From: Miklos Szeredi <miklos@szeredi.hu>
To: Ian Kent <raven@themaw.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Tejun Heo <tj@kernel.org>, Eric Sandeen <sandeen@sandeen.net>,
Fox Chen <foxhlchen@gmail.com>,
Brice Goglin <brice.goglin@gmail.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Rick Lindsley <ricklind@linux.vnet.ibm.com>,
David Howells <dhowells@redhat.com>,
Marcelo Tosatti <mtosatti@redhat.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Carlos Maiolino <cmaiolino@redhat.com>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 3/7] kernfs: use VFS negative dentry caching
Date: Fri, 11 Jun 2021 15:07:08 +0200 [thread overview]
Message-ID: <CAJfpegunvr-0b1SW2FDNRdaExr=A9OFH1K-g7d0+UiS+9j5V_w@mail.gmail.com> (raw)
In-Reply-To: <162322862726.361452.10114120072438540655.stgit@web.messagingengine.com>
On Wed, 9 Jun 2021 at 10:50, Ian Kent <raven@themaw.net> wrote:
>
> 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@themaw.net>
> ---
> fs/kernfs/dir.c | 52 ++++++++++++++++++++++++++++++++--------------------
> 1 file changed, 32 insertions(+), 20 deletions(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index b3d1bc0f317d0..4f037456a8e17 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -1039,9 +1039,28 @@ 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;
> + /* Negative hashed dentry? */
> + if (d_really_is_negative(dentry)) {
> + 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)) {
Perhaps add a note about this being dependent on parent of a negative
dentry never changing.
If this was backported to a kernel where this assumption doesn't hold,
there would be a mathematical chance of a false negative.
Thanks,
Miklos
next prev parent reply other threads:[~2021-06-11 13:08 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-09 8:49 [PATCH v6 0/7] kernfs: proposed locking and concurrency improvement Ian Kent
2021-06-09 8:49 ` [PATCH v6 1/7] kernfs: move revalidate to be near lookup Ian Kent
2021-06-11 12:45 ` Miklos Szeredi
2021-06-09 8:49 ` [PATCH v6 2/7] kernfs: add a revision to identify directory node changes Ian Kent
2021-06-11 12:49 ` Miklos Szeredi
2021-06-11 12:56 ` Ian Kent
2021-06-11 13:11 ` Greg Kroah-Hartman
2021-06-11 13:31 ` Ian Kent
2021-06-11 14:05 ` Greg Kroah-Hartman
2021-06-11 14:16 ` Ian Kent
2021-06-09 8:50 ` [PATCH v6 3/7] kernfs: use VFS negative dentry caching Ian Kent
2021-06-11 13:07 ` Miklos Szeredi [this message]
2021-06-12 0:47 ` Ian Kent
2021-06-12 1:48 ` Al Viro
2021-06-13 1:16 ` Ian Kent
2021-06-12 0:07 ` Al Viro
2021-06-12 0:43 ` Ian Kent
2021-06-12 1:08 ` Ian Kent
2021-06-12 1:51 ` Al Viro
2021-06-13 1:57 ` Ian Kent
2021-06-09 8:50 ` [PATCH v6 4/7] kernfs: switch kernfs to use an rwsem Ian Kent
2021-06-11 13:10 ` Miklos Szeredi
2021-06-12 1:24 ` Al Viro
2021-06-09 8:51 ` [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode updates Ian Kent
2021-06-11 13:11 ` Miklos Szeredi
2021-06-12 1:45 ` Al Viro
2021-06-13 1:31 ` Ian Kent
2021-06-14 1:32 ` Ian Kent
2021-06-14 6:52 ` Ian Kent
2021-06-14 7:16 ` Ian Kent
2021-06-09 8:52 ` [PATCH v6 6/7] kernfs: add kernfs_need_inode_refresh() Ian Kent
2021-06-11 13:13 ` Miklos Szeredi
2021-06-09 8:52 ` [PATCH v6 7/7] kernfs: dont call d_splice_alias() under kernfs node lock Ian Kent
2021-06-11 13:14 ` Miklos Szeredi
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