Re: [PATCH 4/7] xfs: Implement fallocate query support mode

Lukas Czerner
2017-09-19T15:33:45Z

On Tue, Sep 19, 2017 at 10:55:20AM -0400, Theodore Ts'o wrote:
> On Mon, Sep 18, 2017 at 03:55:40PM -0600, Andreas Dilger wrote:
> > > And like hch said, why not {f,}pathconf?
> >
> > The problem with pathconf() is that this is just made up by GlibC, and
> > doesn't actually communicate with the kernel or the filesystem at all.
> > For example, _PC_LINK_MAX does not return different values for ext2/ext4
> > filesystems based on feature flags, only what is hard-coded into GlibC
> > based on the filesystem magic. This is not going to work as a per-file
> > source of information.
> >
> > For example, even though EXT4_LINK_MAX has been 65000 for many years,
> > GlibC reports 32000 for an ext4 filesystem since it only uses the magic:
>
> Maybe the right answer is we should define a new pathconfat(2) system
> call which can be used as part of a C library's implementation of
> pathconf() and fpathconf()? glibc probably won't use it for years, of
> course. But we can at least provide the information via an interface
> which we can control, and which is capable of returning correct
> results?
>
> - Ted

Right, *I think* this is what Christoph suggested in his recent reply.
It sounds fair enough to me and this is much more usefull beyond just
fallocate, so I can look into it.

-Lukas