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

[Michal Hocko <mhocko@suse.com>]

On Tue 26-01-21 12:56:48, David Hildenbrand wrote:
> On 26.01.21 12:46, Michal Hocko wrote:
> > On Thu 21-01-21 14:27:19, Mike Rapoport wrote:
> > > From: Mike Rapoport <rppt@linux.ibm.com>
> > > 
> > > Removing a PAGE_SIZE page from the direct map every time such page is
> > > allocated for a secret memory mapping will cause severe fragmentation of
> > > the direct map. This fragmentation can be reduced by using PMD-size pages
> > > as a pool for small pages for secret memory mappings.
> > > 
> > > Add a gen_pool per secretmem inode and lazily populate this pool with
> > > PMD-size pages.
> > > 
> > > As pages allocated by secretmem become unmovable, use CMA to back large
> > > page caches so that page allocator won't be surprised by failing attempt to
> > > migrate these pages.
> > > 
> > > The CMA area used by secretmem is controlled by the "secretmem=" kernel
> > > parameter. This allows explicit control over the memory available for
> > > secretmem and provides upper hard limit for secretmem consumption.
> > 
> > OK, so I have finally had a look at this closer and this is really not
> > acceptable. I have already mentioned that in a response to other patch
> > but any task is able to deprive access to secret memory to other tasks
> > and cause OOM killer which wouldn't really recover ever and potentially
> > panic the system. Now you could be less drastic and only make SIGBUS on
> > fault but that would be still quite terrible. There is a very good
> > reason why hugetlb implements is non-trivial reservation system to avoid
> > exactly these problems.
> > 
> > So unless I am really misreading the code
> > Nacked-by: Michal Hocko <mhocko@suse.com>
> > 
> > That doesn't mean I reject the whole idea. There are some details to
> > sort out as mentioned elsewhere but you cannot really depend on
> > pre-allocated pool which can fail at a fault time like that.
> 
> So, to do it similar to hugetlbfs (e.g., with CMA), there would have to be a
> mechanism to actually try pre-reserving (e.g., from the CMA area), at which
> point in time the pages would get moved to the secretmem pool, and a
> mechanism for mmap() etc. to "reserve" from these secretmem pool, such that
> there are guarantees at fault time?

yes, reserve at mmap time and use during the fault. But this all sounds
like a self inflicted problem to me. Sure you can have a pre-allocated
or more dynamic pool to reduce the direct mapping fragmentation but you
can always fall back to regular allocatios. In other ways have the pool
as an optimization rather than a hard requirement. With a careful access
control this sounds like a manageable solution to me.
-- 
Michal Hocko
SUSE Labs

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