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

[Mel Gorman <mgorman@techsingularity.net>]

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@techsingularity.net>
> 
> Yeah the irq disabling here is clearly bogus, so:
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 

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