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

[Andrew Morton]

On Sat, 21 Mar 2020 21:35:25 -0400 Rafael Aquini <aquini@xxxxxxxxxx> 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);