Am 17.06.2016 um 20:27 schrieb Michal Hocko:

On Fri 17-06-16 18:55:45, Richard Weinberger wrote:

quoted

Am 17.06.2016 um 18:28 schrieb Michal Hocko:

quoted

But doesn't this disable the page migration and so potentially reduce
the compaction success rate for the large pile of filesystems? Without
any hint about that?

The WARN_ON_ONCE() is the hint. ;)

Right. My reply turned a different way than I meant... I meant to say
that there might be different regressions caused by this change without much
hint that a particular warning would be the smoking gun... 

Okay, what about something like that?
That way everything works as before and we don't have regressions
but FS maintainers will notice the WARN_ON_ONCE() and hopefully review
whether generic_migrate_page() is really suitable.
If so, they can set their a_ops->migratepage to generic_migrate_page().

@@ -771,8 +773,15 @@ static int move_to_new_page(struct page *newpage, struct page *page,
                 * is the most common path for page migration.
                 */
                rc = mapping->a_ops->migratepage(mapping, newpage, page, mode);
-       else
-               rc = fallback_migrate_page(mapping, newpage, page, mode);
+       else {
+               /*
+                * Dear filesystem maintainer, please verify whether
+                * generic_migrate_page() is suitable for your
+                * filesystem, especially wrt. page flag handling.
+                */
+               WARN_ON_ONCE(1);
+               rc = generic_migrate_page(mapping, newpage, page, mode);
+       }

        /*
         * When successful, old pagecache page->mapping must be cleared before

Thanks,
//richard

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