linux-kernel - Re: [PATCH v3 07/12] man/man2/fsmount.2: document "new" mount API

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Message-ID: <2025-08-12.1755007445-rural-feudal-spacebar-forehead-28QkCN@cyphar.com>
Date: Wed, 13 Aug 2025 00:33:04 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Askar Safin <safinaskar@...omail.com>
Cc: Alejandro Colomar <alx@...nel.org>, 
	"Michael T. Kerrisk" <mtk.manpages@...il.com>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Jan Kara <jack@...e.cz>, "G. Branden Robinson" <g.branden.robinson@...il.com>, 
	linux-man <linux-man@...r.kernel.org>, linux-api <linux-api@...r.kernel.org>, 
	linux-fsdevel <linux-fsdevel@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org>, 
	David Howells <dhowells@...hat.com>, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v3 07/12] man/man2/fsmount.2: document "new" mount API

On 2025-08-12, Askar Safin <safinaskar@...omail.com> wrote:
> fsmount:
> > Unlike open_tree(2) with OPEN_TREE_CLONE, fsmount() can only be called once in the lifetime of a filesystem instance to produce a mount object.
>
> I don't understand what you meant here. This phrase in its current form is wrong.
> Consider this scenario: we did this:
> fsopen(...)
> fsconfig(..., FSCONFIG_SET_STRING, "source", ...)
> fsconfig(..., FSCONFIG_CMD_CREATE, ...)
> fsmount(...)
> fsopen(...)
> fsconfig(..., FSCONFIG_SET_STRING, "source", ...)
> fsconfig(..., FSCONFIG_CMD_CREATE, ...)
> fsmount(...)
> 
> We used FSCONFIG_CMD_CREATE here as opposed to FSCONFIG_CMD_CREATE_EXCL, thus
> it is possible that second fsmount will return mount for the same superblock.
> Thus that statement "fsmount() can only be called once in the lifetime of a filesystem instance to produce a mount object"
> is not true.

Yeah, the superblock reuse behaviour makes this description less
coherent than what I was going for. My thinking was that a reused
superblock is (to userspace) conceptually a new filesystem instance
because they create it the same way as any other filesystem instance.
(In fact, the rest of the VFS treats them the same way too -- only
sget_fc() knows about superblock reuse.)

But yeah, "filesystem context" is more accurate here, so probably just:

  Unlike open_tree(2) with OPEN_TREE_CLONE, fsmount() can only be called
  once in the lifetime of a filesystem context.

Though maybe we should mention that it's fsopen(2)-only (even though
it's mentioned earlier in the DESCRIPTION)? If you read the sentence in
isolation you might get the wrong impression. Do you have any
alternative suggestions?

FWIW, superblock reuse is one of those things that is a fairly hairy
implementation detail of the VFS, and as such it has quite odd
semantics. I probably wouldn't have documented it as heavily if it
wasn't for the addition of FSCONFIG_CMD_CREATE_EXCL (maybe an entry in
BUGS or CAVEATS at most -- this behaviour has an even worse impact on
mount(2) but it's completely undocumented there).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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