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

[Rafael Aquini]

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@xxxxxxxxxx> 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);
        }
        lru_cache_add(page);
 }


I'll take a closer look into it, as well as test it properly, tomorrow. 
Thanks for the heads up, Andrew.
-- Rafael