linux-kernel - Re: [PATCH] cifs: Fix reacquisition of volume cookie on still-live connection

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Message-ID: <564520.1713447839@warthog.procyon.org.uk>
Date: Thu, 18 Apr 2024 14:43:59 +0100
From: David Howells <dhowells@...hat.com>
To: Paulo Alcantara <pc@...guebit.com>
Cc: dhowells@...hat.com, Steve French <sfrench@...ba.org>,
    Shyam Prasad N <sprasad@...rosoft.com>, linux-cifs@...r.kernel.org,
    linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cifs: Fix reacquisition of volume cookie on still-live connection

Paulo Alcantara <pc@...guebit.com> wrote:

> I don't know why it was designed that way, but the reason we have two
> different superblocks with ${opts} being the same is because cifs.ko
> relies on the value of cifs_sb_info::prepath to build paths out of
> dentries.  See build_path_from_dentry().  So, when you access
> /mnt/2/foo, cifs.ko will build a path like '[optional tree name prefix]
> + cifs_sb_info::prepath + \foo' and then reuse connections
> (server+session+tcon) from first superblock to perform I/O on that file.

Yep.  You don't *need* prepath.  You could always build from the sb->s_root
without a prepath and have mnt->mnt_root offset the place the VFS thinks you
are:

	[rootdir]/	<--- s_root points here
	|
	v
	foo/
	|
	v
	bar/		<--- mnt_root points here
	|
	v
	a

Without prepath, you build back up the tree { a, bar/, foo/, [rootdir] } with
prepath you insert the prepath at the end.

Bind mounts just make the VFS think it's starting midway down, but you build
up back to s_root.

Think of a mount as just referring to a subtree of the tree inside the
superblock.  The prepath is just an optimisation - but possibly one that makes
sense for cifs if you're having to do pathname fabrication a lot.

David