[PATCH v16 07/11] secretmem: use PMD-size pages to amortize direct map fragmentation

[James Bottomley]

On Fri, 2021-01-29 at 09:23 +0100, Michal Hocko wrote:
> On Thu 28-01-21 13:05:02, James Bottomley wrote:
> > Obviously the API choice could be revisited
> > but do you have anything to add over the previous discussion, or is
> > this just to get your access control?
> 
> Well, access control is certainly one thing which I still believe is
> missing. But if there is a general agreement that the direct map
> manipulation is not that critical then this will become much less of
> a problem of course.

The secret memory is a scarce resource but it's not a facility that
should only be available to some users.

> It all boils down whether secret memory is a scarce resource. With
> the existing implementation it really is. It is effectivelly
> repeating same design errors as hugetlb did. And look now, we have a
> subtle and convoluted reservation code to track mmap requests and we
> have a cgroup controller to, guess what, have at least some control
> over distribution if the preallocated pool. See where am I coming
> from?

I'm fairly sure rlimit is the correct way to control this.  The
subtlety in both rlimit and memcg tracking comes from deciding to
account under an existing category rather than having our own new one. 
People don't like new stuff in accounting because it requires
modifications to everything in userspace.  Accounting under and
existing limit keeps userspace the same but leads to endless arguments
about which limit it should be under.  It took us several patch set
iterations to get to a fragile consensus on this which you're now
disrupting for reasons you're not making clear.

> If the secret memory is more in line with mlock without any imposed
> limit (other than available memory) in the end then, sure, using the
> same access control as mlock sounds reasonable. Btw. if this is
> really just a more restrictive mlock then is there any reason to not
> hook this into the existing mlock infrastructure (e.g.
> MCL_EXCLUSIVE)? Implications would be that direct map would be
> handled on instantiation/tear down paths, migration would deal with
> the same (if possible). Other than that it would be mlock like.

In the very first patch set we proposed a mmap flag to do this.  Under
detailed probing it emerged that this suffers from several design
problems: the KVM people want VMM to be able to remove the secret
memory range from the process; there may be situations where sharing is
useful and some people want to be able to seal the operations.  All of
this ended up convincing everyone that a file descriptor based approach
was better than a mmap one.

James