On 02.02.21 13:48, 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 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...
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.
I am not really concerned about fragmenting/breaking up the direct map
as long as the feature has to be explicitly enabled (similar to
fragmenting the vmemmap).
As already expressed, I dislike allowing user space to consume an
unlimited number unmovable/unmigratable allocations. We already have
that in some cases with huge pages (when the arch does not support
migration) - but there we can at least manage the consumption using the
whole max/reserved/free/... infrastructure. In addition, adding arch
support for migration shouldn't be too complicated.
The idea of using CMA is quite good IMHO, because there we can locally
limit the direct map fragmentation and don't have to bother about
migration at all. We own the area, so we can place as many unmovable
allocations on it as we can fit.
But it sounds like, we would also need some kind of reservation
mechanism in either scenario (CMA vs. no CMA).
If we don't want to go full-circle on max/reserved/free/..., allowing
for migration of secretmem pages would make sense. Then, these pages
become "less special". Map source, copy, unmap destination. The security
implementations are the ugly part. I wonder if we could temporarily map
somewhere else, so avoiding to touch the direct map during migration.
--
Thanks,
David / dhildenb
