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[71.184.142.128]) by smtp.gmail.com with ESMTPSA id t11-20020a05620a034b00b006fa31bf2f3dsm24317491qkm.47.2023.01.19.12.57.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Jan 2023 12:57:25 -0800 (PST) From: Eric Chanudet To: Alexander Viro Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Larsson , Andrew Halaney , Eric Chanudet Subject: [RFC PATCH 1/1] fs/namespace: defer free_mount from namespace_unlock Date: Thu, 19 Jan 2023 15:55:21 -0500 Message-Id: <20230119205521.497401-2-echanude@redhat.com> X-Mailer: git-send-email 2.39.0 In-Reply-To: <20230119205521.497401-1-echanude@redhat.com> References: <20230119205521.497401-1-echanude@redhat.com> MIME-Version: 1.0 Content-type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org From: Alexander Larsson Use call_rcu to defer releasing the umount'ed or detached filesystem when calling namepsace_unlock(). Calling synchronize_rcu_expedited() has a significant cost on RT kernel that default to rcupdate.rcu_normal_after_boot=1. For example, on a 6.2-rt1 kernel: perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt 0.07464 +- 0.00396 seconds time elapsed ( +- 5.31% ) With this change applied: perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt 0.00162604 +- 0.00000637 seconds time elapsed ( +- 0.39% ) Waiting for the grace period before completing the syscall does not seem mandatory. The struct mount umount'ed are queued up for release in a separate list and no longer accessible to following syscalls. Signed-off-by: Alexander Larsson Signed-off-by: Eric Chanudet --- fs/namespace.c | 42 +++++++++++++++++++++++++++++++++++------- 1 file changed, 35 insertions(+), 7 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index ab467ee58341..11d219a6e83c 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -44,6 +44,11 @@ static unsigned int m_hash_shift __read_mostly; static unsigned int mp_hash_mask __read_mostly; static unsigned int mp_hash_shift __read_mostly; +struct mount_delayed_release { + struct rcu_head rcu; + struct hlist_head release_list; +}; + static __initdata unsigned long mhash_entries; static int __init set_mhash_entries(char *str) { @@ -1582,11 +1587,31 @@ int may_umount(struct vfsmount *mnt) EXPORT_SYMBOL(may_umount); -static void namespace_unlock(void) +static void free_mounts(struct hlist_head *mount_list) { - struct hlist_head head; struct hlist_node *p; struct mount *m; + + hlist_for_each_entry_safe(m, p, mount_list, mnt_umount) { + hlist_del(&m->mnt_umount); + mntput(&m->mnt); + } +} + +static void delayed_mount_release(struct rcu_head *head) +{ + struct mount_delayed_release *drelease = + container_of(head, struct mount_delayed_release, rcu); + + free_mounts(&drelease->release_list); + kfree(drelease); +} + +static void namespace_unlock(void) +{ + struct hlist_head head; + struct mount_delayed_release *drelease; + LIST_HEAD(list); hlist_move_list(&unmounted, &head); @@ -1599,12 +1624,15 @@ static void namespace_unlock(void) if (likely(hlist_empty(&head))) return; - synchronize_rcu_expedited(); - - hlist_for_each_entry_safe(m, p, &head, mnt_umount) { - hlist_del(&m->mnt_umount); - mntput(&m->mnt); + drelease = kmalloc(sizeof(*drelease), GFP_KERNEL); + if (unlikely(!drelease)) { + synchronize_rcu_expedited(); + free_mounts(&head); + return; } + + hlist_move_list(&head, &drelease->release_list); + call_rcu(&drelease->rcu, delayed_mount_release); } static inline void namespace_lock(void)