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

[Reinette Chatre]

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@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
> Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

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@xxxxxxxxx>

Reinette