[PATCH] Speed up direct-io hugetlbpage handling

This patch short-circuits all the direct-io page dirtying logic for
higher-order pages.  Without this, we pointlessly bounce BIOs up to keventd
all the time.

fs/bio.c

@@ -532,6 +532,12 @@ void bio_unmap_user(struct bio *bio, int write_to_vm)
  * check that the pages are still dirty.   If so, fine.  If not, redirty them
  * in process context.
  *
+ * We special-case compound pages here: normally this means reads into hugetlb
+ * pages.  The logic in here doesn't really work right for compound pages
+ * because the VM does not uniformly chase down the head page in all cases.
+ * But dirtiness of compound pages is pretty meaningless anyway: the VM doesn't
+ * handle them at all.  So we skip compound pages here at an early stage.
+ *
  * Note that this code is very hard to test under normal circumstances because
  * direct-io pins the pages with get_user_pages().  This makes
  * is_page_cache_freeable return false, and the VM will not clean the pages.
@@ -553,8 +559,8 @@ void bio_set_pages_dirty(struct bio *bio)
 	for (i = 0; i < bio->bi_vcnt; i++) {
 		struct page *page = bvec[i].bv_page;
 
-		if (page)
-			set_page_dirty_lock(bvec[i].bv_page);
+		if (page && !PageCompound(page))
+			set_page_dirty_lock(page);
 	}
 }
 
@@ -620,7 +626,7 @@ void bio_check_pages_dirty(struct bio *bio)
 	for (i = 0; i < bio->bi_vcnt; i++) {
 		struct page *page = bvec[i].bv_page;
 
-		if (PageDirty(page)) {
+		if (PageDirty(page) || PageCompound(page)) {
 			page_cache_release(page);
 			bvec[i].bv_page = NULL;
 		} else {