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

Michal Hocko
2021-04-09T12:48:46Z

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

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 754026a9164d..7169342f5474 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1810,7 +1818,6 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
init_per_zone_wmark_min();

if (!populated_zone(zone)) {
- zone_pcp_reset(zone);
build_all_zonelists(NULL);
} else
zone_pcp_update(zone);
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);
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);
-}
-
#ifdef CONFIG_MEMORY_HOTREMOVE
/*
* All pages in the range must be in a single zone, must not contain holes,
--
Michal Hocko
SUSE Labs