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

Andrew Morton
2026-01-22T19:29:22Z

On Thu, 22 Jan 2026 13:43:43 -0500 Waiman Long <longman@redhat.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.