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

[Mike Rapoport]

On Tue, Jan 26, 2021 at 01:08:23PM +0100, Michal Hocko wrote:
> 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 at 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, if I understand your concerns correct this implementation has two
issues:
1) allocation failure at page fault that causes unrecoverable OOM and
2) a possibility for an unprivileged user to deplete secretmem pool and
cause (1) to others

I'm not really familiar with OOM internals, but when I simulated an
allocation failure in my testing only the allocating process and it's
parent were OOM-killed and then the system continued normally. 

You are right, it would be better if we SIGBUS instead of OOM but I don't
agree SIGBUS is terrible. As we started to draw parallels with hugetlbfs
even despite it's complex reservation system, hugetlb_fault() may fail to
allocate pages from CMA and this still will cause SIGBUS.

And hugetlb pools may be also depleted by anybody by calling
mmap(MAP_HUGETLB) and there is no any limiting knob for this, while
secretmem has RLIMIT_MEMLOCK.

That said, simply replacing VM_FAULT_OOM with VM_FAULT_SIGBUS makes
secretmem at least as controllable and robust than hugeltbfs even without
complex reservation at mmap() time.

> > > So unless I am really misreading the code
> > > Nacked-by: Michal Hocko <mhocko at 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.

I'd really wish we had this discussion for earlier spins of this series,
but since this didn't happen let's refresh the history a bit.

One of the major pushbacks on the first RFC [1] of the concept was about
the direct map fragmentation. I tried really hard to find data that shows
what is the performance difference with different page sizes in the direct
map and I didn't find anything.

So presuming that large pages do provide advantage the first implementation
of secretmem used PMD_ORDER allocations to amortise the effect of the
direct map fragmentation and then handed out 4k pages at each fault. In
addition there was an option to reserve a finite pool at boot time and
limit secretmem allocations only to that pool.

At some point David suggested to use CMA to improve overall flexibility
[3], so I switched secretmem to use CMA.

Now, with the data we have at hand (my benchmarks and Intel's report David
mentioned) I'm even not sure this whole pooling even required.

I like the idea to have a pool as an optimization rather than a hard
requirement but I don't see why would it need a careful access control. As
the direct map fragmentation is not necessarily degrades the performance
(and even sometimes it actually improves it) and even then the degradation
is small, trying a PMD_ORDER allocation for a pool and then falling back to
4K page may be just fine.

I think we could have something like this (error handling is mostly
omitted):

static int secretmem_pool_increase(struct secretmem_ctx *ctx, gfp_t gfp)
{
	struct page *page = alloc_pages(gfp, PMD_PAGE_ORDER);

	if (!page)
		return -ENOMEM;

	/* add large page to pool */
	
	return 0;
}

static struct page *secretmem_alloc_page(struct secretmem_ctx *ctx,
					 gfp_t gfp)
{
	struct page *page;
	...

	if (gen_pool_avail(pool) < PAGE_SIZE) {
		err = secretmem_pool_increase(ctx, gfp);
		if (!err) {
			addr = gen_pool_alloc(pool, PAGE_SIZE);
			if (addr)
				page = virt_to_page(addr);
		}
	}

	if (!page)
		page = alloc_page(gfp);

	return page;	
}

[1] https://lore.kernel.org/lkml/1572171452-7958-1-git-send-email-rppt@kernel.org/
[2] https://lore.kernel.org/lkml/20200720092435.17469-1-rppt@kernel.org/
[3] https://lore.kernel.org/lkml/03ec586d-c00c-c57e-3118-7186acb7b823@redhat.com/#t

-- 
Sincerely yours,
Mike.