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SubjectRe: [PATCH v6] x86/sgx: Free backing memory after faulting the enclave page
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Hi,

On 3/3/2022 2:38 PM, Jarkko Sakkinen wrote:
> There is a limited amount of SGX memory (EPC) on each system. When that
> memory is used up, SGX has its own swapping mechanism which is similar
> in concept but totally separate from the core mm/* code. Instead of
> swapping to disk, SGX swaps from EPC to normal RAM. That normal RAM
> comes from a shared memory pseudo-file and can itself be swapped by the
> core mm code. There is a hierarchy like this:
>
> EPC <-> shmem <-> disk
>
> After data is swapped back in from shmem to EPC, the shmem backing
> storage needs to be freed. Currently, the backing shmem is not freed.
> This effectively wastes the shmem while the enclave is running. The
> memory is recovered when the enclave is destroyed and the backing
> storage freed.
>
> Sort this out by freeing memory with shmem_truncate_range(), as soon as
> a page is faulted back to the EPC. In addition, free the memory for
> PCMD pages as soon as all PCMD's in a page have been marked as unused
> by zeroing its contents.
>
> Reported-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: stable@vger.kernel.org
> Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

I can reliably reproduce the issue this patch aims to solve by creating
a virtual machine that has a significant portion of its memory consumed
by EPC:

qemu-system-x86_64 -smp 4 -m 4G\
-enable-kvm \
-cpu host,+sgx-provisionkey \
-object memory-backend-ram,size=2G,host-nodes=0,policy=bind,id=node0 \
-object memory-backend-epc,id=mem0,size=1536M,prealloc=on,host-nodes=0,policy=bind \
-numa node,nodeid=0,cpus=0-1,memdev=node0 \
-object memory-backend-ram,size=2G,host-nodes=1,policy=bind,id=node1 \
-object memory-backend-epc,id=mem1,size=1536M,prealloc=on,host-nodes=1,policy=bind \
-numa node,nodeid=1,cpus=2-3,memdev=node1 \
-M sgx-epc.0.memdev=mem0,sgx-epc.0.node=0,sgx-epc.1.memdev=mem1,sgx-epc.1.node=1 \
...

Before this patch, running the very stressful SGX2 over subscription test case
(unclobbered_vdso_oversubscribed_remove) in this environment always triggers
the oom-killer but no amount of tasks killed can save the system
with it always ending deadlocked on memory:

[ 58.642719] Tasks state (memory values in pages):
[ 58.644324] [ pid ] uid tgid total_vm rss pgtables_bytes swapents oom_score_adj name
[ 58.647237] [ 195] 0 195 3153 197 45056 0 -1000 systemd-udevd
[ 58.650238] [ 281] 0 281 1836367 0 10817536 0 0 test_sgx
[ 58.653088] Out of memory and no killable processes...
[ 58.654832] Kernel panic - not syncing: System is deadlocked on memory

After applying this patch I was able to run SGX2 selftest
unclobbered_vdso_oversubscribed_remove ten times successfully.

Tested-by: Reinette Chatre <reinette.chatre@intel.com>

Reinette


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