On Mon, Apr 12, 2021 at 02:40:18PM +0200, 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> > Thanks! > 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. > After v1 of the patch, the race was reduced to the point between the zone watermark check and the rmqueue_pcplist but yes, it still existed. Closing it completely was either complex or expensive. Setting zone->pageset = &boot_pageset before the free would shrink the race further but that still leaves a potential memory ordering issue. While fixable, it's either complex, expensive or both so yes, just leaving the pageset structures in place would be much more straight-forward assuming the structures were not allocated in the zone that is being hot-removed. As things stand, I had trouble even testing zone hot-remove as there was always a few pages left behind and I did not chase down why. The focus was getting rid of the bogus local_irq_save() because it was clearly wrong and offering a false sense of safety and the last problematic local_irq_save() user in page_alloc.c when local_lock is used to protect the PCP structures. -- Mel Gorman SUSE Labs
