Re: [PATCH v2] x86/sgx: Set active memcg prior to shmem allocation

[Greg KH]

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.
> 

<formletter>

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.

</formletter>