Remove page migration fallback (was: UBIFS and page migration)

[Richard Weinberger <richard@nod.at>]

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.

In order to address this issue implement a convenient ->migratepage()
function for UBIFS and disable the automatic assignment of
fallback_migrate_page(). Filesystems maintains should decide themselves
whether they have to implement ->migratepage() or can use the generic function.

Another interesting topic is testing ->migratepage(). So far the only reliable
test to trigger the UBIFS issue we have is real hardware and CMA.
I was able to trigger it a few times in KVM using the migrate_pages() system call.
But not reliable at all.

Thanks,
//richard

[PATCH 1/3] mm: Don't blindly assign fallback_migrate_page()
[PATCH 2/3] mm: Export migrate_page_move_mapping and
[PATCH 3/3] UBIFS: Implement ->migratepage()

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