[PATCH 2/5] mm: Split handling old page for migration

[Keith Busch <keith.busch@intel.com>]

Refactor unmap_and_move() handling for the new page into a separate
function from locking and preparing the old page.

No functional change here: this is just making it easier to reuse this
part of the page migration from contexts that already locked the old page.

Signed-off-by: Keith Busch <keith.busch@intel.com>
---
 mm/migrate.c | 115 +++++++++++++++++++++++++++++++----------------------------
 1 file changed, 61 insertions(+), 54 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index ac6f4939bb59..705b320d4b35 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1000,57 +1000,14 @@ static int move_to_new_page(struct page *newpage, struct page *page,
 	return rc;
 }
 
-static int __unmap_and_move(struct page *page, struct page *newpage,
-				int force, enum migrate_mode mode)
+static int __unmap_and_move_locked(struct page *page, struct page *newpage,
+				   enum migrate_mode mode)
 {
 	int rc = -EAGAIN;
 	int page_was_mapped = 0;
 	struct anon_vma *anon_vma = NULL;
 	bool is_lru = !__PageMovable(page);
 
-	if (!trylock_page(page)) {
-		if (!force || mode == MIGRATE_ASYNC)
-			goto out;
-
-		/*
-		 * It's not safe for direct compaction to call lock_page.
-		 * For example, during page readahead pages are added locked
-		 * to the LRU. Later, when the IO completes the pages are
-		 * marked uptodate and unlocked. However, the queueing
-		 * could be merging multiple pages for one bio (e.g.
-		 * mpage_readpages). If an allocation happens for the
-		 * second or third page, the process can end up locking
-		 * the same page twice and deadlocking. Rather than
-		 * trying to be clever about what pages can be locked,
-		 * avoid the use of lock_page for direct compaction
-		 * altogether.
-		 */
-		if (current->flags & PF_MEMALLOC)
-			goto out;
-
-		lock_page(page);
-	}
-
-	if (PageWriteback(page)) {
-		/*
-		 * Only in the case of a full synchronous migration is it
-		 * necessary to wait for PageWriteback. In the async case,
-		 * the retry loop is too short and in the sync-light case,
-		 * the overhead of stalling is too much
-		 */
-		switch (mode) {
-		case MIGRATE_SYNC:
-		case MIGRATE_SYNC_NO_COPY:
-			break;
-		default:
-			rc = -EBUSY;
-			goto out_unlock;
-		}
-		if (!force)
-			goto out_unlock;
-		wait_on_page_writeback(page);
-	}
-
 	/*
 	 * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
 	 * we cannot notice that anon_vma is freed while we migrates a page.
@@ -1077,11 +1034,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 	 * This is much like races on refcount of oldpage: just don't BUG().
 	 */
 	if (unlikely(!trylock_page(newpage)))
-		goto out_unlock;
+		goto out;
 
 	if (unlikely(!is_lru)) {
 		rc = move_to_new_page(newpage, page, mode);
-		goto out_unlock_both;
+		goto out_unlock;
 	}
 
 	/*
@@ -1100,7 +1057,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		VM_BUG_ON_PAGE(PageAnon(page), page);
 		if (page_has_private(page)) {
 			try_to_free_buffers(page);
-			goto out_unlock_both;
+			goto out_unlock;
 		}
 	} else if (page_mapped(page)) {
 		/* Establish migration ptes */
@@ -1110,22 +1067,19 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
 		page_was_mapped = 1;
 	}
-
 	if (!page_mapped(page))
 		rc = move_to_new_page(newpage, page, mode);
 
 	if (page_was_mapped)
 		remove_migration_ptes(page,
 			rc == MIGRATEPAGE_SUCCESS ? newpage : page, false);
-
-out_unlock_both:
-	unlock_page(newpage);
 out_unlock:
+	unlock_page(newpage);
 	/* Drop an anon_vma reference if we took one */
+out:
 	if (anon_vma)
 		put_anon_vma(anon_vma);
-	unlock_page(page);
-out:
+
 	/*
 	 * If migration is successful, decrease refcount of the newpage
 	 * which will not free the page because new page owner increased
@@ -1141,7 +1095,60 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		else
 			putback_lru_page(newpage);
 	}
+	return rc;
+}
+
+static int __unmap_and_move(struct page *page, struct page *newpage,
+				int force, enum migrate_mode mode)
+{
+	int rc = -EAGAIN;
+
+	if (!trylock_page(page)) {
+		if (!force || mode == MIGRATE_ASYNC)
+			goto out;
+
+		/*
+		 * It's not safe for direct compaction to call lock_page.
+		 * For example, during page readahead pages are added locked
+		 * to the LRU. Later, when the IO completes the pages are
+		 * marked uptodate and unlocked. However, the queueing
+		 * could be merging multiple pages for one bio (e.g.
+		 * mpage_readpages). If an allocation happens for the
+		 * second or third page, the process can end up locking
+		 * the same page twice and deadlocking. Rather than
+		 * trying to be clever about what pages can be locked,
+		 * avoid the use of lock_page for direct compaction
+		 * altogether.
+		 */
+		if (current->flags & PF_MEMALLOC)
+			goto out;
+
+		lock_page(page);
+	}
 
+	if (PageWriteback(page)) {
+		/*
+		 * Only in the case of a full synchronous migration is it
+		 * necessary to wait for PageWriteback. In the async case,
+		 * the retry loop is too short and in the sync-light case,
+		 * the overhead of stalling is too much
+		 */
+		switch (mode) {
+		case MIGRATE_SYNC:
+		case MIGRATE_SYNC_NO_COPY:
+			break;
+		default:
+			rc = -EBUSY;
+			goto out_unlock;
+		}
+		if (!force)
+			goto out_unlock;
+		wait_on_page_writeback(page);
+	}
+	rc = __unmap_and_move_locked(page, newpage, mode);
+out_unlock:
+	unlock_page(page);
+out:
 	return rc;
 }
 
-- 
2.14.4