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	Fri, 04 Jul 2025 20:23:37 +0000
Date: Fri, 4 Jul 2025 21:23:37 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-fsdevel@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>,
	Christian Brauner <brauner@kernel.org>
Subject: Re: [RFC] MNT_WRITE_HOLD mess
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 <CAHk-=wgurLEukSdbUPk28rW=hsVGMxE4zDOCZ3xxY3ee3oGyoQ@mail.gmail.com>
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On Fri, Jul 04, 2025 at 12:57:39PM -0700, Linus Torvalds wrote:

> Ugh. I don't hate the concept, but if we do this, I think it needs to
> be better abstracted out.
> 
> And you may be right that things like list_for_each_entry() won't
> care, but I would not be surprised there is list debugging code that
> could care deeply. Or if anybody uses things like "list_is_first()",
> it will work 99+_% of the time, but then break horribly if the low bit
> of the prev pointer is set.
> 
> So we obviously use the low bits of pointers in many other situations,
> but I do think that it needs to have some kind of clear abstraction
> and type safety to make sure that people don't use the "normal" list
> handling helpers silently by mistake when they won't actually work.

Point, but in this case I'd be tempted to turn the damn thing into
pointer + unsigned long right in the struct mount, and deal with it
explicitly.  And put a big note on it, along the lines of "we might want
to abstract that someday".

Backporting would be easier that way, if nothing else...