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

[Michal Hocko]

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.

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?

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.
-- 
Michal Hocko
SUSE Labs