UBIFS and page migration (take 3)

[Richard Weinberger]

Christoph,

Am 12.05.2016 um 13:49 schrieb Christoph Hellwig:
> Hi Richard,
> 
> the series looks fine to me, but it fails to address the root cause:

Is this a Reviewed-by? :-)

> that we have an inherently dangerous default for ->migratepage that
> assumes that file systems are implemented a certain way.  I think the
> series should also grow a third patch to remove the default and just
> wire it up for the known good file systems, although we'd need some
> input on what known good is.
>
> Any idea what filesystems do get regular testing with code that's using
> CMA? A good approximation might be those that use the bufer_head
> based aops from fs/buffer.c

No idea how much is being tested.
I fear most issues are unknown. At least for UBIFS it took
years to get aware of the issue.
Thanks again to Maxime and Boris for providing a reproducer.

There are two classes of issues:
a) filesystems that use buffer_migrate_page() but shouldn't
b) filesystems that don't implement ->migratepage() and fallback_migrate_page()
   is not suitable.

As starter we could kill the automatic assignment of fallback_migrate_page() and
non-buffer_head filesystems need to figure out whether fallback_migrate_page()
is suitable or not.
UBIFS found out the hard way. ;-\

MM folks, do we have a way to force page migration?
Maybe we can create a generic stress test.

Thanks,
//richard