[PATCH] mm/shmem: report RCU-tasks quiescent states while undoing a range

[Breno Leitao]

This shows up in the Meta fleet on ftruncate() of large tmpfs files:

  INFO: rcu_tasks detected stalls on tasks:
  00000000752fd185: .. nvcsw: 59455/59455 holdout: 1 idle_cpu: -1/0
  task:rocksdb:bottom  state:R  running task
    __folio_split
    find_get_entry
    find_get_entries
    truncate_inode_partial_folio
    shmem_undo_range
    shmem_setattr
    notify_change
    do_ftruncate
    __x64_sys_ftruncate
    do_syscall_64

shmem_undo_range() walks the whole of the requested range in folio_batch
sized steps, twice, and its two cond_resched() calls are the only
reschedule points in that walk.

cond_resched() is not an RCU-tasks quiescent state.  Use
cond_resched_tasks_rcu_qs() at both points so the walk reports an
RCU-tasks quiescent state as it proceeds.

Fixes: 8315f42295d2 ("rcu: Add call_rcu_tasks()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 mm/shmem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index b572c60f2af85..bf48cb49565ea 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1367,7 +1367,7 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 		}
 		folio_batch_remove_exceptionals(&fbatch);
 		folio_batch_release(&fbatch);
-		cond_resched();
+		cond_resched_tasks_rcu_qs();
 	}
 
 	/*
@@ -1408,7 +1408,7 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 
 	index = start;
 	while (index < end) {
-		cond_resched();
+		cond_resched_tasks_rcu_qs();
 
 		if (!find_get_entries(mapping, &index, end - 1, &fbatch,
 				indices)) {

---
base-commit: e657a464aba873f5eaf3ca821b6e4016674492d6
change-id: 20260918-shmem-tasks-rcu-6be37ae730f3

Best regards,
--  
Breno Leitao <leitao@xxxxxxxxxx>