On Thu, Sep 24, 2020 at 04:29:03PM +0300, Mike Rapoport wrote:

From: Mike Rapoport <redacted>

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.