On Sat, 21 Mar 2020 22:03:26 -0400 Rafael Aquini aquini@redhat.com wrote:
- In order to sort out that race, and get the after fault checks consistent,
- the "quick and dirty" trick below is required in order to force a call to
- lru_add_drain_all() to get the recently MLOCK_ONFAULT pages moved to
- the unevictable LRU, as expected by the checks in this selftest.
- */
+static void force_lru_add_drain_all(void) +{
- sched_yield();
- system("echo 1 > /proc/sys/vm/compact_memory");
+}
What is the sched_yield() for?
Mostly it's there to provide a sleeping gap after the fault, whithout actually adding an arbitrary value with usleep().
It's not a hard requirement, but, in some of the tests I performed (whithout that sleeping gap) I would still see around 1% chance of hitting the false-negative. After adding it I could not hit the issue anymore.
It's concerning that such deep machinery as pagevec draining is visible to userspace.
I suppose that for consistency and correctness we should perform a drain prior to each read from /proc/*/pagemap. Presumably this would be far too expensive.
Is there any other way? One such might be to make the MLOCK_ONFAULT pages bypass the lru_add_pvecs?