[PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize direct map fragmentation

[Peter Zijlstra]

On Thu, Sep 24, 2020 at 04:29:03PM +0300, Mike Rapoport wrote:
> From: Mike Rapoport <rppt at linux.ibm.com>
> 
> Removing a PAGE_SIZE page from the direct map every time such page is
> allocated for a secret memory mapping will cause severe fragmentation of
> the direct map. This fragmentation can be reduced by using PMD-size pages
> as a pool for small pages for secret memory mappings.
> 
> Add a gen_pool per secretmem inode and lazily populate this pool with
> PMD-size pages.

What's the actual efficacy of this? Since the pmd is per inode, all I
need is a lot of inodes and we're in business to destroy the directmap,
no?

Afaict there's no privs needed to use this, all a process needs is to
stay below the mlock limit, so a 'fork-bomb' that maps a single secret
page will utterly destroy the direct map.

I really don't like this, at all.

IIRC Kirill looked at merging the directmap. I think he ran into
performance issues there, but we really need something like that before
something like this lands.