mm, page_alloc: simplify pageset_update()

mm, page_alloc: simplify pageset_update()

pageset_update() attempts to update pcplist's high and batch values in a
way that readers don't observe batch > high.  It uses smp_wmb() to order
the updates in a way to achieve this.  However, without proper pairing
read barriers in readers this guarantee doesn't hold, and there are no
such barriers in e.g.  free_unref_page_commit().

Commit 88e8ac11d2ea ("mm, page_alloc: fix core hung in
free_pcppages_bulk()") already showed this is problematic, and solved this
by ultimately only trusing pcp->count of the current cpu with interrupts
disabled.

The update dance with unpaired write barriers thus makes no sense.
Replace them with plain WRITE_ONCE to prevent store tearing, and document
that the values can change asynchronously and should not be trusted for
correctness.

All current readers appear to be OK after 88e8ac11d2ea.  Convert them to
READ_ONCE to prevent unnecessary read tearing, but mainly to alert anybody
making future changes to the code that special care is needed.

Link: https://lkml.kernel.org/r/20201111092812.11329-5-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>