On Thu, 26 Jan 2023 20:15:35 -0500
Rik van Riel <riel@xxxxxxxxxxx> wrote:
> Instead of waiting for an RCU grace period between each ipc_namespace
> structure that is being freed, wait an RCU grace period for every batch
> of ipc_namespace structures.
>
> Thanks to Al Viro for the suggestion of the helper function.
... and of course I forgot the "git add" before the "git commit --amend".
Sorry about that.
The real v2 of this patch, with Al's suggestions included is below.
---8<---
>From 479378a794c5312acdb555b3deedc403134b2a70 Mon Sep 17 00:00:00 2001
From: Rik van Riel <riel@xxxxxxxxxxx>
Date: Thu, 26 Jan 2023 15:45:06 -0500
Subject: [PATCH] ipc,namespace: batch free ipc_namespace structures
Instead of waiting for an RCU grace period between each ipc_namespace
structure that is being freed, wait an RCU grace period for every batch
of ipc_namespace structures.
Thanks to Al Viro for the suggestion of the helper function.
This speeds up the run time of the test case that allocates ipc_namespaces
in a loop from 6 minutes, to a little over 1 second:
real 0m1.192s
user 0m0.038s
sys 0m1.152s
Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
Reported-by: Chris Mason <clm@xxxxxxxx>
Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
fs/namespace.c | 18 ++++++++++++++----
include/linux/mount.h | 1 +
ipc/namespace.c | 13 ++++++++++---
3 files changed, 25 insertions(+), 7 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index ab467ee58341..1ad4e5acef06 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1397,6 +1397,17 @@ struct vfsmount *mntget(struct vfsmount *mnt)
}
EXPORT_SYMBOL(mntget);
+/*
+ * Make a mount point inaccessible to new lookups.
+ * Because there may still be current users, the caller MUST WAIT
+ * for an RCU grace period before destroying the mount point.
+ */
+void mnt_make_shortterm(struct vfsmount *mnt)
+{
+ if (mnt)
+ real_mount(mnt)->mnt_ns = NULL;
+}
+
/**
* path_is_mountpoint() - Check if path is a mount in the current namespace.
* @path: path to check
@@ -4573,8 +4584,8 @@ EXPORT_SYMBOL_GPL(kern_mount);
void kern_unmount(struct vfsmount *mnt)
{
/* release long term mount so mount point can be released */
- if (!IS_ERR_OR_NULL(mnt)) {
- real_mount(mnt)->mnt_ns = NULL;
+ if (!IS_ERR(mnt)) {
+ mnt_make_shortterm(mnt);
synchronize_rcu(); /* yecchhh... */
mntput(mnt);
}
@@ -4586,8 +4597,7 @@ void kern_unmount_array(struct vfsmount *mnt[], unsigned int num)
unsigned int i;
for (i = 0; i < num; i++)
- if (mnt[i])
- real_mount(mnt[i])->mnt_ns = NULL;
+ mnt_make_shortterm(mnt[i]);
synchronize_rcu_expedited();
for (i = 0; i < num; i++)
mntput(mnt[i]);
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 62475996fac6..ec55a031aa8c 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -88,6 +88,7 @@ extern void mnt_drop_write(struct vfsmount *mnt);
extern void mnt_drop_write_file(struct file *file);
extern void mntput(struct vfsmount *mnt);
extern struct vfsmount *mntget(struct vfsmount *mnt);
+extern void mnt_make_shortterm(struct vfsmount *mnt);
extern struct vfsmount *mnt_clone_internal(const struct path *path);
extern bool __mnt_is_readonly(struct vfsmount *mnt);
extern bool mnt_may_suid(struct vfsmount *mnt);
diff --git a/ipc/namespace.c b/ipc/namespace.c
index a26860a41dac..6ecc30effd3e 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -145,10 +145,11 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,
static void free_ipc_ns(struct ipc_namespace *ns)
{
- /* mq_put_mnt() waits for a grace period as kern_unmount()
- * uses synchronize_rcu().
+ /*
+ * Caller needs to wait for an RCU grace period to have passed
+ * after making the mount point inaccessible to new accesses.
*/
- mq_put_mnt(ns);
+ mntput(ns->mq_mnt);
sem_exit_ns(ns);
msg_exit_ns(ns);
shm_exit_ns(ns);
@@ -168,6 +169,12 @@ static void free_ipc(struct work_struct *unused)
struct llist_node *node = llist_del_all(&free_ipc_list);
struct ipc_namespace *n, *t;
+ llist_for_each_entry_safe(n, t, node, mnt_llist)
+ mnt_make_shortterm(n->mq_mnt);
+
+ /* Wait for any last users to have gone away. */
+ synchronize_rcu();
+
llist_for_each_entry_safe(n, t, node, mnt_llist)
free_ipc_ns(n);
}
--
2.38.1
