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

[Michal Hocko]

On Tue 02-02-21 21:10:40, Mike Rapoport wrote:
> On Tue, Feb 02, 2021 at 02:27:14PM +0100, Michal Hocko wrote:
> > 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.
>   
> Let me reiterate to make sure I don't misread your suggestion.
> 
> If we make secretmem an opt-in feature with, e.g. kernel parameter, the
> pooling of large pages is unnecessary. In this case there is no limited
> resource we need to protect because secretmem will allocate page by page.

Yes.

> Since there is no limited resource, we don't need special permissions
> to access secretmem so we can move forward with a system call that creates
> a mmapable file descriptor and save the hassle of a chardev.

Yes, I assume you implicitly assume mlock rlimit here. Also memcg
accounting should be in place. Wrt to the specific syscall, please
document why existing interfaces are not a good fit as well. It would be
also great to describe interaction with mlock itself (I assume the two
to be incompatible - mlock will fail on and mlockall will ignore it).

> I cannot say I don't like this as it cuts roughly half of mm/secretmem.c :)
> 
> But I must say I am still a bit concerned about that we have no provisions
> here for dealing with the direct map fragmentation even with the set goal
> to improve the direct map management in the long run...

Yes that is something that will be needed long term. I do not think this
is strictly necessary for the initial submission, though. The
implementation should be as simple as possible now and complexity added
on top.
-- 
Michal Hocko
SUSE Labs