LRU pagevec holds refcount of pages until the pagevec are drained.  It
could prevent migration since the refcount of the page is greater than
the expection in migration logic.  To mitigate the issue, callers of
migrate_pages drains LRU pagevec via migrate_prep or lru_add_drain_all
before migrate_pages call.

However, it's not enough because pages coming into pagevec after the
draining call still could stay at the pagevec so it could keep
preventing page migration.  Since some callers of migrate_pages have
retrial logic with LRU draining, the page would migrate at next trail
but it is still fragile in that it doesn't close the fundamental race
between upcoming LRU pages into pagvec and migration so the migration
failure could cause contiguous memory allocation failure in the end.

To close the race, this patch disables lru caches(i.e, pagevec) during
ongoing migration until migrate is done.

Since it's really hard to reproduce, I measured how many times
migrate_pages retried with force mode(it is about a fallback to a sync
migration) with below debug code.

int migrate_pages(struct list_head *from, new_page_t get_new_page,
			..
			..

  if (rc && reason == MR_CONTIG_RANGE && pass > 2) {
         printk(KERN_ERR, "pfn 0x%lx reason %d", page_to_pfn(page), rc);
         dump_page(page, "fail to migrate");
  }

The test was repeating android apps launching with cma allocation in
background every five seconds.  Total cma allocation count was about 500
during the testing.  With this patch, the dump_page count was reduced
from 400 to 30.

The new interface is also useful for memory hotplug which currently
drains lru pcp caches after each migration failure.  This is rather
suboptimal as it has to disrupt others running during the operation.
With the new interface the operation happens only once.  This is also in
line with pcp allocator cache which are disabled for the offlining as
well.

Link: https://lkml.kernel.org/r/20210319175127.886124-1-minchan@kernel.orgSigned-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Chris Goldsworthy <cgoldswo@codeaurora.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: John Dias <joaodias@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Oliver Sang <oliver.sang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>