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SubjectRe: [PATCH] tools/testing/selftests/vm/mlock2-tests: fix mlock2 false-negative errors
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);

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