linux-kernel - Re: [PATCH 00/13] cross rename v4

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Date:	Thu, 13 Feb 2014 21:28:50 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Howells <dhowells@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Bruce Fields <bfields@...ldses.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zach Brown <zab@...hat.com>, Jan Kara <jack@...e.cz>,
	Andy Lutomirski <luto@...capital.net>,
	"mszeredi@...e.cz" <mszeredi@...e.cz>
Subject: Re: [PATCH 00/13] cross rename v4

On Thu, Feb 13, 2014 at 8:32 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Feb 13, 2014 at 11:02 AM, David Howells <dhowells@...hat.com> wrote:
>>
>> Whilst that does seem reasonable, what about all the other software that
>> iterates over a directory?  Some of that is surely not going to know about
>> DT_WHT.
>
> So?
>
> Remeber: whiteout entries do not exist "normally". No normal apps
> should care or see them, since the whole and only point of them is
> when they are part of a union mount (in which case they are not
> visible).
>
> So the "how do you see whiteouts" is really only about the raw
> filesystem mount when *not* in the normal place.
>
> IOW, it's not like these guys are going to show up in users home
> directories etc. It's more like a special device node than a file - we
> need to care about some basic system management interfaces, not about
> "random apps". So "coreutils" is the primary user, although I guess a
> few IT people would prefer for things like Nautilus etc random file
> managers to be able to show them nicely too. But if they show up as an
> icon with a question mark on them or whatever, that's really not a big
> deal either.
>
> Sure, maybe they'll look odd in some graphical file chooser *if*
> somebody makes them show up, but I think creation of a whiteout - if
> we allow it at all outside of the union mount itself - should be a
> root-only thing (the same way mknod is) so quite frankly, it falls
> under "filesystem corruption makes my directory listings look odd -
> cry me a river".
>
> (I do think we should allow creation - but for root only - for
> management and testing purposes, but I really think it's a secondary
> issue, and I do think we should literally use "mknod()" - either with
> a new S_IFWHT or even just making use of existing S_IFCHR just so you
> could use the user-space "mknod" to create it with some magic
> major/minor combination.

And IMO the magic S_IFCHR is a lot better in many respects than a new
filetype, since now all backup tools automatically work.  And I think
that's a lot more important than looking like a nice new design.
Sure, if S_IFWHT was there from the start, it would be wonderful.  But
as it stands, it's a lot more difficult to add support for such a
thing to userspace than adding a hack, using the existing intefaces,
to the kernel.

Thanks,
Miklos




>
>                Linus
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