RE: [PATCH V2] x86/sgx: Fix free page accounting
From: Luck, Tony
Date: Wed Nov 10 2021 - 14:16:16 EST
>>> The consequence of sgx_nr_free_pages not being protected is that
>>> its value may not accurately reflect the actual number of free
>>> pages on the system, impacting the availability of free pages in
>>> support of many flows. The problematic scenario isu when the
>>
> > In non-atomicity is not a problem, when it is not a problem :-)
This is most definitely a problem.
start with sgx_nr_free_pages == 100
Now have a cpu on node0 allocate a page at the same time as another
cpu on node1 allcoates a page. Each holds the relevent per-node lock,
but that doesn't stop both CPUs from accessing the global together:
CPU on node0 CPU on node1
sgx_nr_free_pages--; sgx_nr_free_pages--;
What is the value of sgx_nr_free_pages now? We want it to be 98,
but it could be 99.
Rinse, repeat thousands of times. Eventually the value of sgx_nr_free_pages
has not even a close connection to the number of free pages.
-Tony