Re: [PATCH v9 00/15] Add Cgroup support for SGX EPC memory

From: Mikko Ylinen
Date: Thu Feb 08 2024 - 03:46:08 EST

On Mon, Feb 05, 2024 at 01:06:23PM -0800, Haitao Huang wrote:
> SGX Enclave Page Cache (EPC) memory allocations are separate from normal
> RAM allocations, and are managed solely by the SGX subsystem. The existing
> cgroup memory controller cannot be used to limit or account for SGX EPC
> memory, which is a desirable feature in some environments, e.g., support
> for pod level control in a Kubernates cluster on a VM or bare-metal host
> [1,2].
> This patchset implements the support for sgx_epc memory within the misc
> cgroup controller. A user can use the misc cgroup controller to set and
> enforce a max limit on total EPC usage per cgroup. The implementation
> reports current usage and events of reaching the limit per cgroup as well
> as the total system capacity.
> Much like normal system memory, EPC memory can be overcommitted via virtual
> memory techniques and pages can be swapped out of the EPC to their backing
> store, which are normal system memory allocated via shmem and accounted by
> the memory controller. Similar to per-cgroup reclamation done by the memory
> controller, the EPC misc controller needs to implement a per-cgroup EPC
> reclaiming process: when the EPC usage of a cgroup reaches its hard limit
> ('sgx_epc' entry in the 'misc.max' file), the cgroup starts swapping out
> some EPC pages within the same cgroup to make room for new allocations.
> For that, this implementation tracks reclaimable EPC pages in a separate
> LRU list in each cgroup, and below are more details and justification of
> this design.
> Track EPC pages in per-cgroup LRUs (from Dave)
> ----------------------------------------------
> tl;dr: A cgroup hitting its limit should be as similar as possible to the
> system running out of EPC memory. The only two choices to implement that
> are nasty changes the existing LRU scanning algorithm, or to add new LRUs.
> The result: Add a new LRU for each cgroup and scans those instead. Replace
> the existing global cgroup with the root cgroup's LRU (only when this new
> support is compiled in, obviously).
> The existing EPC memory management aims to be a miniature version of the
> core VM where EPC memory can be overcommitted and reclaimed. EPC
> allocations can wait for reclaim. The alternative to waiting would have
> been to send a signal and let the enclave die.
> This series attempts to implement that same logic for cgroups, for the same
> reasons: it's preferable to wait for memory to become available and let
> reclaim happen than to do things that are fatal to enclaves.
> There is currently a global reclaimable page SGX LRU list. That list (and
> the existing scanning algorithm) is essentially useless for doing reclaim
> when a cgroup hits its limit because the cgroup's pages are scattered
> around that LRU. It is unspeakably inefficient to scan a linked list with
> millions of entries for what could be dozens of pages from a cgroup that
> needs reclaim.
> Even if unspeakably slow reclaim was accepted, the existing scanning
> algorithm only picks a few pages off the head of the global LRU. It would
> either need to hold the list locks for unreasonable amounts of time, or be
> taught to scan the list in pieces, which has its own challenges.
> Unreclaimable Enclave Pages
> ---------------------------
> There are a variety of page types for enclaves, each serving different
> purposes [5]. Although the SGX architecture supports swapping for all
> types, some special pages, e.g., Version Array(VA) and Secure Enclave
> Control Structure (SECS)[5], holds meta data of reclaimed pages and
> enclaves. That makes reclamation of such pages more intricate to manage.
> The SGX driver global reclaimer currently does not swap out VA pages. It
> only swaps the SECS page of an enclave when all other associated pages have
> been swapped out. The cgroup reclaimer follows the same approach and does
> not track those in per-cgroup LRUs and considers them as unreclaimable
> pages. The allocation of these pages is counted towards the usage of a
> specific cgroup and is subject to the cgroup's set EPC limits.
> Earlier versions of this series implemented forced enclave-killing to
> reclaim VA and SECS pages. That was designed to enforce the 'max' limit,
> particularly in scenarios where a user or administrator reduces this limit
> post-launch of enclaves. However, subsequent discussions [3, 4] indicated
> that such preemptive enforcement is not necessary for the misc-controllers.
> Therefore, reclaiming SECS/VA pages by force-killing enclaves were removed,
> and the limit is only enforced at the time of new EPC allocation request.
> When a cgroup hits its limit but nothing left in the LRUs of the subtree,
> i.e., nothing to reclaim in the cgroup, any new attempt to allocate EPC
> within that cgroup will result in an 'ENOMEM'.
> Unreclaimable Guest VM EPC Pages
> --------------------------------
> The EPC pages allocated for guest VMs by the virtual EPC driver are not
> reclaimable by the host kernel [6]. Therefore an EPC cgroup also treats
> those as unreclaimable and returns ENOMEM when its limit is hit and nothing
> reclaimable left within the cgroup. The virtual EPC driver translates the
> ENOMEM error resulted from an EPC allocation request into a SIGBUS to the
> user process exactly the same way handling host running out of physical
> EPC.
> This work was originally authored by Sean Christopherson a few years ago,
> and previously modified by Kristen C. Accardi to utilize the misc cgroup
> controller rather than a custom controller. I have been updating the
> patches based on review comments since V2 [7-13], simplified the
> implementation/design, added selftest scripts, fixed some stability issues
> found from testing.
> Thanks to all for the review/test/tags/feedback provided on the previous
> versions.
> I appreciate your further reviewing/testing and providing tags if
> appropriate.

I'be been running this patchset on my (single node) Kubernetes cluster
with EPC cgroups limits enforced for each SGX (Gramine) container and
everything seems to be working well and as expected.

Tested-by: Mikko Ylinen <mikko.ylinen@xxxxxxxxxxxxxxx>

Regards, Mikko

> ---
> V9:
> - Add comments for static variables outside functions. (Jarkko)
> - Remove unnecessary ifs. (Tim)
> - Add more Reviewed-By: tags from Jarkko and TJ.
> V8:
> - Style fixes. (Jarkko)
> - Abstract _misc_res_free/alloc() (Jarkko)
> - Remove unneeded NULL checks. (Jarkko)
> V7:
> - Split the large patch for the final EPC implementation, #10 in V6, into
> smaller ones. (Dave, Kai)
> - Scan and reclaim one cgroup at a time, don't split sgx_reclaim_pages()
> into two functions (Kai)
> - Removed patches to introduce the EPC page states, list for storing
> candidate pages for reclamation. (not needed due to above changes)
> - Make ops one per resource type and store them in array (Michal)
> - Rename the ops struct to misc_res_ops, and enforce the constraints of
> required callback functions (Jarkko)
> - Initialize epc cgroup in sgx driver init function. (Kai)
> - Moved addition of priv field to patch 4 where it was used first. (Jarkko)
> - Split sgx_get_current_epc_cg() out of sgx_epc_cg_try_charge() (Kai)
> - Use a static for root cgroup (Kai)
> [1]
> [2]
> [3]
> [4]
> [5]Documentation/arch/x86/sgx.rst, Section"Enclave Page Types"
> [6]Documentation/arch/x86/sgx.rst, Section "Virtual EPC"
> [7]v2:
> [8]v3:
> [9]v4:
> [10]v5:
> [11]v6:
> [12]v7:
> [13]v8:
> Haitao Huang (2):
> x86/sgx: Charge mem_cgroup for per-cgroup reclamation
> selftests/sgx: Add scripts for EPC cgroup testing
> Kristen Carlson Accardi (10):
> cgroup/misc: Add per resource callbacks for CSS events
> cgroup/misc: Export APIs for SGX driver
> cgroup/misc: Add SGX EPC resource type
> x86/sgx: Implement basic EPC misc cgroup functionality
> x86/sgx: Abstract tracking reclaimable pages in LRU
> x86/sgx: Implement EPC reclamation flows for cgroup
> x86/sgx: Add EPC reclamation in cgroup try_charge()
> x86/sgx: Abstract check for global reclaimable pages
> x86/sgx: Expose sgx_epc_cgroup_reclaim_pages() for global reclaimer
> x86/sgx: Turn on per-cgroup EPC reclamation
> Sean Christopherson (3):
> x86/sgx: Add sgx_epc_lru_list to encapsulate LRU list
> x86/sgx: Expose sgx_reclaim_pages() for cgroup
> Docs/x86/sgx: Add description for cgroup support
> Documentation/arch/x86/sgx.rst | 83 ++++++
> arch/x86/Kconfig | 13 +
> arch/x86/kernel/cpu/sgx/Makefile | 1 +
> arch/x86/kernel/cpu/sgx/encl.c | 38 ++-
> arch/x86/kernel/cpu/sgx/encl.h | 3 +-
> arch/x86/kernel/cpu/sgx/epc_cgroup.c | 276 ++++++++++++++++++
> arch/x86/kernel/cpu/sgx/epc_cgroup.h | 83 ++++++
> arch/x86/kernel/cpu/sgx/main.c | 180 +++++++++---
> arch/x86/kernel/cpu/sgx/sgx.h | 22 ++
> include/linux/misc_cgroup.h | 41 +++
> kernel/cgroup/misc.c | 109 +++++--
> .../selftests/sgx/ | 246 ++++++++++++++++
> .../selftests/sgx/ | 13 +
> 13 files changed, 1019 insertions(+), 89 deletions(-)
> create mode 100644 arch/x86/kernel/cpu/sgx/epc_cgroup.c
> create mode 100644 arch/x86/kernel/cpu/sgx/epc_cgroup.h
> create mode 100755 tools/testing/selftests/sgx/
> create mode 100755 tools/testing/selftests/sgx/
> base-commit: 54be6c6c5ae8e0d93a6c4641cb7528eb0b6ba478
> --
> 2.25.1