linux-kernel - Re: A Plumber’s Wish List for Linux

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Message-Id: <8E8162FF-C301-40C1-9FBC-9B6ADB29BC59@mit.edu>
Date:	Fri, 7 Oct 2011 09:39:49 -0400
From:	Theodore Tso <tytso@....EDU>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	lennart@...ttering.net, harald@...hat.com, david@...ar.dk,
	greg@...ah.com
Subject: Re: A Plumber’s Wish List for Linux

On Oct 7, 2011, at 8:46 AM, Kay Sievers wrote:
> On Fri, Oct 7, 2011 at 12:38, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> On Fri, 7 Oct 2011 12:28:46 +0200 Kay Sievers <kay.sievers@...y.org> wrote:
>>> What do you mean would be ugly?
>> 
>> I have an ext4fs. It supports every possible file name allowed by POSIX
>> and SuS. What name are you going to use for your 'hidden directory' that
>> won't clash with a real file ?
> 
> Ah, no. The label on FAT (similar on NTFS) are 'magic entries' in the
> root dir list, not a real file in the root dir.
> 
> We need kernel support for changing a mounted fs, because, unlike
> ext4, the blocks containing the strings are inside the fs, which the
> kernel might change any time.

I'd suggest a syscall, not an ioctl, and if a file system has some limitation on what is a valid name (even ext4 has length limitations which might be different from other file systems), we just simply return an error if it's not a valid label name.

As it turns out I went to great lengths in both the kernel and userspace implementations of e2label/tune2fs to make sure it would be safe to directly edit the superblock while the file system is mounted, but that depends on implementation details of the buffer cache in the kernel.  Better to have a formally supported interface which is file system independent.

-- Ted

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