linux-kernel - Re: [PATCH 3/3] avoid extra path_get/path_put cycle in path

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Message-ID: <o2uu6mzozjaruja5udzfnv2p5fwfoeud5movtve7gyuj57xlz3@aqy6sil3o2ze>
Date: Thu, 22 Aug 2024 12:21:39 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: brauner@...nel.org, jack@...e.cz, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/3] avoid extra path_get/path_put cycle in path_openat()

On Thu, Aug 22, 2024 at 01:41:49AM +0100, Al Viro wrote:
> Once we'd opened the file, nd->path and file->f_path have the
> same contents.  Rather than having both pinned and nd->path
> dropped by terminate_walk(), let's have them share the
> references from the moment when FMODE_OPENED is set and
> clear nd->path just before the terminate_walk() in such case.
> 
> To do that, we
> 	* add a variant of vfs_open() that does *not* do conditional
> path_get() (vfs_open_borrow()); use it in do_open().
> 	* don't grab f->f_path.mnt in finish_open() - only
> f->f_path.dentry.  Have atomic_open() drop the child dentry
> in FMODE_OPENED case and return f->path.dentry without grabbing it.
> 	* adjust vfs_tmpfile() for finish_open() change (it
> is called from ->tmpfile() instances).
> 	* make do_o_path() use vfs_open_borrow(), collapse path_put()
> there with the conditional path_get() we would've get in vfs_open().
> 	* in FMODE_OPENED case clear nd->path before calling
> terminate_walk().
> 
> diff --git a/fs/open.c b/fs/open.c
> index 0ec2e9a33856..f9988427fb97 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -1046,7 +1046,7 @@ int finish_open(struct file *file, struct dentry *dentry,
>  	file->f_path.dentry = dentry;
>  	err = do_dentry_open(file, open);
>  	if (file->f_mode & FMODE_OPENED)
> -		path_get(&file->f_path);
> +		dget(&file->f_path.dentry);
>  	return err;
>  }

There are numerous consumers of finish_open(), I don't see how they got
adjusted to cope with this (or why they would not need adjustment).

For example fuse_create_open().

If this is sorted out I would argue it needs to be explained in the
commit message.

fwiw I don't think patching up the convention of finish_open() is needed
for avoiding the extra ref cycle to work.