Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs

Mel Gorman
2021-05-28T10:09:27Z

On Fri, May 28, 2021 at 11:52:53AM +0200, David Hildenbrand wrote:
> > > "Disable pcplists so that page isolation cannot race with freeing
> > > in a way that pages from isolated pageblock are left on pcplists."
> > >
> > > Guess we'd then want to move the draining before start_isolate_page_range()
> > > in alloc_contig_range().
> > >
> >
> > Or instead of draining, validate the PFN range in alloc_contig_range
> > is within the same zone and if so, call zone_pcp_disable() before
> > start_isolate_page_range and enable after __alloc_contig_migrate_range.
> >
>
> We require the caller to only pass a range within a single zone, so that
> should be fine.
>
> The only ugly thing about zone_pcp_disable() is
> mutex_lock(&pcp_batch_high_lock) which would serialize all
> alloc_contig_range() and even with offline_pages().
>

True so it would have to be accessed if that is bad or not. If racing
against offline_pages, memory is potentially being offlined in the
target zone which may cause allocation failure. If racing with other
alloc_contig_range calls, the two callers are potentially racing to
isolate and allocate the same range. The arguement could be made that
alloc_contig_ranges should be serialised within one zone to improve the
allocation success rate at the potential cost of allocation latency.

--
Mel Gorman
SUSE Labs