Re: [PATCH v2] x86/sgx: Set active memcg prior to shmem allocation

[Kristen Carlson Accardi <kristen@linux.intel.com>]

On Thu, 2022-05-19 at 18:24 -0700, Roman Gushchin wrote:
> On Thu, May 19, 2022 at 02:04:45PM -0700, Kristen Carlson Accardi
> wrote:
> > When the system runs out of enclave memory, SGX can reclaim EPC
> > pages
> > by swapping to normal RAM. These backing pages are allocated via a
> > per-enclave shared memory area. Since SGX allows unlimited over
> > commit on EPC memory, the reclaimer thread can allocate a large
> > number of backing RAM pages in response to EPC memory pressure.
> > 
> > When the shared memory backing RAM allocation occurs during
> > the reclaimer thread context, the shared memory is charged to
> > the root memory control group, and the shmem usage of the enclave
> > is not properly accounted for, making cgroups ineffective at
> > limiting the amount of RAM an enclave can consume.
> > 
> > For example, when using a cgroup to launch a set of test
> > enclaves, the kernel does not properly account for 50% - 75% of
> > shmem page allocations on average. In the worst case, when
> > nearly all allocations occur during the reclaimer thread, the
> > kernel accounts less than a percent of the amount of shmem used
> > by the enclave's cgroup to the correct cgroup.
> > 
> > SGX stores a list of mm_structs that are associated with
> > an enclave. Pick one of them during reclaim and charge that
> > mm's memcg with the shmem allocation. The one that gets picked
> > is arbitrary, but this list almost always only has one mm. The
> > cases where there is more than one mm with different memcg's
> > are not worth considering.
> > 
> > Create a new function - sgx_encl_alloc_backing(). This function
> > is used whenever a new backing storage page needs to be
> > allocated. Previously the same function was used for page
> > allocation as well as retrieving a previously allocated page.
> > Prior to backing page allocation, if there is a mm_struct
> > associated
> > with the enclave that is requesting the allocation, it is set
> > as the active memory control group.
> > 
> > Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
> > ---
> > V1 -> V2:
> >  Changed sgx_encl_set_active_memcg() to simply return the correct
> >  memcg for the enclave and renamed to sgx_encl_get_mem_cgroup().
> > 
> >  Created helper function current_is_ksgxd() to improve readability.
> > 
> >  Use mmget_not_zero()/mmput_async() when searching mm_list.
> > 
> >  Move call to set_active_memcg() to sgx_encl_alloc_backing() and
> >  use mem_cgroup_put() to avoid leaking a memcg reference.
> > 
> >  Address review feedback regarding comments and commit log.
> > 
> >  arch/x86/kernel/cpu/sgx/encl.c | 109
> > ++++++++++++++++++++++++++++++++-
> >  arch/x86/kernel/cpu/sgx/encl.h |  11 +++-
> >  arch/x86/kernel/cpu/sgx/main.c |   4 +-
> >  3 files changed, 118 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/sgx/encl.c
> > b/arch/x86/kernel/cpu/sgx/encl.c
> > index 001808e3901c..6d10202612d6 100644
> > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > @@ -32,7 +32,7 @@ static int __sgx_encl_eldu(struct sgx_encl_page
> > *encl_page,
> >  	else
> >  		page_index = PFN_DOWN(encl->size);
> >  
> > -	ret = sgx_encl_get_backing(encl, page_index, &b);
> > +	ret = sgx_encl_lookup_backing(encl, page_index, &b);
> >  	if (ret)
> >  		return ret;
> >  
> > @@ -574,7 +574,7 @@ static struct page
> > *sgx_encl_get_backing_page(struct sgx_encl *encl,
> >   *   0 on success,
> >   *   -errno otherwise.
> >   */
> > -int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long
> > page_index,
> > +static int sgx_encl_get_backing(struct sgx_encl *encl, unsigned
> > long page_index,
> >  			 struct sgx_backing *backing)
> >  {
> >  	pgoff_t pcmd_index = PFN_DOWN(encl->size) + 1 + (page_index >>
> > 5);
> > @@ -601,6 +601,111 @@ int sgx_encl_get_backing(struct sgx_encl
> > *encl, unsigned long page_index,
> >  	return 0;
> >  }
> >  
> > +/*
> > + * When called from ksgxd, returns the mem_cgroup of a struct mm
> > stored
> > + * in the enclave's mm_list. When not called from ksgxd, just
> > returns
> > + * the mem_cgroup of the current task.
> > + */
> > +static struct mem_cgroup *sgx_encl_get_mem_cgroup(struct sgx_encl
> > *encl)
> > +{
> > +	struct mem_cgroup *memcg = NULL;
> > +	struct sgx_encl_mm *encl_mm;
> > +	int idx;
> > +
> > +	/*
> > +	 * If called from normal task context, return the mem_cgroup
> > +	 * of the current task's mm. The remainder of the handling is
> > for
> > +	 * ksgxd.
> > +	 */
> > +	if (!current_is_ksgxd())
> > +		return get_mem_cgroup_from_mm(current->mm);
> > +
> > +	/*
> > +	 * Search the enclave's mm_list to find an mm associated with
> > +	 * this enclave to charge the allocation to.
> > +	 */
> > +	idx = srcu_read_lock(&encl->srcu);
> > +
> > +	list_for_each_entry_rcu(encl_mm, &encl->mm_list, list) {
> > +		if (!mmget_not_zero(encl_mm->mm))
> > +			continue;
> > +
> > +		memcg = get_mem_cgroup_from_mm(encl_mm->mm);
> > +
> > +		mmput_async(encl_mm->mm);
> > +
> > +		break;
> > +	}
> > +
> > +	srcu_read_unlock(&encl->srcu, idx);
> > +
> > +	/*
> > +	 * In the rare case that there isn't an mm associated with
> > +	 * the enclave, set memcg to the current active mem_cgroup.
> > +	 * This will be the root mem_cgroup if there is no active
> > +	 * mem_cgroup.
> > +	 */
> > +	if (!memcg)
> > +		return get_mem_cgroup_from_mm(NULL);
> > +
> > +	return memcg;
> > +}
> 
> You can simplify the function a bit. But it's up to you, not a strong
> opinion.
> 
> static struct mem_cgroup *sgx_encl_get_mem_cgroup(struct sgx_encl
> *encl)
> {
> 	struct mem_cgroup *memcg = NULL;
> 	struct sgx_encl_mm *encl_mm;
> 	int idx;
> 
> 	if (current_is_ksgxd()) {
> 		/*
> 		 * Search the enclave's mm_list to find an mm
> associated with
> 		 * this enclave to charge the allocation to.
> 		 */
> 		idx = srcu_read_lock(&encl->srcu);
> 		list_for_each_entry_rcu(encl_mm, &encl->mm_list, list)
> {
> 			if (!mmget_not_zero(encl_mm->mm))
> 				continue;
> 
> 			memcg = get_mem_cgroup_from_mm(encl_mm->mm);
> 			mmput_async(encl_mm->mm);
> 			break;
> 		}
> 		srcu_read_unlock(&encl->srcu, idx);
> 	}
> 
> 	return memcg ? memcg : get_mem_cgroup_from_mm(current->mm);
> }
> 

I don't have strong opinions on this either - I actually had it written
this way originally but then decided maybe other people would find it
more readable the other way. I definitely don't care either way.

> --
> 
> The rest of the patch looks good to me. Please, feel free to add:
> 
> Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
> 
> Thanks!

Thanks so much for your review, I will add this to v3.

Kristen