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.
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This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.
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