UBIFS and page migration (take 2)

[Vlastimil Babka]

On 03/31/2016 11:58 PM, Richard Weinberger wrote:
> 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.
> 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 they 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.

Yes, we could document more thoroughly the expectations of 
fallback_migrate_page() and audit the existing users, but still relying on every 
new address_space_operations instance to verify them isn't without risk. And I 
doubt there can be a default fallback that's guaranteed safe for all filesystems.

> Comments? Flames? :-)
>
> Thanks,
> //richard
>
> [PATCH 1/2] mm: Export migrate_page_move_mapping and
> [PATCH 2/2] UBIFS: Implement ->migratepage()
>