Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory

Huang, Kai
2022-12-13T23:49:38Z

>
> memfd_restricted() itself is implemented as a shim layer on top of real
> memory file systems (currently tmpfs). Pages in restrictedmem are marked
> as unmovable and unevictable, this is required for current confidential
> usage. But in future this might be changed.
>
>
I didn't dig full histroy, but I interpret this as we don't support page
migration and swapping for restricted memfd for now. IMHO "page marked as
unmovable" can be confused with PageMovable(), which is a different thing from
this series. It's better to just say something like "those pages cannot be
migrated and swapped".

[...]

> +
> + /*
> + * These pages are currently unmovable so don't place them into movable
> + * pageblocks (e.g. CMA and ZONE_MOVABLE).
> + */
> + mapping = memfd->f_mapping;
> + mapping_set_unevictable(mapping);
> + mapping_set_gfp_mask(mapping,
> + mapping_gfp_mask(mapping) & ~__GFP_MOVABLE);

But, IIUC removing __GFP_MOVABLE flag here only makes page allocation from non-
movable zones, but doesn't necessarily prevent page from being migrated. My
first glance is you need to implement either a_ops->migrate_folio() or just
get_page() after faulting in the page to prevent.

So I think the comment also needs improvement -- IMHO we can just call out
currently those pages cannot be migrated and swapped, which is clearer (and the
latter justifies mapping_set_unevictable() clearly).