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

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Message-ID: <5c6e2e516f19b0a030eae9bf073d555c57ca1f21.camel@intel.com>
Date:   Tue, 13 Dec 2022 23:49:13 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
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Subject: Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to
 create restricted user memory

> 
> 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).