From:   Stephen Brennan <stephen.s.brennan@oracle.com>
To:     Jan Kara <jack@suse.cz>, Amir Goldstein <amir73il@gmail.com>
Cc:     Stephen Brennan <stephen.s.brennan@oracle.com>,
        Gautham Ananthakrishna <gautham.ananthakrishna@oracle.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 2/4] fsnotify: stop walking child dentries if remaining tail is negative
Date:   Mon, 13 Dec 2021 16:53:35 -0800
Message-Id: <20211214005337.161885-3-stephen.s.brennan@oracle.com>
In-Reply-To: <20211214005337.161885-1-stephen.s.brennan@oracle.com>
References: <20211214005337.161885-1-stephen.s.brennan@oracle.com>
Content-Transfer-Encoding: 8bit
Content-Type: text/plain
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?DjNs2joLNUXo12mpCAUyqyXJmN/wL6gxMdYuPn9+l4qu8HPuANRLz0TCbtTe?=
 =?us-ascii?Q?jDbsI83DBnubsQ0o9EY9TQGYHbd2ZchLWucLSMcmUvLJajG8vuR9KowAuFuA?=
 =?us-ascii?Q?j9uWQI6kPPLSGbqxcPcCdbWw0R80Ly/i95qMJCoufcLcrpVEh0/wDv2mQ9go?=
 =?us-ascii?Q?9Jnp9bvUVzeRLPd5LZIgVS2u9GMvRMnEbe7woeAn3eO6hD7CBUEfo7LVXFWX?=
 =?us-ascii?Q?KR7YuqJo/+24HkAz10fp0W85WKOwbyk2dVK3MHsdW6vadBuiXKDh7uwfeHPh?=
 =?us-ascii?Q?hlH3BI+lJMPQ4u9kZ0jd9hEu1cL5hpDeOHKO1SPDVnXihAjruz3wNRgh00RG?=
 =?us-ascii?Q?5ZEAZz4OzI9irk9L0kGVsjC7QiezqZ0ErLO152ULMEZ4KoZEfqT0kEVzDE6E?=
 =?us-ascii?Q?qVCiYZsI9B0LusQGMpBHnAOlHNfFegmv9N4LRT6C0lPeFTZxodIv27sH/Oq3?=
 =?us-ascii?Q?jbTOBL3gTKwk5tmR+aWdqXuwOYFtwGvdiMHH23c+f10UYYKk0DqocxlXQM04?=
 =?us-ascii?Q?1sZntKEEjJbqbSy0bkpu2Ej10vQ0LYcc66GeN5NqqVUIKmdKRFEU3vy6/8og?=
 =?us-ascii?Q?QDwoR0wqQZL0LkRPlecntvBZqc7yhUbfl6/VB8Ecb8FPR9PgqMh2z5wLmDzC?=
 =?us-ascii?Q?N1oEjb/Ol7Ge9H+WXIRdFpjZtTHN1+ahjygci9QTG7MwPYZ+1/b8XJMmBWcT?=
 =?us-ascii?Q?Q9WZ55w5BW0ZYO+Z4jKaqMSDAW90TwRuqCIqc2XYVHCNrxgdp4A5hp8xxQ8+?=
 =?us-ascii?Q?c73Eur1sntHZfDImOkHAe3DvdE2cIy/7NW03ef6vMbteMB8zhk9HcerhPcw+?=
 =?us-ascii?Q?8Er2pzkhoop/XIRtuwLjJojKDNl9uh32bMkDNryPD9aFaj9xV1bW6s6i2mdq?=
 =?us-ascii?Q?P8pRXyq7wJYaqt07xsZi8Qg1jcq/gWyR6kG4z3NcdoPJlmnXAotoKtzWo6Jg?=
 =?us-ascii?Q?u6ektfH82yTYGNrUlTDigNDWe8gPm0Sx7DUXh3c13BdUqfsGuQ3lNdnHlnC4?=
 =?us-ascii?Q?xQHpZOSWW8vduArI3NrHSU3zQLXZMOYYUHqHQf3uHMsaB+k7Fg8QbKKvhjW2?=
 =?us-ascii?Q?aLAqr8GLzzHSDqXxUOnVyayZuh2T/aBMn5he9koQRG4PXPej9JGx/duVWYNw?=
 =?us-ascii?Q?nTINwVy6JLWABkfMYnBIWWgt34QdpruVykiTMHZM1JjnhjQpHGwp/5wAtvqk?=
 =?us-ascii?Q?KQ1RsZGWBswREQC6KxTstPKbr5cURu6r+O/2XifjG446PCwsLTNv1wRR/feV?=
 =?us-ascii?Q?ovPVpVDpqMEc6O9+7Q4YTJYL1IUTUwJtq/9ux4AqBNJ66mnyyf+TW3KzgmAJ?=
 =?us-ascii?Q?juOFUjSo1irLdr1vZ57HVy/itWfes62AXnFokxx6havWfFWtQRT9TEOmA2Qw?=
 =?us-ascii?Q?2fPoPTToQ2St1syovWvAtmj4F0Ts7h/hBTQFg5su5L+JVlqUfHHgEgwG1r/y?=
 =?us-ascii?Q?GO9X4FTQ2gqFMK79lua/NyMspoEjclA5hLvMSbffcJkwPXoXtJzgD2MkCwFx?=
 =?us-ascii?Q?1DNk7M/RjiJI6HJ3IR60Dpoi8k/jTIPOUYPcEqoBHlG4CDmZtYJCszg9R1cG?=
 =?us-ascii?Q?Nq6QvP8SuFN1SfYpjUXUkAtF8Sg9MdmM0D/cfRPjZR4znqAEY7lQrNsp1N+r?=
 =?us-ascii?Q?+WAwVgwHhsEHhEoCqI4VDghhQQmT9WdemF3468Ui9QP7zgw2+MalvV02lmr/?=
 =?us-ascii?Q?u44iXQ=3D=3D?=
X-OriginatorOrg: oracle.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 45239b2b-1602-4196-bba0-08d9be9c2e9a
X-MS-Exchange-CrossTenant-AuthSource: CH2PR10MB4166.namprd10.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Dec 2021 00:53:45.6797
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: YZX3NymnrUkTd72eWyzpPcmT+0ItCVwMBW2010uORIBADlNnQ1tUQdEavocYcAuA49TuWGDcro+/jmIU3AYfeW4+vGk8+KD35ePz9/t3vcQ=
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH2PR10MB3893
X-Proofpoint-Virus-Version: vendor=nai engine=6300 definitions=10197 signatures=668683
X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 adultscore=0
 malwarescore=0 bulkscore=0 spamscore=0 mlxlogscore=999 mlxscore=0
 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1
 engine=8.12.0-2110150000 definitions=main-2112140001
X-Proofpoint-ORIG-GUID: 0gsDhyom31aHsqAv3MZpChdIQ17kVwJY
X-Proofpoint-GUID: 0gsDhyom31aHsqAv3MZpChdIQ17kVwJY
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

When notification starts/stops listening events from inode's children it
has to update dentry->d_flags of all positive child dentries. Scanning
may take a long time if the directory has a lot of negative child
dentries. Use the new tail negative flag to detect when the remainder of
the children are negative, and skip them. This speeds up
fsnotify/inotify watch creation, and in some extreme cases can avoid a
soft lockup, for example, with 200 million negative dentries in a single
directory:

 watchdog: BUG: soft lockup - CPU#20 stuck for 9s! [inotifywait:9528]
 CPU: 20 PID: 9528 Comm: inotifywait Kdump: loaded Not tainted 5.16.0-rc4.20211208.el8uek.rc1.x86_64 #1
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.4.1 12/03/2020
 RIP: 0010:__fsnotify_update_child_dentry_flags+0xad/0x120
 Call Trace:
  <TASK>
  fsnotify_add_mark_locked+0x113/0x160
  inotify_new_watch+0x130/0x190
  inotify_update_watch+0x11a/0x140
  __x64_sys_inotify_add_watch+0xef/0x140
  do_syscall_64+0x3b/0x90
  entry_SYSCALL_64_after_hwframe+0x44/0xae

Co-authored-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Co-authored-by: Gautham Ananthakrishna <gautham.ananthakrishna@oracle.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
---
 fs/notify/fsnotify.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 4034ca566f95..b754cd9efbad 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -127,8 +127,12 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
 		 * original inode) */
 		spin_lock(&alias->d_lock);
 		list_for_each_entry(child, &alias->d_subdirs, d_child) {
-			if (!child->d_inode)
+			if (!child->d_inode) {
+				/* all remaining children are negative */
+				if (d_is_tail_negative(child))
+					break;
 				continue;
+			}
 
 			spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
 			if (watched)
-- 
2.30.2