linux-kernel - Re: [PATCH] mm/page_alloc: fix deadlock on cpu_hotplug_lock in __accept

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Message-ID: <b44cef03-46ef-4153-b21a-98aa6ff43a08@intel.com>
Date: Mon, 31 Mar 2025 12:07:07 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Mike Rapoport <rppt@...nel.org>, David Hildenbrand <david@...hat.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...hsingularity.net>,
 Tom Lendacky <thomas.lendacky@....com>, "Kalra, Ashish"
 <ashish.kalra@....com>, Rick Edgecombe <rick.p.edgecombe@...el.com>,
 linux-mm@...ck.org, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Srikanth Aithal <sraithal@....com>
Subject: Re: [PATCH] mm/page_alloc: fix deadlock on cpu_hotplug_lock in
 __accept_page()

On 3/29/25 10:10, Kirill A. Shutemov wrote:
> +		if (system_wq)
> +			schedule_work(&zone->unaccepted_cleanup);
> +		else
> +			unaccepted_cleanup_work(&zone->unaccepted_cleanup);
> +	}
>  }

The 'system_wq' check seems like an awfully big hack. No other
schedule_work() user does anything similar that I can find across the tree.

Instead of hacking in some internal state, could you use 'system_state',
like:

	if (system_state == SYSTEM_BOOTING)
		unaccepted_cleanup_work(&zone->unaccepted_cleanup);
	else
		schedule_work(&zone->unaccepted_cleanup);

The other method would be to make it more opportunistic? Basically,
detect when it might deadlock:

bool try_to_dec()
{
	if (!cpus_read_trylock())
		return false;

	static_branch_dec_cpuslocked(&zones_with_unaccepted_pages);
	cpus_read_unlock();

	return true;
}

That still requires a bit in the zone to say whether the
static_branch_dec() was deferred or not, though. It's kinda open-coding
schedule_work().