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@xxxxxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxx>
>
> Thanks!
Thanks so much for your review, I will add this to v3.
Kristen
