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

[David Hildenbrand]

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@xxxxxxxxxxxxx>
>>
>> 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.

But my suggestion was not well received :)

-- 
Thanks,

David / dhildenb