Date:   Fri, 20 May 2022 08:43:19 -0700
From:   Kristen Carlson Accardi <kristen@...ux.intel.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     linux-sgx@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...e.com, roman.gushchin@...ux.dev,
        hannes@...xchg.org, shakeelb@...gle.com
Subject: Re: [PATCH v2] x86/sgx: Set active memcg prior to shmem allocation

On Fri, 2022-05-20 at 01:31 +0300, Jarkko Sakkinen 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@...ux.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;
> > +}
> > +
> > +/**
> > + * sgx_encl_alloc_backing() - allocate a new backing storage page
> > + * @encl:	an enclave pointer
> > + * @page_index:	enclave page index
> > + * @backing:	data for accessing backing storage for the page
> > + *
> > + * When called from ksgxd, sets the active memcg from one of the
> > + * mms in the enclave's mm_list prior to any backing page
> > allocation,
> > + * in order to ensure that shmem page allocations are charged to
> > the
> > + * enclave.
> > + *
> > + * Return:
> > + *   0 on success,
> > + *   -errno otherwise.
> > + */
> > +int sgx_encl_alloc_backing(struct sgx_encl *encl, unsigned long
> > page_index,
> > +			   struct sgx_backing *backing)
> > +{
> > +	struct mem_cgroup *memcg, *old_memcg;
> > +	int ret;
> > +
> > +	memcg = sgx_encl_get_mem_cgroup(encl);
> > +
> > +	old_memcg = set_active_memcg(memcg);
> > +
> > +	ret = sgx_encl_get_backing(encl, page_index, backing);
> > +
> > +	set_active_memcg(old_memcg);
> > +
> > +	mem_cgroup_put(memcg);
> 
> This is too sparse IMHO.
> 
> I would rewrite it as:
> 
> 	struct mem_cgroup *encl_memcg = sgx_encl_get_mem_cgroup(encl);
> 	struct mem_cgroup *memcg = set_active_memcg(encl_memcg);
> 	int ret;
> 
> 	ret = sgx_encl_get_backing(encl, page_index, backing);
> 
> 	set_active_memcg(memcg);
> 	mem_cgroup_put(encl_memcg);
> 
> I think old_memcg is not very documentative, whereas this also
> dclearly
> tells what is going on: enclave's memcg is temporarily swapped in
> order to
> perform page allocation, so that the allocations gets accounted from
> them
> correct cgroup.

Thanks for your feedback, I will incorporate it into v3.

--Kristen

> 
> > +
> > +	return ret;
> > +}
> > +
> > +/**
> > + * sgx_encl_lookup_backing() - retrieve an existing backing
> > storage page
> > + * @encl:	an enclave pointer
> > + * @page_index:	enclave page index
> > + * @backing:	data for accessing backing storage for the page
> > + *
> > + * Retrieve a backing page for loading data back into an EPC page
> > with ELDU.
> > + * It is the caller's responsibility to ensure that it is
> > appropriate to use
> > + * sgx_encl_lookup_backing() rather than sgx_encl_alloc_backing().
> > If lookup is
> > + * not used correctly, this will cause an allocation which is not
> > accounted for.
> > + *
> > + * Return:
> > + *   0 on success,
> > + *   -errno otherwise.
> > + */
> > +int sgx_encl_lookup_backing(struct sgx_encl *encl, unsigned long
> > page_index,
> > +			   struct sgx_backing *backing)
> > +{
> > +	return sgx_encl_get_backing(encl, page_index, backing);
> > +}
> > +
> >  /**
> >   * sgx_encl_put_backing() - Unpin the backing storage
> >   * @backing:	data for accessing backing storage for the page
> > diff --git a/arch/x86/kernel/cpu/sgx/encl.h
> > b/arch/x86/kernel/cpu/sgx/encl.h
> > index fec43ca65065..2de3b150ab00 100644
> > --- a/arch/x86/kernel/cpu/sgx/encl.h
> > +++ b/arch/x86/kernel/cpu/sgx/encl.h
> > @@ -100,13 +100,20 @@ static inline int sgx_encl_find(struct
> > mm_struct *mm, unsigned long addr,
> >  	return 0;
> >  }
> >  
> > +static inline bool current_is_ksgxd(void)
> > +{
> > +	return current->mm ? false : true;
> > +}
> > +
> >  int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
> >  		     unsigned long end, unsigned long vm_flags);
> >  
> >  void sgx_encl_release(struct kref *ref);
> >  int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm);
> > -int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long
> > page_index,
> > -			 struct sgx_backing *backing);
> > +int sgx_encl_lookup_backing(struct sgx_encl *encl, unsigned long
> > page_index,
> > +			    struct sgx_backing *backing);
> > +int sgx_encl_alloc_backing(struct sgx_encl *encl, unsigned long
> > page_index,
> > +			   struct sgx_backing *backing);
> >  void sgx_encl_put_backing(struct sgx_backing *backing, bool
> > do_write);
> >  int sgx_encl_test_and_clear_young(struct mm_struct *mm,
> >  				  struct sgx_encl_page *page);
> > diff --git a/arch/x86/kernel/cpu/sgx/main.c
> > b/arch/x86/kernel/cpu/sgx/main.c
> > index 4b41efc9e367..7d41c8538795 100644
> > --- a/arch/x86/kernel/cpu/sgx/main.c
> > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > @@ -310,7 +310,7 @@ static void sgx_reclaimer_write(struct
> > sgx_epc_page *epc_page,
> >  	encl->secs_child_cnt--;
> >  
> >  	if (!encl->secs_child_cnt && test_bit(SGX_ENCL_INITIALIZED,
> > &encl->flags)) {
> > -		ret = sgx_encl_get_backing(encl, PFN_DOWN(encl->size),
> > +		ret = sgx_encl_alloc_backing(encl, PFN_DOWN(encl-
> > >size),
> >  					   &secs_backing);
> >  		if (ret)
> >  			goto out;
> > @@ -381,7 +381,7 @@ static void sgx_reclaim_pages(void)
> >  			goto skip;
> >  
> >  		page_index = PFN_DOWN(encl_page->desc - encl_page-
> > >encl->base);
> > -		ret = sgx_encl_get_backing(encl_page->encl, page_index,
> > &backing[i]);
> > +		ret = sgx_encl_alloc_backing(encl_page->encl,
> > page_index, &backing[i]);
> >  		if (ret)
> >  			goto skip;
> >  
> > -- 
> > 2.20.1
> > 
> 
> BR, Jarkko

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