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

From: Al Viro
Date: Fri Feb 03 2012 - 12:03:07 EST

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.


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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