On Fri, Sep 25, 2020 at 11:00:30AM +0200, David Hildenbrand wrote:

On 25.09.20 09:41, Peter Zijlstra wrote:

quoted

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

quoted

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.

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.

The prevois version contained a patch that allowed reserving a memory
pool for the secretmem at boot time to avpoid splitting pages from the
direct map

But my suggestion was not well received :)

The disagreemet was only whether to use CMA or simple boot time
reservation :-P

-- 
Thanks,

David / dhildenb

-- 
Sincerely yours,
Mike.