On Thu, Sep 24, 2020 at 04:29:03PM +0300, Mike Rapoport wrote:
From: Mike Rapoport firstname.lastname@example.org
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.