FAILED: patch "[PATCH] mm/shmem: report RCU-tasks quiescent states while undoing a range" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x ebd1a08c192109c83a87ddf42793ed6283180faf
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107060459-failed-5.15-ebd1a08c1921@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From ebd1a08c192109c83a87ddf42793ed6283180faf Mon Sep 17 00:00:00 2001
From: Breno Leitao <leitao@debian.org>
Date: Fri, 18 Sep 2026 04:24:37 -0700
Subject: [PATCH] mm/shmem: report RCU-tasks quiescent states while undoing a
 range

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.

Link: https://lore.kernel.org/20260918-shmem-tasks-rcu-v1-1-79acf91a2569@debian.org
Fixes: 8315f42295d2 ("rcu: Add call_rcu_tasks()")
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: SJ Park <sj@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: <stable@vger.kernel.org>
---
 mm/shmem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index f33dbf5af2cb7..f2a36a1b53750 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)) {
-- 
2.53.0