linux-kernel - Re: [PATCH v3] mm/mm_init: Don't cond_resched() in deferred_init_memmap_chunk() if called from deferred_grow

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Message-Id: <20260122112920.2b435873a0cc5f396df5d1a7@linux-foundation.org>
Date: Thu, 22 Jan 2026 11:29:20 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Waiman Long <longman@...hat.com>
Cc: Mike Rapoport <rppt@...nel.org>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>, Steven
 Rostedt <rostedt@...dmis.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev, Wei Yang
 <richard.weiyang@...il.com>, David Hildenbrand <david@...nel.org>,
 "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v3] mm/mm_init: Don't cond_resched() in
 deferred_init_memmap_chunk() if called from deferred_grow_zone()

On Thu, 22 Jan 2026 13:43:43 -0500 Waiman Long <longman@...hat.com> wrote:

> Commit 3acb913c9d5b ("mm/mm_init: use deferred_init_memmap_chunk()
> in deferred_grow_zone()") made deferred_grow_zone() call
> deferred_init_memmap_chunk() within a pgdat_resize_lock() critical
> section with irqs disabled.
>
> It did check for irqs_disabled() in
> deferred_init_memmap_chunk() to avoid calling cond_resched(). For a
> PREEMPT_RT kernel build, however, spin_lock_irqsave() does not disable
> interrupt but rcu_read_lock() is called. This leads to the following
> bug report.
> 
>   BUG: sleeping function called from invalid context at mm/mm_init.c:2091
>   in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
>   preempt_count: 0, expected: 0
>
> @@ -2085,10 +2085,10 @@ deferred_init_memmap_chunk(unsigned long start_pfn, unsigned long end_pfn,
>  
>  			spfn = chunk_end;
>  
> -			if (irqs_disabled())
> -				touch_nmi_watchdog();
> -			else
> +			if (can_resched)
>  				cond_resched();
> +			else
> +				touch_nmi_watchdog();
>  		}
>  	}

Disables the cond_resched() in some situations.  Can this reintroduce
the watchdog warnings which that cond_resched() was intended to
prevent?

The cond_resched() was added by <dig, dig> da97f2d56bbd ("mm: call
cond_resched() from deferred_init_memmap()").

Pasha's 2020 patch replaced touch_nmi_watchdog() with cond_resched() to
prevent RCU stall warnings.  So I think the answer to my question is
yes, going back to touch_nmi_watchdog() could reintroduce those RCU
warnings.