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

[Peter Zijlstra]

On Fri, Sep 25, 2020 at 11:00:30AM +0200, David Hildenbrand wrote:
> On 25.09.20 09:41, Peter Zijlstra wrote:
> > 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.
> 
> As I expressed earlier, I would prefer allowing allocation of secretmem
> only from a previously defined CMA area. This would physically locally
> limit the pain.

Given that this thing doesn't have a migrate hook, that seems like an
eminently reasonable contraint. Because not only will it mess up the
directmap, it will also destroy the ability of the page-allocator /
compaction to re-form high order blocks by sprinkling holes throughout.

Also, this is all very close to XPFO, yet I don't see that mentioned
anywhere.

Further still, it has this HAVE_SECRETMEM_UNCACHED nonsense which is
completely unused. I'm not at all sure exposing UNCACHED to random
userspace is a sane idea.