[PATCH 2/4] mm/mempolicy: kernel_migrate_pages: use find_get_task_by_vpid()

[Oleg Nesterov <oleg@redhat.com>]

kernel_migrate_pages() calls ptrace_may_access() under rcu_read_lock() for
no reason. This is a leftover from before the commit 313674661925 ("Unify
migrate_pages and move_pages access checks"), where rcu_read_lock() was
needed to protect __task_cred(task).

So we can drop the RCU lock right after get_task_struct(). Better yet, if
pid != 0, we can use find_get_task_by_vpid() which does get_task_struct()
itself, and avoid get/put_task_struct() otherwise.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 mm/mempolicy.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4326dff16aa6..2ec14001e4dc 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1883,15 +1883,11 @@ static int kernel_migrate_pages(pid_t pid, unsigned long maxnode,
 	if (err)
 		goto out;
 
-	/* Find the mm_struct */
-	rcu_read_lock();
-	task = pid ? find_task_by_vpid(pid) : current;
+	task = pid ? find_get_task_by_vpid(pid) : current;
 	if (!task) {
-		rcu_read_unlock();
 		err = -ESRCH;
 		goto out;
 	}
-	get_task_struct(task);
 
 	err = -EINVAL;
 
@@ -1900,11 +1896,9 @@ static int kernel_migrate_pages(pid_t pid, unsigned long maxnode,
 	 * Use the regular "ptrace_may_access()" checks.
 	 */
 	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
-		rcu_read_unlock();
 		err = -EPERM;
 		goto out_put;
 	}
-	rcu_read_unlock();
 
 	task_nodes = cpuset_mems_allowed(task);
 	/* Is the user allowed to access the target nodes? */
@@ -1932,7 +1926,8 @@ static int kernel_migrate_pages(pid_t pid, unsigned long maxnode,
 
 	mmput(mm);
 out_put:
-	put_task_struct(task);
+	if (pid)
+		put_task_struct(task);
 out:
 	NODEMASK_SCRATCH_FREE(scratch);
 	return err;
-- 
2.52.0