On Sat, Mar 21, 2020 at 10:41 PM Rafael Aquini aquini@redhat.com wrote:
On Sat, Mar 21, 2020 at 09:31:42PM -0700, Andrew Morton wrote:
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?
Well,
I admit I wasn't taking the approach of changing the kernel because I was thinking it would require a partial, or even full, revert of commit 9c4e6b1a7027f, and that would be increasing complexity, but on a second thought, it seems that we might just be missing:
diff --git a/mm/swap.c b/mm/swap.c index cf39d24ada2a..b1601228ded4 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -473,6 +473,7 @@ void lru_cache_add_active_or_unevictable(struct page *page, __mod_zone_page_state(page_zone(page), NR_MLOCK, hpage_nr_pages(page)); count_vm_event(UNEVICTABLE_PGMLOCKED);
SetPageUnevictable(page);
No, this is not correct. Check __pagevec_lru_add_fn() for TestClearPageUnevictable().
As I mentioned in the other email, I think the better solution would be to introduce a sysctl to drain the pageves. That way there will not be any dependency on compaction as was in your original patch.
Shakeel