On Sat, 21 Mar 2020 21:35:25 -0400 Rafael Aquini aquini@redhat.com wrote:
Changes for commit 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs") break this test expectations on the behavior of mlock syscall family immediately inserting the recently faulted pages into the UNEVICTABLE_LRU, when MCL_ONFAULT is passed to the syscall as part of its flag-set.
There is no functional error introduced by the aforementioned commit, but it opens up a time window where the recently faulted and locked pages might yet not be put back into the UNEVICTABLE_LRU, thus causing a subsequent and immediate PFN flag check for the UNEVICTABLE bit to trip on false-negative errors, as it happens with this test.
This patch fix the false negative by forcefully resorting to a code path that will call a CPU pagevec drain right after the fault but before the PFN flag check takes place, sorting out the race that way.
+/*
- After commit 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")
- changes made by calls to mlock* family might not be immediately reflected
- on the LRUs, thus checking the PFN flags might race against pagevec drain.
- 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?
static int onfault_check(char *map) { unsigned long page_size = getpagesize(); @@ -343,6 +360,9 @@ static int onfault_check(char *map) } *map = 'a';
- force_lru_add_drain_all();
- page1_flags = get_pageflags((unsigned long)map); page2_flags = get_pageflags((unsigned long)map + page_size);
@@ -465,6 +485,8 @@ static int test_lock_onfault_of_present() goto unmap; }
- force_lru_add_drain_all();
- page1_flags = get_pageflags((unsigned long)map); page2_flags = get_pageflags((unsigned long)map + page_size); page1_flags = get_kpageflags(page1_flags & PFN_MASK);