On Sat, Mar 21, 2020 at 6:35 PM 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.
mlock* syscalls do not provide any guarantee that the pages will be in unevictable LRU, only that the pages will not be paged-out. The test is checking something very internal to the kernel and this is expected to break.
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.
Fixes: 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")
This is fixing the actual test and not about fixing the mentioned patch. So, this Fixes line is not needed.