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

[Jarkko Sakkinen]

On Tue, Mar 08, 2022 at 11:16:19AM -0800, Reinette Chatre wrote:
> 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>

Thank you.

> Reinette


BR, Jarkko