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Date: Wed, 16 Jul 2014 12:44:17 +0800
From: Ian Kent <raven@...maw.net>
To: NeilBrown <neilb@...e.de>
Cc: autofs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] autofs4: allow RCU-walk to walk through autofs4.
On Thu, 2014-07-10 at 09:41 +1000, NeilBrown wrote:
> Any attempt to look up a pathname that passes though an
> autofs4 mount is currently forced out of RCU-walk into
> REF-walk.
>
> This can significantly hurt performance of many-thread work
> loads on many-core systems, especially if the automounted
> filesystem supports RCU-walk but doesn't get to benefit from
> it.
>
> So if autofs4_d_manage is called with rcu_walk set, only
> fail with -ECHILD if it is necessary to wait longer than
> a spinlock, and avoid even the spinlock in one trivial case.
I've looked more closely at this one now and mostly it looks good to me.
But I'm probably a bit slow, there's just one place where I can't work
out the reasoning ....
>
> Signed-off-by: NeilBrown <neilb@...e.de>
> ---
> fs/autofs4/autofs_i.h | 2 +-
> fs/autofs4/dev-ioctl.c | 2 +-
> fs/autofs4/expire.c | 4 +++-
> fs/autofs4/root.c | 44 +++++++++++++++++++++++++++++---------------
> 4 files changed, 34 insertions(+), 18 deletions(-)
>
> diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
> index 22a280151e45..99dbb05d6148 100644
> --- a/fs/autofs4/autofs_i.h
> +++ b/fs/autofs4/autofs_i.h
> @@ -148,7 +148,7 @@ void autofs4_free_ino(struct autofs_info *);
>
> /* Expiration */
> int is_autofs4_dentry(struct dentry *);
> -int autofs4_expire_wait(struct dentry *dentry);
> +int autofs4_expire_wait(struct dentry *dentry, int rcu_walk);
> int autofs4_expire_run(struct super_block *, struct vfsmount *,
> struct autofs_sb_info *,
> struct autofs_packet_expire __user *);
> diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c
> index 5b570b6efa28..aaf96cb25452 100644
> --- a/fs/autofs4/dev-ioctl.c
> +++ b/fs/autofs4/dev-ioctl.c
> @@ -450,7 +450,7 @@ static int autofs_dev_ioctl_requester(struct file *fp,
> ino = autofs4_dentry_ino(path.dentry);
> if (ino) {
> err = 0;
> - autofs4_expire_wait(path.dentry);
> + autofs4_expire_wait(path.dentry, 0);
> spin_lock(&sbi->fs_lock);
> param->requester.uid = from_kuid_munged(current_user_ns(), ino->uid);
> param->requester.gid = from_kgid_munged(current_user_ns(), ino->gid);
> diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
> index a7be57e39be7..7e2f22ce6954 100644
> --- a/fs/autofs4/expire.c
> +++ b/fs/autofs4/expire.c
> @@ -467,7 +467,7 @@ found:
> return expired;
> }
>
> -int autofs4_expire_wait(struct dentry *dentry)
> +int autofs4_expire_wait(struct dentry *dentry, int rcu_walk)
> {
> struct autofs_sb_info *sbi = autofs4_sbi(dentry->d_sb);
> struct autofs_info *ino = autofs4_dentry_ino(dentry);
> @@ -477,6 +477,8 @@ int autofs4_expire_wait(struct dentry *dentry)
> spin_lock(&sbi->fs_lock);
> if (ino->flags & AUTOFS_INF_EXPIRING) {
> spin_unlock(&sbi->fs_lock);
> + if (rcu_walk)
> + return -ECHILD;
>
> DPRINTK("waiting for expire %p name=%.*s",
> dentry, dentry->d_name.len, dentry->d_name.name);
> diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
> index cc87c1abac97..1ad119407e2f 100644
> --- a/fs/autofs4/root.c
> +++ b/fs/autofs4/root.c
> @@ -208,7 +208,8 @@ next:
> return NULL;
> }
>
> -static struct dentry *autofs4_lookup_expiring(struct dentry *dentry)
> +static struct dentry *autofs4_lookup_expiring(struct dentry *dentry,
> + bool rcu_walk)
> {
> struct autofs_sb_info *sbi = autofs4_sbi(dentry->d_sb);
> struct dentry *parent = dentry->d_parent;
> @@ -225,6 +226,11 @@ static struct dentry *autofs4_lookup_expiring(struct dentry *dentry)
> struct dentry *expiring;
> struct qstr *qstr;
>
> + if (rcu_walk) {
> + spin_unlock(&sbi->lookup_lock);
> + return ERR_PTR(-ECHILD);
> + }
> +
> ino = list_entry(p, struct autofs_info, expiring);
> expiring = ino->dentry;
>
> @@ -260,13 +266,15 @@ next:
> return NULL;
> }
>
> -static int autofs4_mount_wait(struct dentry *dentry)
> +static int autofs4_mount_wait(struct dentry *dentry, bool rcu_walk)
> {
> struct autofs_sb_info *sbi = autofs4_sbi(dentry->d_sb);
> struct autofs_info *ino = autofs4_dentry_ino(dentry);
> int status = 0;
>
> if (ino->flags & AUTOFS_INF_PENDING) {
> + if (rcu_walk)
> + return -ECHILD;
> DPRINTK("waiting for mount name=%.*s",
> dentry->d_name.len, dentry->d_name.name);
> status = autofs4_wait(sbi, dentry, NFY_MOUNT);
> @@ -276,20 +284,22 @@ static int autofs4_mount_wait(struct dentry *dentry)
> return status;
> }
>
> -static int do_expire_wait(struct dentry *dentry)
> +static int do_expire_wait(struct dentry *dentry, bool rcu_walk)
> {
> struct dentry *expiring;
>
> - expiring = autofs4_lookup_expiring(dentry);
> + expiring = autofs4_lookup_expiring(dentry, rcu_walk);
> + if (IS_ERR(expiring))
> + return PTR_ERR(expiring);
> if (!expiring)
> - return autofs4_expire_wait(dentry);
> + return autofs4_expire_wait(dentry, rcu_walk);
> else {
> /*
> * If we are racing with expire the request might not
> * be quite complete, but the directory has been removed
> * so it must have been successful, just wait for it.
> */
> - autofs4_expire_wait(expiring);
> + autofs4_expire_wait(expiring, 0);
> autofs4_del_expiring(expiring);
> dput(expiring);
> }
> @@ -341,7 +351,7 @@ static struct vfsmount *autofs4_d_automount(struct path *path)
> * and the directory was removed, so just go ahead and try
> * the mount.
> */
> - status = do_expire_wait(dentry);
> + status = do_expire_wait(dentry, 0);
> if (status && status != -EAGAIN)
> return NULL;
>
> @@ -349,7 +359,7 @@ static struct vfsmount *autofs4_d_automount(struct path *path)
> spin_lock(&sbi->fs_lock);
> if (ino->flags & AUTOFS_INF_PENDING) {
> spin_unlock(&sbi->fs_lock);
> - status = autofs4_mount_wait(dentry);
> + status = autofs4_mount_wait(dentry, 0);
> if (status)
> return ERR_PTR(status);
> goto done;
> @@ -390,7 +400,7 @@ static struct vfsmount *autofs4_d_automount(struct path *path)
> }
> ino->flags |= AUTOFS_INF_PENDING;
> spin_unlock(&sbi->fs_lock);
> - status = autofs4_mount_wait(dentry);
> + status = autofs4_mount_wait(dentry, 0);
> spin_lock(&sbi->fs_lock);
> ino->flags &= ~AUTOFS_INF_PENDING;
> if (status) {
> @@ -426,21 +436,25 @@ static int autofs4_d_manage(struct dentry *dentry, bool rcu_walk)
> return 0;
> }
>
> - /* We need to sleep, so we need pathwalk to be in ref-mode */
> - if (rcu_walk)
> - return -ECHILD;
> -
> /* Wait for pending expires */
> - do_expire_wait(dentry);
> + if (do_expire_wait(dentry, rcu_walk) == -ECHILD)
> + return -ECHILD;
>
> /*
> * This dentry may be under construction so wait on mount
> * completion.
> */
> - status = autofs4_mount_wait(dentry);
> + status = autofs4_mount_wait(dentry, rcu_walk);
> if (status)
> return status;
>
> + if (rcu_walk)
> + /* No need to consider returning -EISDIR as
> + * there is no risk that ->d_automount will
> + * be called
> + */
> + return status;
> +
This bit here.
The test below says (accepting that d_mountpoint() is unreliable in a
multi-namespace environment and needs to be fixed), if this dentry isn't
being expired and it's not a mount and the directory isn't empty then we
can walk straight through the dentry.
For certain automount map constructs it's possible for a dentry that is
not itself a mount point to be expired in the same way a mount or mount
tree is expired.
What's the thinking behind this one please?
> spin_lock(&sbi->fs_lock);
> /*
> * If the dentry has been selected for expire while we slept
>
>
--
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