Re: [PATCH 1/3] mm: Don't blindly assign fallback_migrate_page()

[Richard Weinberger]

Am 17.06.2016 um 20:27 schrieb Michal Hocko:
> On Fri 17-06-16 18:55:45, Richard Weinberger wrote:
>> Am 17.06.2016 um 18:28 schrieb Michal Hocko:
>>> 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