Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline

Pavel Tatashin
2020-09-03T18:24:42Z

On Thu, Sep 3, 2020 at 2:20 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 03.09.20 08:38, Michal Hocko wrote:
> > On Wed 02-09-20 19:51:45, Vlastimil Babka wrote:
> >> On 9/2/20 5:13 PM, Michal Hocko wrote:
> >>> On Wed 02-09-20 16:55:05, Vlastimil Babka wrote:
> >>>> On 9/2/20 4:26 PM, Pavel Tatashin wrote:
> >>>>> On Wed, Sep 2, 2020 at 10:08 AM Michal Hocko <mhocko@suse.com> wrote:
> >>>>>>
> >>>>>>>
> >>>>>>> Thread#1 - continue
> >>>>>>> free_unref_page_commit
> >>>>>>> migratetype = get_pcppage_migratetype(page);
> >>>>>>> // get old migration type
> >>>>>>> list_add(&page->lru, &pcp->lists[migratetype]);
> >>>>>>> // add new page to already drained pcp list
> >>>>>>>
> >>>>>>> Thread#2
> >>>>>>> Never drains pcp again, and therefore gets stuck in the loop.
> >>>>>>>
> >>>>>>> The fix is to try to drain per-cpu lists again after
> >>>>>>> check_pages_isolated_cb() fails.
> >>>>>>
> >>>>>> But this means that the page is not isolated and so it could be reused
> >>>>>> for something else. No?
> >>>>>
> >>>>> The page is in a movable zone, has zero references, and the section is
> >>>>> isolated (i.e. set_pageblock_migratetype(page, MIGRATE_ISOLATE);) is
> >>>>> set. The page should be offlinable, but it is lost in a pcp list as
> >>>>> that list is never drained again after the first failure to migrate
> >>>>> all pages in the range.
> >>>>
> >>>> Yeah. To answer Michal's "it could be reused for something else" - yes, somebody
> >>>> could allocate it from the pcplist before we do the extra drain. But then it
> >>>> becomes "visible again" and the loop in __offline_pages() should catch it by
> >>>> scan_movable_pages() - do_migrate_range(). And this time the pageblock is
> >>>> already marked as isolated, so the page (freed by migration) won't end up on the
> >>>> pcplist again.
> >>>
> >>> So the page block is marked MIGRATE_ISOLATE but the allocation itself
> >>> could be used for non migrateable objects. Or does anything prevent that
> >>> from happening?
> >>
> >> In a movable zone, the allocation should not be used for non migrateable
> >> objects. E.g. if the zone was not ZONE_MOVABLE, the offlining could fail
> >> regardless of this race (analogically for migrating away from CMA pageblocks).
> >>
> >>> We really do depend on isolation to not allow reuse when offlining.
> >>
> >> This is not really different than if the page on pcplist was allocated just a
> >> moment before the offlining, thus isolation started. We ultimately rely on being
> >> able to migrate any allocated pages away during the isolation. This "freeing to
> >> pcplists" race doesn't fundamentally change anything in this regard. We just
> >> have to guarantee that pages on pcplists will be eventually flushed, to make
> >> forward progress, and there was a bug in this aspect.
> >
> > You are right. I managed to confuse myself yesterday. The race is
> > impossible for !ZONE_MOVABLE because we do PageBuddy check there. And on
> > the movable zone we are not losing the migrateability property.
> >
> > Pavel I think this will be a useful information to add to the changelog.
> > We should also document this in the code to prevent from further
> > confusion. I would suggest something like the following:
> >
> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > index 242c03121d73..56d4892bceb8 100644
> > --- a/mm/page_isolation.c
> > +++ b/mm/page_isolation.c
> > @@ -170,6 +170,14 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
> > * pageblocks we may have modified and return -EBUSY to caller. This
> > * prevents two threads from simultaneously working on overlapping ranges.
> > *
> > + * Please note that there is no strong synchronization with the page allocator
> > + * either. Pages might be freed while their page blocks are marked ISOLATED.
> > + * In some cases pages might still end up on pcp lists and that would allow
> > + * for their allocation even when they are in fact isolated already. Depending on
> > + * how strong of a guarantee the caller needs drain_all_pages might be needed
> > + * (e.g. __offline_pages will need to call it after check for isolated range for
> > + * a next retry).
> > + *
>
> As expressed in reply to v2, I dislike this hack. There is strong
> synchronization, just PCP is special. Allocating from MIGRATE_ISOLATE is
> just plain ugly.
>
> Can't we temporarily disable PCP (while some pageblock in the zone is
> isolated, which we know e.g., due to the counter), so no new pages get
> put into PCP lists after draining, and re-enable after no pageblocks are
> isolated again? We keep draining the PCP, so it doesn't seem to be of a
> lot of use during that period, no? It's a performance hit already.
>
> Then, we would only need exactly one drain. And we would only have to
> check on the free path whether PCP is temporarily disabled.

Hm, we could use a static branches to disable it, that would keep
release code just as fast, but I am worried it will make code even
uglier. Let's see what others in this thread think about this idea.

Thank you,
Pasha