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mm-migrate-report-rcu-tasks-quiescent-states-in-migrate_pages_batch.patch removed from -mm tree

merged mm-stable Andrew Morton 1 participant 1 message Aug 7 Stable branch

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The quilt patch titled
Subject: mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()
has been removed from the -mm tree. Its filename was
mm-migrate-report-rcu-tasks-quiescent-states-in-migrate_pages_batch.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Breno Leitao <[email protected]>
Subject: mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()
Date: Mon, 27 Jul 2026 06:50:19 -0700

migrate_pages_batch() unmaps each folio before moving it, and every unmap runs the mmu_notifier invalidate callbacks. On KVM hosts try_to_migrate() ends up in kvm_mmu_notifier_invalidate_range_start() -> tdp_mmu_zap_leafs(), which is expensive, so unmapping a large batch keeps the CPU busy for a long time.

The loop already calls cond_resched(), but on PREEMPTION kernels that is a no-op, and involuntary preemption is not a Tasks-RCU quiescent state.

A long batch therefore never reports a quiescent state, and the migrating task (e.g. kcompactd) becomes a Tasks-RCU holdout, stalling the Tasks-RCU grace period for minutes, which is common at Meta fleet:

  INFO: rcu_tasks detected stalls on tasks:
  0000000055349ecc: .. nvcsw: 1157401/1157401 holdout: 1 idle_cpu: -1/56 task:kcompactd0      state:R  running task
  Call Trace:
   tdp_mmu_zap_leafs
   tdp_mmu_next_root
   gfn_to_pfn_cache_invalidate_start
   kvm_mmu_notifier_invalidate_range_start
   __mmu_notifier_invalidate_range_start
   try_to_migrate_one
   try_to_migrate
   migrate_pages_batch
   migrate_pages
   compact_zone
   compact_node
   kcompactd
   kthread

Use cond_resched_tasks_rcu_qs() so a quiescent state is reported even when cond_resched() does nothing.

This has also been discussed at [1]

Link https://lore.kernel.org/[email protected] open here
Link https://lore.kernel.org/all/[email protected]/ [1] open here
Signed-off-by Breno Leitao <[email protected]>
Acked-by Zi Yan <[email protected]>
Reviewed-by Gregory Price <[email protected]>
Reviewed-by Paul E. McKenney <[email protected]>
Acked-by David Hildenbrand (Arm) <[email protected]>
Cc Alistair Popple <[email protected]>
Cc Byungchul Park <[email protected]>
Cc "Huang, Ying" <[email protected]>
Cc Joshua Hahn <[email protected]>
Cc Matthew Brost <[email protected]>
Cc Rakie Kim <[email protected]>
Cc <[email protected]>
Signed-off-by Andrew Morton <[email protected]>

---

 mm/migrate.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
mm/migrate.c renamed from mm/migrate.c~mm-migrate-report-rcu-tasks-quiescent-states-in-migrate_pages_batch +1 -1
@@ -1841,7 +1841,7 @@ static int migrate_pages_batch(struct li
 			is_thp = folio_test_pmd_mappable(folio);
 			nr_pages = folio_nr_pages(folio);
 
-			cond_resched();
+			cond_resched_tasks_rcu_qs();
 
 			/*
 			 * The rare folio on the deferred split list should

_

Patches currently in -mm which might be from [email protected] are

mm-kmemleak-report-leaks-only-after-n-consecutive-unreferenced-scans.patch mm-kmemleak-factor-leak-confirmation-into-a-helper.patch selftests-mm-test-kmemleaks-n-consecutive-scan-leak-confirmation.patch radix-tree-fix-kmemleak-false-positives-on-tree-head-reassignment.patch mm-kmemleak-report-rcu-tasks-quiescent-states-during-the-scan.patch mm-kmemleak-default-min_unref_scans-to-2-for-verbose-auto-scan.patch documentation-kmemleak-document-the-conditional-min_unref_scans-default.patch selftests-mm-kmemleak-drop-stale-min_unref_scans-default-from-comments.patch