After commit 907ec5fc ("mm: zero remaining unavailable struct
pages"), struct page of reserved memory is zeroed.  This causes
page->flags to be 0 and fixes issues related to reading
/proc/kpageflags, for example, of reserved memory.

The VM_BUG_ON() in move_freepages_block(), however, assumes that
page_zone() is meaningful even for reserved memory.  That assumption is
no longer true after the aforementioned commit.

There's no reason why move_freepages_block() should be testing the
legitimacy of page_zone() for reserved memory; its scope is limited only
to pages on the zone's freelist.

Note that pfn_valid() can be true for reserved memory: there is a
backing struct page.  The check for page_to_nid(page) is also buggy but
reserved memory normally only appears on node 0 so the zeroing doesn't
affect this.

Move the debug checks to after verifying PageBuddy is true.  This
isolates the scope of the checks to only be for buddy pages which are on
the zone's freelist which move_freepages_block() is operating on.  In
this case, an incorrect node or zone is a bug worthy of being warned
about (and the examination of struct page is acceptable bcause this
memory is not reserved).

Why does move_freepages_block() gets called on reserved memory? It's
simply math after finding a valid free page from the per-zone free area
to use as fallback.  We find the beginning and end of the pageblock of
the valid page and that can bring us into memory that was reserved per
the e820.  pfn_valid() is still true (it's backed by a struct page), but
since it's zero'd we shouldn't make any inferences here about comparing
its node or zone.  The current node check just happens to succeed most
of the time by luck because reserved memory typically appears on node 0.

The fix here is to validate that we actually have buddy pages before
testing if there's any type of zone or node strangeness going on.

We noticed it almost immediately after bringing 907ec5fc in on
CONFIG_DEBUG_VM builds.  It depends on finding specific free pages in
the per-zone free area where the math in move_freepages() will bring the
start or end pfn into reserved memory and wanting to claim that entire
pageblock as a new migratetype.  So the path will be rare, require
CONFIG_DEBUG_VM, and require fallback to a different migratetype.

Some struct pages were already zeroed from reserve pages before
907ec5fca3c so it theoretically could trigger before this commit.  I
think it's rare enough under a config option that most people don't run
that others may not have noticed.  I wouldn't argue against a stable tag
and the backport should be easy enough, but probably wouldn't single out
a commit that this is fixing.

Mel said:

: The overhead of the debugging check is higher with this patch although
: it'll only affect debug builds and the path is not particularly hot.
: If this was a concern, I think it would be reasonable to simply remove
: the debugging check as the zone boundaries are checked in
: move_freepages_block and we never expect a zone/node to be smaller than
: a pageblock and stuck in the middle of another zone.

Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1908122036560.10779@chino.kir.corp.google.comSigned-off-by: David Rientjes <rientjes@google.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Pavel Tatashin <pavel.tatashin@microsoft.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>