Re: [PATCH v5 12/18] x86/sgx: Add EPC OOM path to forcefully reclaim EPC

[Huang, Kai]

On Fri, 2023-09-22 at 20:06 -0700, Haitao Huang wrote:
> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> 
> Introduce the OOM path for killing an enclave with a reclaimer that is no
> longer able to reclaim enough EPC pages. Find a victim enclave, which
> will be an enclave with only "unreclaimable" EPC pages left in the
> cgroup LRU lists. Once a victim is identified, mark the enclave as OOM
> and zap the enclave's entire page range, and drain all mm references in
> encl->mm_list. Block allocating any EPC pages in #PF handler, or
> reloading any pages in all paths, or creating any new mappings.
> 
> The OOM killing path may race with the reclaimers: in some cases, the
> victim enclave is in the process of reclaiming the last EPC pages when
> OOM happens, that is, all pages other than SECS and VA pages are in
> RECLAIMING_IN_PROGRESS state. The reclaiming process requires access to
> the enclave backing, VA pages as well as SECS. So the OOM killer does
> not directly release those enclave resources, instead, it lets all
> reclaiming in progress to finish, and relies (as currently done) on
> kref_put on encl->refcount to trigger sgx_encl_release() to do the
> final cleanup.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Co-developed-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> Co-developed-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> Signed-off-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> V5:
> - Rename SGX_ENCL_OOM to SGX_ENCL_NO_MEMORY
> 
> V4:
> - Updates for patch reordering and typo fixes.
> 
> V3:
> - Rebased to use the new VMA_ITERATOR to zap VMAs.
> - Fixed the racing cases by blocking new page allocation/mapping and
> reloading when enclave is marked for OOM. And do not release any enclave
> resources other than draining mm_list entries, and let pages in
> RECLAIMING_IN_PROGRESS to be reaped by reclaimers.
> - Due to above changes, also removed the no-longer needed encl->lock in
> the OOM path which was causing deadlocks reported by the lock prover.
> 

[...]

> +
> +/**
> + * sgx_epc_oom() - invoke EPC out-of-memory handling on target LRU
> + * @lru:	LRU that is low
> + *
> + * Return:	%true if a victim was found and kicked.
> + */
> +bool sgx_epc_oom(struct sgx_epc_lru_lists *lru)
> +{
> +	struct sgx_epc_page *victim;
> +
> +	spin_lock(&lru->lock);
> +	victim = sgx_oom_get_victim(lru);
> +	spin_unlock(&lru->lock);
> +
> +	if (!victim)
> +		return false;
> +
> +	if (victim->flags & SGX_EPC_OWNER_PAGE)
> +		return sgx_oom_encl_page(victim->encl_page);
> +
> +	if (victim->flags & SGX_EPC_OWNER_ENCL)
> +		return sgx_oom_encl(victim->encl);

I hate to bring this up, at least at this stage, but I am wondering why we need
to put VA and SECS pages to the unreclaimable list, but cannot keep an
"enclave_list" instead?

So by looking the patch (" x86/sgx: Limit process EPC usage with misc cgroup
controller"), if I am not missing anything, the whole "unreclaimable" list is
just used to find the victim enclave when OOM needs to be done.  Thus, I don't
see why "enclave_list" cannot be used to achieve this.

The reason that I am asking is because it seems using "enclave_list" we can
simplify the code.  At least the patches related to track VA/SECS pages, and the
SGX_EPC_OWNER_PAGE/SGX_EPC_OWNER_ENCL thing can be eliminated completely.  

Using "enclave_list", I guess you just need to put the enclave to the current
EPC cgroup when SECS page is allocated.

In fact, putting "unreclaimable" list to LRU itself is a little bit confusing
because: 1) you cannot really reclaim anything from the list; 2) VA/SECS pages
don't have the concept of "young" at all, thus makes no sense to annotate they
as LRU.

Thus putting VA/SECS to "unreclaimable" list, instead of keeping an
"enclave_list" seems won't have any benefit but will only make the code more
complicated.

Or am I missing anything?