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

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Message-ID: <20250225150940.GB1499716@cmpxchg.org>
Date: Tue, 25 Feb 2025 10:09:40 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] mm: page_alloc: remove remnants of unlocked
 migratetype updates

On Tue, Feb 25, 2025 at 01:43:35PM +0000, Brendan Jackman wrote:
> 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@...xchg.org>
> 
> Aside from my WARN bikeshedding, which isn't really about this patch
> anyway:
> 
> Reviewed-by: Brendan Jackman <jackmanb@...gle.com>

Thanks

> > -			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?

Yeah I think that would be a nice additional cleanup. Do you want to
send a patch? Otherwise, I can.