+ mm-mm_init-dont-call-cond_resched-in-deferred_init_memmap_chunk-if-rcu_preempt_depth-set.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/mm_init: don't call cond_resched() in deferred_init_memmap_chunk() if rcu_preempt_depth() set
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-mm_init-dont-call-cond_resched-in-deferred_init_memmap_chunk-if-rcu_preempt_depth-set.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mm_init-dont-call-cond_resched-in-deferred_init_memmap_chunk-if-rcu_preempt_depth-set.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Waiman Long <longman@redhat.com>
Subject: mm/mm_init: don't call cond_resched() in deferred_init_memmap_chunk() if rcu_preempt_depth() set
Date: Wed, 21 Jan 2026 14:10:36 -0500

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
  RCU nest depth: 1, expected: 0
  3 locks held by swapper/0/1:
   #0: ffff80008471b7a0 (sched_domains_mutex){+.+.}-{4:4}, at: sched_domains_mutex_lock+0x28/0x40
   #1: ffff003bdfffef48 (&pgdat->node_size_lock){+.+.}-{3:3}, at: deferred_grow_zone+0x140/0x278
   #2: ffff800084acf600 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1b4/0x408
  CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Tainted: G        W           6.19.0-rc6-test #1 PREEMPT_{RT,(full)
}
  Tainted: [W]=WARN
  Call trace:
   show_stack+0x20/0x38 (C)
   dump_stack_lvl+0xdc/0xf8
   dump_stack+0x1c/0x28
   __might_resched+0x384/0x530
   deferred_init_memmap_chunk+0x560/0x688
   deferred_grow_zone+0x190/0x278
   _deferred_grow_zone+0x18/0x30
   get_page_from_freelist+0x780/0xf78
   __alloc_frozen_pages_noprof+0x1dc/0x348
   alloc_slab_page+0x30/0x110
   allocate_slab+0x98/0x2a0
   new_slab+0x4c/0x80
   ___slab_alloc+0x5a4/0x770
   __slab_alloc.constprop.0+0x88/0x1e0
   __kmalloc_node_noprof+0x2c0/0x598
   __sdt_alloc+0x3b8/0x728
   build_sched_domains+0xe0/0x1260
   sched_init_domains+0x14c/0x1c8
   sched_init_smp+0x9c/0x1d0
   kernel_init_freeable+0x218/0x358
   kernel_init+0x28/0x208
   ret_from_fork+0x10/0x20

Fix it by checking rcu_preempt_depth() as well to prevent calling
cond_resched(). Note that CONFIG_PREEMPT_RCU should always be enabled
in a PREEMPT_RT kernel.

Link: https://lkml.kernel.org/r/20260121191036.461389-1-longman@redhat.com
Fixes: 3acb913c9d5b ("mm/mm_init: use deferred_init_memmap_chunk() in deferred_grow_zone()")
Signed-off-by: Waiman Long <longman@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/mm_init.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/mm/mm_init.c~mm-mm_init-dont-call-cond_resched-in-deferred_init_memmap_chunk-if-rcu_preempt_depth-set
+++ a/mm/mm_init.c
@@ -2085,7 +2085,12 @@ deferred_init_memmap_chunk(unsigned long
 
 			spfn = chunk_end;
 
-			if (irqs_disabled())
+			/*
+			 * pgdat_resize_lock() only disables irqs in non-RT
+			 * kernels but calls rcu_read_lock() in a PREEMPT_RT
+			 * kernel.
+			 */
+			if (irqs_disabled() || rcu_preempt_depth())
 				touch_nmi_watchdog();
 			else
 				cond_resched();
_

Patches currently in -mm which might be from longman@redhat.com are

mm-mm_init-dont-call-cond_resched-in-deferred_init_memmap_chunk-if-rcu_preempt_depth-set.patch