[PATCH v2] mm/migrate: fix softlockup in do_pages_stat() with huge page arrays

[Qi Xi]

From: Qi Xi <xiqi2@xxxxxxxxxx>

do_pages_stat() walks the user page array in 16-page chunks but never
reschedules, so a huge nr_pages can hold a CPU past the watchdog
threshold on non-preemptible kernels.  Add cond_resched() to the loop so
long move_pages() node-query calls yield periodically.

With KASAN and function tracing enabled, a move_pages() node query
(nodes == NULL) over 10 million page entries (~40GB of mapped memory)
reproduces the softlockup on a 6.6-based arm64 kernel with
CONFIG_PREEMPT_NONE=y.

In current mainline, only alpha, hexagon and classic m68k can still
select PREEMPT_NONE; stable kernels older than v7.0 still allow it, so
the fix mainly targets pre-v7.0 stable users.

Fixes: 80bba1290ab5 ("mm: no get_user/put_user while holding mmap_sem in do_pages_stat?")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Qi Xi <xiqi2@xxxxxxxxxx>
---
v1 -> v2: describe the repro environment; add Cc stable; no code change.

 mm/migrate.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 7bdcdb57652f..6c2371020cc5 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2545,6 +2545,7 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
 
 		chunk_offset += chunk_nr;
 		nr_pages -= chunk_nr;
+		cond_resched();
 	}
 	return nr_pages ? -EFAULT : 0;
 }
-- 
2.53.0