Re: [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove

[Michal Hocko]

On Mon 12-04-21 14:40:18, Vlastimil Babka wrote:
> 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@xxxxxxxxxxxxxxxxxxx>
> 
> Yeah the irq disabling here is clearly bogus, so:
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 
> 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.

Completely agreed here. Not an urgent thing to work on but something to
look into long term.

-- 
Michal Hocko
SUSE Labs