Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized memmaps in memory_failure()

From: David Hildenbrand
Date: Mon Oct 21 2019 - 05:44:29 EST

On 19.10.19 04:05, Andrew Morton wrote:
On Thu, 10 Oct 2019 09:17:42 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:

- pgmap = get_dev_pagemap(pfn, NULL);
- if (pgmap)
- return memory_failure_dev_pagemap(pfn, flags, pgmap);
- p = pfn_to_page(pfn);

This change seems to assume that memory_failure_dev_pagemap() is never
called for online pages. Is it an intended behavior?
Or the concept "online pages" is not applicable to zone device pages?

Yes, that's the real culprit. ZONE_DEVICE/devmem pages are never online
(SECTION_IS_ONLINE). The terminology "online" only applies to pages that
were given to the buddy. And as we support sup-section hotadd for
devmem, we cannot easily make use of the section flag it. I already
proposed somewhere to convert SECTION_IS_ONLINE to a subsection bitmap
and call it something like pfn_active().

pfn_online() would then be "pfn_active() && zone != ZONE_DEVICE". And we
could use pfn_active() everywhere to test for initialized memmaps (well,
besides some special cases like device reserved memory that does not
span full sub-sections). Until now, nobody volunteered and I have other
things to do.

Is it worth a code comment or two to make this clearer?

You mean something like

/* Only pages managed by the buddy are online (not ZONE_DEVICE). */




David / dhildenb