Re: [PATCH] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove

Michal Hocko
2021-04-09T14:38:09Z

On Fri 09-04-21 14:42:21, Mel Gorman wrote:
> On Fri, Apr 09, 2021 at 02:48:12PM +0200, Michal Hocko wrote:
> > On Fri 09-04-21 14:42:58, Michal Hocko wrote:
> > > On Fri 09-04-21 13:09:57, Mel Gorman wrote:
> > > > zone_pcp_reset allegedly protects against a race with drain_pages
> > > > using local_irq_save but this is bogus. local_irq_save only operates
> > > > on the local CPU. If memory hotplug is running on CPU A and drain_pages
> > > > is running on CPU B, disabling IRQs on CPU A does not affect CPU B and
> > > > offers no protection.
> > >
> > > Yes, the synchronization aspect is bogus indeed.
> > >
> > > > This patch reorders memory hotremove such that the PCP structures
> > > > relevant to the zone are no longer reachable by the time the structures
> > > > are freed. With this reordering, no protection is required to prevent
> > > > a use-after-free and the IRQs can be left enabled. zone_pcp_reset is
> > > > renamed to zone_pcp_destroy to make it clear that the per-cpu structures
> > > > are deleted when the function returns.
> > >
> > > Wouldn't it be much easier to simply not destroy/reset pcp of an empty
> > > zone at all? The whole point of this exercise seems to be described in
> > > 340175b7d14d5. setup_zone_pageset can check for an already allocated pcp
> > > and simply reinitialize it.
> >
> > I meant this
> >
>
> It might be simplier but if the intention is to free as much memory
> as possible during hot-remove, it seems wasteful to leave the per-cpu
> structures behind if we do not have to.

We do leave the whole pgdat behind. I do not think pagesets really do
matter.

> If a problem with my patch can
> be spotted then I'm happy to go with an alternative fix but there are
> two minor issues with your proposed fix.

I will not insist but this code has proven to bitrot and I just find it
much simpler to drop it altogether rather than conserve it in some form.
Not something I would insist though.

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index e6a602e82860..b0fdda77e570 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -6496,7 +6496,13 @@ void __meminit setup_zone_pageset(struct zone *zone)
> > struct per_cpu_pageset *p;
> > int cpu;
> >
> > - zone->pageset = alloc_percpu(struct per_cpu_pageset);
> > + /*
> > + * zone could have gone completely offline during memory hotplug
> > + * when the pgdat is left behind for simplicity. On a next onlining
> > + * we do not need to reallocate pcp state.
> > + */
> > + if (!zone->pageset)
> > + zone->pageset = alloc_percpu(struct per_cpu_pageset);
>
> Should be "if (zone->pageset != &boot_pageset)" ?

Memory hotplug really wants the NULL
check. it doesn't use boot_pageset (if we drop rest to boot_pageset).
But you are right that the boot time initialization first sets
boot_pageset (zone_pcp_init) and initializes real pagesets later
(setup_per_cpu_pageset). But this can be handled at the memory hotplug
layer I believe

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 754026a9164d..1cadfec323fc 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -883,7 +883,8 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
*/
if (!populated_zone(zone)) {
need_zonelists_rebuild = 1;
- setup_zone_pageset(zone);
+ if (!zone->pageset)
+ setup_zone_pageset(zone);
}

> > for_each_possible_cpu(cpu) {
> > p = per_cpu_ptr(zone->pageset, cpu);
> > pageset_init(p);
> > @@ -8803,25 +8809,6 @@ void zone_pcp_enable(struct zone *zone)
> > mutex_unlock(&pcp_batch_high_lock);
> > }
> >
> > -void zone_pcp_reset(struct zone *zone)
> > -{
> > - unsigned long flags;
> > - int cpu;
> > - struct per_cpu_pageset *pset;
> > -
> > - /* avoid races with drain_pages() */
> > - local_irq_save(flags);
> > - if (zone->pageset != &boot_pageset) {
> > - for_each_online_cpu(cpu) {
> > - pset = per_cpu_ptr(zone->pageset, cpu);
> > - drain_zonestat(zone, pset);
> > - }
> > - free_percpu(zone->pageset);
> > - zone->pageset = &boot_pageset;
> > - }
> > - local_irq_restore(flags);
> > -}
> > -
>
> zone_pcp_reset still needs to exist to drain the remaining vmstats or
> it'll break 5a883813845a ("memory-hotplug: fix zone stat
> mismatch").

Are you sure we are reseting vmstats in the hotremove. I do not see
anything like that. Maybe this was needed at the time. I will double
check.
--
Michal Hocko
SUSE Labs