linux-kernel - Re: [RFC] killing boilerplate checks in ->link/->mkdir/->rename

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Date:	Fri, 3 Feb 2012 17:03:00 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC] killing boilerplate checks in ->link/->mkdir/->rename

On Fri, Feb 03, 2012 at 01:25:26AM -0700, Andreas Dilger wrote:
> On 2012-02-02, at 2:24 PM, Al Viro wrote:
> > FWIW, there's something we really should've done a long time ago: putting
> > that limit into sb->s_max_links.  With 0 meaning "leave all checks to
> > ->link/->mkdir/->rename".  Something like the following would make a
> > reasonable start - just the conversion of obvious cases.  As the next
> > step I'd probably initialize it as ~0U instead of 0 and let the filesystems
> > that want something trickier (reiserfs, ext4, gfs2, ocfs2) explicitly set
> > it to 0 in their foo_fill_super().  That would take care of a bunch of cases
> > where we forgot to do those checks (ubifs, hfsplus, jffs2, ramfs, etc.) and
> > it's probably a saner default anyway.
> 
> This would also give userspace some hope of pathconf(path, _PC_LINK_MAX)
> returning the actual value from the filesystem, instead of hard-coding
> this into glibc itself based on the statfs-returned f_type magic value.

*snort*

Even skipping the standard flame about pathconf() as an API, this will
not work.
	* we have filesystems that do not allow link creation at all and
do keep track of subdirectories count in i_nlink of directories.  What
would you have them store?  As it is, ~0U works just fine, but pathconf()
users won't be happy with it.
	* we have filesystems that allow unlimited subdirectories, while
limiting the number of links to non-directories; ->s_max_links == 0 will
work just fine, but won't make pathconf() happy.
	* we have filesystems that have more complex rules re links to
non-directory (see mail from Chris in this thread).  What would you have
pathconf() do?
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