linux-kernel - Re: [PATCH 1/2] autofs: revert take more care to not update last

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Date:   Wed, 29 Nov 2017 09:48:09 +1100
From:   NeilBrown <neilb@...e.com>
To:     Ian Kent <raven@...maw.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Colin Walters <walters@...hat.com>, Ondrej Holy <oholy@...hat.com>,
        autofs mailing list <autofs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH 1/2] autofs: revert take more care to not update last_used on path walk

On Mon, Nov 27 2017, Ian Kent wrote:

> While the patch of commit 092a53452b helped (partially) resolve a
> problem where automounts were not expiring due to aggressive accesses
> from user space it has a side effect for very large environments.
>
> This change helps with the expire problem by making the expire more
> aggressive but, for very large environments, that means more mount
> requests from clients. When there are a lot of clients that can mean
> fairly significant server load increases.
>
> It turns out I put the last_used in this position to solve this
> very problem and failed to update my own thinking of the autofs
> expire policy. So the patch being reverted introduces a regression
> which should be fixed.
>
> Reverts: 092a53452b

I would add:
 Fixes: 092a53452bb7 ("autofs: take more care to not update last_used on path walk")
 Cc: stable@...r.kernel.org (v4.11+)

to ensure these are picked up as needed (different Fixes line for second
of course).

Reviewed-by: NeilBrown <neilb@...e.com>

Thanks,
NeilBrown

>
> Signed-off-by: Ian Kent <raven@...maw.net>
> Cc: Neil Brown <neilb@...e.com>
> Cc: Al Viro <viro@...IV.linux.org.uk>
> ---
>  fs/autofs4/root.c |   17 ++++++-----------
>  1 file changed, 6 insertions(+), 11 deletions(-)
>
> diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
> index d79ced925861..82e8f6edfb48 100644
> --- a/fs/autofs4/root.c
> +++ b/fs/autofs4/root.c
> @@ -281,8 +281,8 @@ static int autofs4_mount_wait(const struct path *path, bool rcu_walk)
>  		pr_debug("waiting for mount name=%pd\n", path->dentry);
>  		status = autofs4_wait(sbi, path, NFY_MOUNT);
>  		pr_debug("mount wait done status=%d\n", status);
> -		ino->last_used = jiffies;
>  	}
> +	ino->last_used = jiffies;
>  	return status;
>  }
>  
> @@ -321,21 +321,16 @@ static struct dentry *autofs4_mountpoint_changed(struct path *path)
>  	 */
>  	if (autofs_type_indirect(sbi->type) && d_unhashed(dentry)) {
>  		struct dentry *parent = dentry->d_parent;
> +		struct autofs_info *ino;
>  		struct dentry *new;
>  
>  		new = d_lookup(parent, &dentry->d_name);
>  		if (!new)
>  			return NULL;
> -		if (new == dentry)
> -			dput(new);
> -		else {
> -			struct autofs_info *ino;
> -
> -			ino = autofs4_dentry_ino(new);
> -			ino->last_used = jiffies;
> -			dput(path->dentry);
> -			path->dentry = new;
> -		}
> +		ino = autofs4_dentry_ino(new);
> +		ino->last_used = jiffies;
> +		dput(path->dentry);
> +		path->dentry = new;
>  	}
>  	return path->dentry;
>  }

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