linux-kernel - [PATCH 1/4] mm/migration: reduce the rcu lock duration

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Date:   Sat, 9 Apr 2022 15:38:43 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     <akpm@...ux-foundation.org>
CC:     <mike.kravetz@...cle.com>, <shy828301@...il.com>,
        <willy@...radead.org>, <ying.huang@...el.com>, <ziy@...dia.com>,
        <minchan@...nel.org>, <apopple@...dia.com>,
        <dave.hansen@...ux.intel.com>, <o451686892@...il.com>,
        <jhubbard@...dia.com>, <peterx@...hat.com>,
        <naoya.horiguchi@....com>, <mhocko@...e.com>, <riel@...hat.com>,
        <osalvador@...e.de>, <david@...hat.com>, <sfr@...b.auug.org.au>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <linmiaohe@...wei.com>
Subject: [PATCH 1/4] mm/migration: reduce the rcu lock duration

rcu_read_lock is required by grabbing the task refcount but it's not
needed for ptrace_may_access. So we could release the rcu lock after
task refcount is successfully grabbed to reduce the rcu holding time.

Reviewed-by: Muchun Song <songmuchun@...edance.com>
Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
---
 mm/migrate.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index a3d8c2be2631..d8aae6c75990 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1907,17 +1907,16 @@ static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
 		return ERR_PTR(-ESRCH);
 	}
 	get_task_struct(task);
+	rcu_read_unlock();
 
 	/*
 	 * Check if this process has the right to modify the specified
 	 * process. Use the regular "ptrace_may_access()" checks.
 	 */
 	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
-		rcu_read_unlock();
 		mm = ERR_PTR(-EPERM);
 		goto out;
 	}
-	rcu_read_unlock();
 
 	mm = ERR_PTR(security_task_movememory(task));
 	if (IS_ERR(mm))
-- 
2.23.0