Re: [PATCH 8/9] mm, page_alloc: drain all pcplists during memory offline

David Hildenbrand
2020-09-25T10:46:40Z

On 22.09.20 16:37, Vlastimil Babka wrote:
> drain_all_pages() is optimized to only execute on cpus where pcplists are not
> empty. The check can however race with a free to pcplist that has not yet
> increased the pcp->count from 0 to 1. Make the drain optionally skip the racy
> check and drain on all cpus, and use it in memory offline context, where we
> want to make sure no isolated pages are left behind on pcplists.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> include/linux/gfp.h | 1 +
> mm/memory_hotplug.c | 4 ++--
> mm/page_alloc.c | 29 ++++++++++++++++++++---------
> 3 files changed, 23 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 67a0774e080b..cc52c5cc9fab 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -592,6 +592,7 @@ extern void page_frag_free(void *addr);
>
> void page_alloc_init(void);
> void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp);
> +void __drain_all_pages(struct zone *zone, bool page_isolation);
> void drain_all_pages(struct zone *zone);
> void drain_local_pages(struct zone *zone);
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 08f729922e18..bbde415b558b 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1524,7 +1524,7 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
> goto failed_removal;
> }
>
> - drain_all_pages(zone);
> + __drain_all_pages(zone, true);
>
> arg.start_pfn = start_pfn;
> arg.nr_pages = nr_pages;
> @@ -1588,7 +1588,7 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
> */
> ret = test_pages_isolated(start_pfn, end_pfn, MEMORY_OFFLINE);
> if (ret)
> - drain_all_pages(zone);
> + __drain_all_pages(zone, true);
> } while (ret);
>
> /* Mark all sections offline and remove free pages from the buddy. */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4e37bc3f6077..33cc35d152b1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2960,14 +2960,7 @@ static void drain_local_pages_wq(struct work_struct *work)
> preempt_enable();
> }
>
> -/*
> - * Spill all the per-cpu pages from all CPUs back into the buddy allocator.
> - *
> - * When zone parameter is non-NULL, spill just the single zone's pages.
> - *
> - * Note that this can be extremely slow as the draining happens in a workqueue.
> - */
> -void drain_all_pages(struct zone *zone)
> +void __drain_all_pages(struct zone *zone, bool force_all_cpus)
> {
> int cpu;
>
> @@ -3006,7 +2999,13 @@ void drain_all_pages(struct zone *zone)
> struct zone *z;
> bool has_pcps = false;
>
> - if (zone) {
> + if (force_all_cpus) {
> + /*
> + * The pcp.count check is racy, some callers need a
> + * guarantee that no cpu is missed.
> + */
> + has_pcps = true;
> + } else if (zone) {
> pcp = per_cpu_ptr(zone->pageset, cpu);
> if (pcp->pcp.count)
> has_pcps = true;
> @@ -3039,6 +3038,18 @@ void drain_all_pages(struct zone *zone)
> mutex_unlock(&pcpu_drain_mutex);
> }
>
> +/*
> + * Spill all the per-cpu pages from all CPUs back into the buddy allocator.
> + *
> + * When zone parameter is non-NULL, spill just the single zone's pages.
> + *
> + * Note that this can be extremely slow as the draining happens in a workqueue.
> + */
> +void drain_all_pages(struct zone *zone)
> +{
> + __drain_all_pages(zone, false);
> +}
> +
> #ifdef CONFIG_HIBERNATION
>
> /*
>

Interesting race. Instead of this ugly __drain_all_pages() with a
boolean parameter, can we have two properly named functions to be used
in !page_alloc.c code without scratching your head what the difference is?

(yeah, coming up with a proper name is difficult. the one gives more
guarantees than the other, that cannot really be deducted from
"force_all_cpus" - maybe we can encode the actual semantics in the name)

--
Thanks,

David / dhildenb