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 email@example.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.
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
David / dhildenb