mm: migration: factor out code to compute expected number of page references

Patch series "mm: migrate: Fix page migration stalls for blkdev pages".

This patchset deals with page migration stalls that were reported by our
customer due to a block device page that had a bufferhead that was in the
bh LRU cache.

The patchset modifies the page migration code so that bufferheads are
completely handled inside buffer_migrate_page() and then provides a new
migration helper for pages with buffer heads that is safe to use even for
block device pages and that also deals with bh lrus.

This patch (of 6):

Factor out function to compute number of expected page references in
migrate_page_move_mapping().  Note that we move hpage_nr_pages() and
page_has_private() checks from under xas_lock_irq() however this is safe
since we hold page lock.

[jack@suse.cz: fix expected_page_refs()]
  Link: http://lkml.kernel.org/r/20181217131710.GB8611@quack2.suse.cz
Link: http://lkml.kernel.org/r/20181211172143.7358-2-jack@suse.czSigned-off-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -424,6 +424,22 @@ static inline bool buffer_migrate_lock_buffers(struct buffer_head *head,
 }
 #endif /* CONFIG_BLOCK */
 
+static int expected_page_refs(struct page *page)
+{
+	int expected_count = 1;
+
+	/*
+	 * Device public or private pages have an extra refcount as they are
+	 * ZONE_DEVICE pages.
+	 */
+	expected_count += is_device_private_page(page);
+	expected_count += is_device_public_page(page);
+	if (page_mapping(page))
+		expected_count += hpage_nr_pages(page) + page_has_private(page);
+
+	return expected_count;
+}
+
 /*
  * Replace the page in the mapping.
  *
@@ -440,14 +456,7 @@ int migrate_page_move_mapping(struct address_space *mapping,
 	XA_STATE(xas, &mapping->i_pages, page_index(page));
 	struct zone *oldzone, *newzone;
 	int dirty;
-	int expected_count = 1 + extra_count;
-
-	/*
-	 * Device public or private pages have an extra refcount as they are
-	 * ZONE_DEVICE pages.
-	 */
-	expected_count += is_device_private_page(page);
-	expected_count += is_device_public_page(page);
+	int expected_count = expected_page_refs(page) + extra_count;
 
 	if (!mapping) {
 		/* Anonymous page without mapping */
@@ -467,8 +476,6 @@ int migrate_page_move_mapping(struct address_space *mapping,
 	newzone = page_zone(newpage);
 
 	xas_lock_irq(&xas);
-
-	expected_count += hpage_nr_pages(page) + page_has_private(page);
 	if (page_count(page) != expected_count || xas_load(&xas) != page) {
 		xas_unlock_irq(&xas);
 		return -EAGAIN;