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

Michal Hocko mhocko at suse.com
Tue Feb 2 08:27:14 EST 2021

On Tue 02-02-21 14:48:57, Mike Rapoport wrote:
> On Tue, Feb 02, 2021 at 10:35:05AM +0100, Michal Hocko wrote:
> > On Mon 01-02-21 08:56:19, James Bottomley wrote:
> > 
> > I have also proposed potential ways out of this. Either the pool is not
> > fixed sized and you make it a regular unevictable memory (if direct map
> > fragmentation is not considered a major problem)
> I think that the direct map fragmentation is not a major problem, and the
> data we have confirms it, so I'd be more than happy to entirely drop the
> pool, allocate memory page by page and remove each page from the direct
> map. 
> Still, we cannot prove negative and it could happen that there is a
> workload that would suffer a lot from the direct map fragmentation, so
> having a pool of large pages upfront is better than trying to fix it
> afterwards. As we get more confidence that the direct map fragmentation is
> not an issue as it is common to believe we may remove the pool altogether.

I would drop the pool altogether and instantiate pages to the
unevictable LRU list and internally treat it as ramdisk/mlock so you
will get an accounting correctly. The feature should be still opt-in
(e.g. a kernel command line parameter) for now. The recent report by
Intel (http://lkml.kernel.org/r/213b4567-46ce-f116-9cdf-bbd0c884eb3c@linux.intel.com)
there is no clear win to have huge mappings in _general_ but there are
still workloads which benefit. 
> I think that using PMD_ORDER allocations for the pool with a fallback to
> order 0 will do the job, but unfortunately I doubt we'll reach a consensus
> about this because dogmatic beliefs are hard to shake...

If this is opt-in then those beliefs can be relaxed somehow. Long term
it makes a lot of sense to optimize for a better direct map management
but I do not think this is a hard requirement for an initial
implementation if it is not imposed to everybody by default.

> A more restrictive possibility is to still use plain PMD_ORDER allocations
> to fill the pool, without relying on CMA. In this case there will be no
> global secretmem specific pool to exhaust, but then it's possible to drain
> high order free blocks in a system, so CMA has an advantage of limiting
> secretmem pools to certain amount of memory with somewhat higher
> probability for high order allocation to succeed. 
> > or you need a careful access control 
> Do you mind elaborating what do you mean by "careful access control"?

As already mentioned, a mechanism to control who can use this feature -
e.g. make it a special device which you can access control by
permissions or higher level security policies. But that is really needed
only if the pool is fixed sized.
> > or you need SIGBUS on the mmap failure (to allow at least some fallback
> > mode to caller).
> As I've already said, I agree that SIGBUS is way better than OOM at #PF
> time.

It would be better than OOM but it would still be a terrible interface.
So I would go that path only as a last resort. I do not even want to
think what kind of security consequences that would have. E.g. think of
somebody depleting the pool and pushing security sensitive workload into
fallback which is not backed by security memory.

> And we can add some means to fail at mmap() time if the pools are running
> low.

Welcome to hugetlb reservation world...

Michal Hocko

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