Re: [PATCH 2/3] mm: page_alloc: remove remnants of unlocked migratetype updates

[Brendan Jackman <jackmanb@google.com>]

On Mon, Feb 24, 2025 at 07:08:25PM -0500, Johannes Weiner wrote:
> The freelist hygiene patches made migratetype accesses fully protected
> under the zone->lock. Remove remnants of handling the race conditions
> that existed before from the MIGRATE_HIGHATOMIC code.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Aside from my WARN bikeshedding, which isn't really about this patch
anyway:

Reviewed-by: Brendan Jackman <jackmanb@google.com>

> -			if (is_migrate_highatomic(mt)) {
> -				unsigned long size;
> -				/*
> -				 * It should never happen but changes to
> -				 * locking could inadvertently allow a per-cpu
> -				 * drain to add pages to MIGRATE_HIGHATOMIC
> -				 * while unreserving so be safe and watch for
> -				 * underflows.
> -				 */
> -				size = max(pageblock_nr_pages, 1UL << order);
> -				size = min(size, zone->nr_reserved_highatomic);
> -				zone->nr_reserved_highatomic -= size;
> -			}
> +			size = max(pageblock_nr_pages, 1UL << order);
> +			size = min(size, zone->nr_reserved_highatomic);
> +			zone->nr_reserved_highatomic -= size;

Now that the locking is a bit cleaner, would it make sense to add a
[VM_]WARN_ON[_ONCE] for underflow?