UBIFS and page migration (take 3)

[Richard Weinberger]

During page migrations UBIFS gets confused. We triggered this by using CMA
on two different targets.
It turned out that fallback_migrate_page() is not suitable for UBIFS as it
does not copy the PagePrivate flag. Non-trivial block based filesystems
do not notice since they can use buffer_migrate_page().
UBIFS is using this flag among with PageChecked to account free space.
One possible solution is implementing a ->migratepage() function in UBIFS
which does more or less the same as fallback_migrate_page() but also
copies PagePrivate. I'm not at all sure whether this is the way to go.
IMHO either page migration should not happen if ->migratepage() is not implement
or fallback_migrate_page() has to work for all filesystems.

Comments? Flames? :-)

Thanks,
//richard

[PATCH 1/2] mm: Export migrate_page_move_mapping and
[PATCH 2/2] UBIFS: Implement ->migratepage()