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Date: Thu, 27 Feb 2025 14:33:02 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand <david@redhat.com>, Oscar Salvador
 <osalvador@suse.de>, Johannes Weiner <hannes@cmpxchg.org>, Vlastimil Babka
 <vbabka@suse.cz>, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type
 change
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On Thu, 27 Feb 2025 16:15:47 +0000 Brendan Jackman <jackmanb@google.com> wrote:

> Since the migratetype hygiene patches [0], the locking here is
> a bit more formalised, so write it down with an assert.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -417,6 +417,10 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>  
>  void set_pageblock_migratetype(struct page *page, int migratetype)
>  {
> +	lockdep_assert_once(system_state == SYSTEM_BOOTING ||
> +		in_mem_hotplug() ||
> +		lockdep_is_held(&page_zone(page)->lock));
> +
>  	if (unlikely(page_group_by_mobility_disabled &&
>  		     migratetype < MIGRATE_PCPTYPES))
>  		migratetype = MIGRATE_UNMOVABLE;
> 

We could add such assertions all over the place.  Why this place in
particular?