[PATCH v2] sched/isolation: Postpone re-allocation of housekeeping cpumask until initcall

[Waiman Long <longman@redhat.com>]

When testing a linux-next kernel with commit 59bd1d914bb5 ("memblock:
warn when freeing reserved memory before memory map is initialized"),
the following warning was hit when there was a "nohz_full" kernel boot
parameter.

  Cannot free reserved memory because of deferred initialization of the memory map
  WARNING: mm/memblock.c:904 at __free_reserved_area+0xde/0xf0, CPU#0: swapper/0/0
    :
  Call Trace:
   <TASK>
   memblock_phys_free+0xcb/0x100
   housekeeping_init+0x14c/0x170
   start_kernel+0x207/0x450
   x86_64_start_reservations+0x24/0x30
   x86_64_start_kernel+0xda/0xe0
   common_startup_64+0x13e/0x141
   </TASK>

IOW, we shouldn't free memblock allocated memory so early
in the boot process when memory map isn't fully initialized in
deferred_init_memmap(). Fix it by adding a new housekeeping_late_init()
helper to defer the re-allocation of the housekeeping cpumasks to
when initcall's are being processed. Also change rcu_reference() by
rcu_reference_check() to prevent incorrect RCU lockdep splat as RCU
will be active in this later boot stage.

This commit also depends on the presence of commit 7c2eee9c1367
("memblock: don't touch memblock arrays when memblock_free() is called
late") to prevent a KASAN UAF bug report [1].

 [1] https://lore.kernel.org/lkml/20260505051821.1107133-1-longman@redhat.com/

Fixes: 27c3a5967f05 ("sched/isolation: Convert housekeeping cpumasks to rcu pointers")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/sched/isolation.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index ef152d401fe2..a947d75b43f1 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -164,8 +164,6 @@ int housekeeping_update(struct cpumask *isol_mask)
 
 void __init housekeeping_init(void)
 {
-	enum hk_type type;
-
 	if (!housekeeping.flags)
 		return;
 
@@ -173,17 +171,27 @@ void __init housekeeping_init(void)
 
 	if (housekeeping.flags & HK_FLAG_KERNEL_NOISE)
 		sched_tick_offload_init();
+}
+
+static int __init housekeeping_late_init(void)
+{
+	enum hk_type type;
+
+	if (!housekeeping.flags)
+		return 0;
+
 	/*
 	 * Realloc with a proper allocator so that any cpumask update
-	 * can indifferently free the old version with kfree().
+	 * can indifferently free the old version with kfree(). This
+	 * should be done after the completion of deferred_init_memmap().
 	 */
 	for_each_set_bit(type, &housekeeping.flags, HK_TYPE_MAX) {
 		struct cpumask *omask, *nmask = kmalloc(cpumask_size(), GFP_KERNEL);
 
 		if (WARN_ON_ONCE(!nmask))
-			return;
+			return 0;
 
-		omask = rcu_dereference(housekeeping.cpumasks[type]);
+		omask = rcu_dereference_check(housekeeping.cpumasks[type], 1);
 
 		/* We need at least one CPU to handle housekeeping work */
 		WARN_ON_ONCE(cpumask_empty(omask));
@@ -191,7 +199,9 @@ void __init housekeeping_init(void)
 		RCU_INIT_POINTER(housekeeping.cpumasks[type], nmask);
 		memblock_free(omask, cpumask_size());
 	}
+	return 0;
 }
+pure_initcall(housekeeping_late_init);
 
 static void __init housekeeping_setup_type(enum hk_type type,
 					   cpumask_var_t housekeeping_staging)
-- 
2.54.0