon driver in a PV guest which will always call vmemmap_populate_basepages() (since Xen PV guests lack superpage support).

Not really sure what the best way to resolve this is. Presumably vmmemmap_verify() needs to switch to using pfn_to_nid() after the initial initialization but there doesn't appear to be anything suitable to distinguish between the early and hotplug cases.

David

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Mel Gorman

2015-07-27 15:41 UTC #1 lore raw

Quoted 8 lines · David Vrabel

Mel,
As of commit 8a942fdea560d4ac0e9d9fabcd5201ad20e0c382 (mm: meminit: make __early_pfn_to_nid SMP-safe and introduce meminit_pfn_in_nid) vmemmap_verify() will BUG_ON() during memory hotplug because of its use of early_pfn_to_nid(). Previously, it would have reported bogus (or failed to report valid) warnings.

Please test "mm, meminit: Allow early_pfn_to_nid to be used during runtime"

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Mel Gorman
SUSE Labs

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David Vrabel

author

2015-07-28 17:09 UTC #2 lore raw

Quoted 10 lines · Mel Gorman

Quoted 8 lines · David Vrabel
Mel,
As of commit 8a942fdea560d4ac0e9d9fabcd5201ad20e0c382 (mm: meminit: make __early_pfn_to_nid SMP-safe and introduce meminit_pfn_in_nid) vmemmap_verify() will BUG_ON() during memory hotplug because of its use of early_pfn_to_nid(). Previously, it would have reported bogus (or failed to report valid) warnings.

Quoted 2 lines

Please test "mm, meminit: Allow early_pfn_to_nid to be used during runtime"

That fixed the BUG_ON() thanks. But I still can't only the new sections because their first page is not reserved, but I've not had time to investigate why this is yet.

David