Re: [PATCH 1/5] fallocate() implementation in i86, x86_64 and powerpc

Andrew Morton
2007-05-04T06:28:37Z

On Fri, 4 May 2007 16:07:31 +1000 David Chinner <dgc@sgi.com> wrote:

> On Thu, May 03, 2007 at 09:29:55PM -0700, Andrew Morton wrote:
> > On Thu, 26 Apr 2007 23:33:32 +0530 "Amit K. Arora" <aarora@linux.vnet.ibm.com> wrote:
> >
> > > This patch implements the fallocate() system call and adds support for
> > > i386, x86_64 and powerpc.
> > >
> > > ...
> > > +{
> > > + struct file *file;
> > > + struct inode *inode;
> > > + long ret = -EINVAL;
> > > +
> > > + if (len == 0 || offset < 0)
> > > + goto out;
> >
> > The posix spec implies that negative `len' is permitted - presumably "allocate
> > ahead of `offset'". How peculiar.
>
> I just checked the man page for posix_fallocate() and it says:
>
> EINVAL offset or len was less than zero.
>
> We should probably follow this lead.

Yes, I think so. I'm suspecting that
http://www.opengroup.org/onlinepubs/009695399/functions/posix_fallocate.html
is just buggy. Or I can't read.

I mean, if we're going to support negative `len' then is the byte at
`offset' inside or outside the segment? Head spins.

However it would be neat if someone could test $OTHER_OS and, perhaps more
importantly, the present glibc emulation (which I assume your manpage is
referring to, so this would be a manpage test ;)).

> > > +
> > > + ret = -ENODEV;
> > > + if (!S_ISREG(inode->i_mode))
> > > + goto out_fput;
> >
> > So we return ENODEV against an S_ISBLK fd, as per the posix spec. That
> > seems a bit silly of them.
>
> Hmmmm - I thought that the intention of sys_fallocate() was to
> be generic enough to eventually allow preallocation on directories.
> If that is the case, then this check will prevent that....

The above opengroup page only permits S_ISREG. Preallocating directories
sounds quite useful to me, although it's something which would be pretty
hard to emulate if the FS doesn't support it. And there's a decent case to
be made for emulating it - run-anywhere reasons. Does glibc emulation support
directories? Quite unlikely.

But yes, sounds like a desirable thing. Would XFS support it easily if the above
check was relaxed?