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
