[PATCH 7/12] mm: page migration trylock newpage at same level as oldpage

[Hugh Dickins <hughd@google.com>]

Clean up page migration a little by moving the trylock of newpage from
move_to_new_page() into __unmap_and_move(), where the old page has been
locked.  Adjust unmap_and_move_huge_page() and balloon_page_migrate()
accordingly.

But make one kind-of-functional change on the way: whereas trylock of
newpage used to BUG() if it failed, now simply return -EAGAIN if so.
Cutting out BUG()s is good, right?  But, to be honest, this is really
to extend the usefulness of the custom put_new_page feature, allowing
a pool of new pages to be shared perhaps with racing uses.

Use an "else" instead of that "skip_unmap" label.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/balloon_compaction.c |   10 +-------
 mm/migrate.c            |   46 +++++++++++++++++++++-----------------
 2 files changed, 28 insertions(+), 28 deletions(-)

--- migrat.orig/mm/balloon_compaction.c	2014-12-07 14:21:05.000000000 -0800
+++ migrat/mm/balloon_compaction.c	2015-10-18 17:53:22.486335020 -0700
@@ -199,23 +199,17 @@ int balloon_page_migrate(struct page *ne
 	struct balloon_dev_info *balloon = balloon_page_device(page);
 	int rc = -EAGAIN;
 
-	/*
-	 * Block others from accessing the 'newpage' when we get around to
-	 * establishing additional references. We should be the only one
-	 * holding a reference to the 'newpage' at this point.
-	 */
-	BUG_ON(!trylock_page(newpage));
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
 
 	if (WARN_ON(!__is_movable_balloon_page(page))) {
 		dump_page(page, "not movable balloon page");
-		unlock_page(newpage);
 		return rc;
 	}
 
 	if (balloon && balloon->migratepage)
 		rc = balloon->migratepage(balloon, newpage, page, mode);
 
-	unlock_page(newpage);
 	return rc;
 }
 #endif /* CONFIG_BALLOON_COMPACTION */
--- migrat.orig/mm/migrate.c	2015-10-18 17:53:20.159332371 -0700
+++ migrat/mm/migrate.c	2015-10-18 17:53:22.487335021 -0700
@@ -727,13 +727,8 @@ static int move_to_new_page(struct page
 	struct address_space *mapping;
 	int rc;
 
-	/*
-	 * Block others from accessing the page when we get around to
-	 * establishing additional references. We are the only one
-	 * holding a reference to the new page at this point.
-	 */
-	if (!trylock_page(newpage))
-		BUG();
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
 
 	/* Prepare mapping for the new page.*/
 	newpage->index = page->index;
@@ -774,9 +769,6 @@ static int move_to_new_page(struct page
 			remove_migration_ptes(page, newpage);
 		page->mapping = NULL;
 	}
-
-	unlock_page(newpage);
-
 	return rc;
 }
 
@@ -861,6 +853,17 @@ static int __unmap_and_move(struct page
 		}
 	}
 
+	/*
+	 * Block others from accessing the new page when we get around to
+	 * establishing additional references. We are usually the only one
+	 * holding a reference to newpage at this point. We used to have a BUG
+	 * here if trylock_page(newpage) fails, but would like to allow for
+	 * cases where there might be a race with the previous use of newpage.
+	 * This is much like races on refcount of oldpage: just don't BUG().
+	 */
+	if (unlikely(!trylock_page(newpage)))
+		goto out_unlock;
+
 	if (unlikely(isolated_balloon_page(page))) {
 		/*
 		 * A ballooned page does not need any special attention from
@@ -870,7 +873,7 @@ static int __unmap_and_move(struct page
 		 * the page migration right away (proteced by page lock).
 		 */
 		rc = balloon_page_migrate(newpage, page, mode);
-		goto out_unlock;
+		goto out_unlock_both;
 	}
 
 	/*
@@ -889,30 +892,27 @@ static int __unmap_and_move(struct page
 		VM_BUG_ON_PAGE(PageAnon(page), page);
 		if (page_has_private(page)) {
 			try_to_free_buffers(page);
-			goto out_unlock;
+			goto out_unlock_both;
 		}
-		goto skip_unmap;
-	}
-
-	/* Establish migration ptes or remove ptes */
-	if (page_mapped(page)) {
+	} else if (page_mapped(page)) {
+		/* Establish migration ptes */
 		try_to_unmap(page,
 			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
 		page_was_mapped = 1;
 	}
 
-skip_unmap:
 	if (!page_mapped(page))
 		rc = move_to_new_page(newpage, page, page_was_mapped, mode);
 
 	if (rc && page_was_mapped)
 		remove_migration_ptes(page, page);
 
+out_unlock_both:
+	unlock_page(newpage);
+out_unlock:
 	/* Drop an anon_vma reference if we took one */
 	if (anon_vma)
 		put_anon_vma(anon_vma);
-
-out_unlock:
 	unlock_page(page);
 out:
 	return rc;
@@ -1056,6 +1056,9 @@ static int unmap_and_move_huge_page(new_
 	if (PageAnon(hpage))
 		anon_vma = page_get_anon_vma(hpage);
 
+	if (unlikely(!trylock_page(new_hpage)))
+		goto put_anon;
+
 	if (page_mapped(hpage)) {
 		try_to_unmap(hpage,
 			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
@@ -1068,6 +1071,9 @@ static int unmap_and_move_huge_page(new_
 	if (rc != MIGRATEPAGE_SUCCESS && page_was_mapped)
 		remove_migration_ptes(hpage, hpage);
 
+	unlock_page(new_hpage);
+
+put_anon:
 	if (anon_vma)
 		put_anon_vma(anon_vma);
 

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