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

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Message-ID: <12ba3f76f06d1336d78e4bfe0f36ba83fbf2f2d1.camel@linux.intel.com>
Date:   Fri, 20 May 2022 08:44:14 -0700
From:   Kristen Carlson Accardi <kristen@...ux.intel.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     linux-sgx@...r.kernel.org, Jarkko Sakkinen <jarkko@...nel.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 <stable@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v2] x86/sgx: Set active memcg prior to shmem allocation

On Thu, 2022-05-19 at 14:22 -0700, Shakeel Butt wrote:
> On Thu, May 19, 2022 at 2:05 PM Kristen Carlson Accardi
> <kristen@...ux.intel.com> 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>
> 
> For the memcg part:
> 
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>

Thanks!