linux-kernel - Re: [PATCH v5 3/6] kernfs: use VFS negative dentry caching

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Date:   Mon, 07 Jun 2021 13:27:29 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Ian Kent <raven@...maw.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, Eric Sandeen <sandeen@...deen.net>,
        Fox Chen <foxhlchen@...il.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3/6] kernfs: use VFS negative dentry caching

Ian Kent <raven@...maw.net> writes:

> If there are many lookups for non-existent paths these negative lookups
> can lead to a lot of overhead during path walks.
>
> The VFS allows dentries to be created as negative and hashed, and caches
> them so they can be used to reduce the fairly high overhead alloc/free
> cycle that occurs during these lookups.
>
> Use the kernfs node parent revision to identify if a change has been
> made to the containing directory so that the negative dentry can be
> discarded and the lookup redone.
>
> Signed-off-by: Ian Kent <raven@...maw.net>
> ---
>  fs/kernfs/dir.c |   53 +++++++++++++++++++++++++++++++----------------------
>  1 file changed, 31 insertions(+), 22 deletions(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index b88432c48851f..5ae95e8d1aea1 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -1039,13 +1039,32 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
>  	if (flags & LOOKUP_RCU)
>  		return -ECHILD;
>  
> -	/* Always perform fresh lookup for negatives */
> -	if (d_really_is_negative(dentry))
> -		goto out_bad_unlocked;
> -
>  	kn = kernfs_dentry_node(dentry);
>  	mutex_lock(&kernfs_mutex);
>  
> +	/* Negative hashed dentry? */
> +	if (!kn) {
> +		struct dentry *d_parent = dget_parent(dentry);
> +		struct kernfs_node *parent;
> +
> +		/* If the kernfs parent node has changed discard and
> +		 * proceed to ->lookup.
> +		 */
> +		parent = kernfs_dentry_node(d_parent);
> +		if (parent) {
> +			if (kernfs_dir_changed(parent, dentry)) {
> +				dput(d_parent);
> +				goto out_bad;
> +			}
> +		}
> +		dput(d_parent);
> +
> +		/* The kernfs node doesn't exist, leave the dentry
> +		 * negative and return success.
> +		 */
> +		goto out;
> +	}

What part of this new negative hashed dentry check needs the
kernfs_mutex?

I guess it is the reading of kn->dir.rev.

Since all you are doing is comparing if two fields are equal it
really should not matter.  Maybe somewhere there needs to be a
sprinkling of primitives like READ_ONCE.

It just seems like such a waste to put all of that under kernfs_mutex
on the off chance kn->dir.rev will change while it is being read.

Eric