block: move bio_associate_blkg_from_page to mm/page_io.c

bio_associate_blkg_from_page is a special purpose helper for swap bios
that doesn't need access to bio internals.  Move it to the swap code
instead of having it in bio.c.
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -1659,32 +1659,6 @@ void bio_associate_blkg_from_css(struct bio *bio,
 }
 EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);
 
-#ifdef CONFIG_MEMCG
-/**
- * bio_associate_blkg_from_page - associate a bio with the page's blkg
- * @bio: target bio
- * @page: the page to lookup the blkcg from
- *
- * Associate @bio with the blkg from @page's owning memcg and the respective
- * request_queue.  If cgroup_e_css returns %NULL, fall back to the queue's
- * root_blkg.
- */
-void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
-{
-	struct cgroup_subsys_state *css;
-
-	if (!page->mem_cgroup)
-		return;
-
-	rcu_read_lock();
-
-	css = cgroup_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);
-	bio_associate_blkg_from_css(bio, css);
-
-	rcu_read_unlock();
-}
-#endif /* CONFIG_MEMCG */
-
 /**
  * bio_associate_blkg - associate a bio with a blkg
  * @bio: target bio

include/linux/bio.h

@@ -489,13 +489,6 @@ do {						\
 #define bio_dev(bio) \
 	disk_devt((bio)->bi_disk)
 
-#if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
-void bio_associate_blkg_from_page(struct bio *bio, struct page *page);
-#else
-static inline void bio_associate_blkg_from_page(struct bio *bio,
-						struct page *page) { }
-#endif
-
 #ifdef CONFIG_BLK_CGROUP
 void bio_associate_blkg(struct bio *bio);
 void bio_associate_blkg_from_css(struct bio *bio,

mm/page_io.c

@@ -277,6 +277,23 @@ static inline void count_swpout_vm_event(struct page *page)
 	count_vm_events(PSWPOUT, hpage_nr_pages(page));
 }
 
+#if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
+static void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
+{
+	struct cgroup_subsys_state *css;
+
+	if (!page->mem_cgroup)
+		return;
+
+	rcu_read_lock();
+	css = cgroup_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);
+	bio_associate_blkg_from_css(bio, css);
+	rcu_read_unlock();
+}
+#else
+#define bio_associate_blkg_from_page(bio, page)		do { } while (0)
+#endif /* CONFIG_MEMCG && CONFIG_BLK_CGROUP */
+
 int __swap_writepage(struct page *page, struct writeback_control *wbc,
 		bio_end_io_t end_write_func)
 {