From: Vlastimil Babka <vbabka@suse.cz>
To: Mel Gorman <mgorman@techsingularity.net>,
Andrew Morton <akpm@linux-foundation.org>
Cc: Oscar Salvador <osalvador@suse.de>,
"Michael S. Tsirkin" <mst@redhat.com>,
Alexander Duyck <alexanderduyck@fb.com>,
Minchan Kim <minchan@kernel.org>, Michal Hocko <mhocko@suse.com>,
David Hildenbrand <david@redhat.com>,
Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove
Date: Mon, 12 Apr 2021 14:40:18 +0200 [thread overview]
Message-ID: <d4e4c3e4-7d47-d634-4374-4cf1e55c7895@suse.cz> (raw)
In-Reply-To: <20210412120842.GY3697@techsingularity.net>
On 4/12/21 2:08 PM, 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.
>
> This patch deletes IRQ disable/enable on the grounds that IRQs protect
> nothing and assumes the existing hotplug paths guarantees the PCP cannot be
> used after zone_pcp_enable(). That should be the case already because all
> the pages have been freed and there is no page to put on the PCP lists.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Yeah the irq disabling here is clearly bogus, so:
Acked-by: Vlastimil Babka <vbabka@suse.cz>
But I think Michal has a point that we might best leave the pagesets around, by
a future change. I'm have some doubts that even with your reordering of the
reset/destroy after zonelist rebuild in v1 they cant't be reachable. We have no
protection between zonelist rebuild and zonelist traversal, and that's why we
just leave pgdats around.
So I can imagine a task racing with memory hotremove might see watermarks as ok
in get_page_from_freelist() for the zone and proceeds to try_this_zone:, then
gets stalled/scheduled out while hotremove rebuilds the zonelist and destroys
the pcplists, then the first task is resumed and proceeds with rmqueue_pcplist().
So that's very rare thus not urgent, and this patch doesn't make it less rare so
not a reason to block it.
> ---
> Resending for email address correction and adding lists
>
> Changelog since v1
> o Minimal fix
>
> mm/page_alloc.c | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5e8aedb64b57..9bf0db982f14 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8952,12 +8952,9 @@ void zone_pcp_enable(struct zone *zone)
>
> 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);
> @@ -8966,7 +8963,6 @@ void zone_pcp_reset(struct zone *zone)
> free_percpu(zone->pageset);
> zone->pageset = &boot_pageset;
> }
> - local_irq_restore(flags);
> }
>
> #ifdef CONFIG_MEMORY_HOTREMOVE
>
next prev parent reply other threads:[~2021-04-12 12:40 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-12 12:08 [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove Mel Gorman
2021-04-12 12:12 ` David Hildenbrand
2021-04-12 12:40 ` Vlastimil Babka [this message]
2021-04-12 14:08 ` Mel Gorman
2021-04-12 14:12 ` David Hildenbrand
2021-04-12 15:27 ` Mel Gorman
2021-04-12 16:02 ` David Hildenbrand
2021-04-13 9:36 ` Vlastimil Babka
2021-04-13 9:49 ` Mel Gorman
2021-04-13 10:16 ` Michal Hocko
2021-04-13 6:44 ` Michal Hocko
2021-04-13 6:40 ` Michal Hocko
2021-04-14 7:18 ` Oscar Salvador
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