Re: [PATCH 4/7][TAKE5] support new modes in fallocate

Amit K. Arora
2007-06-26T19:09:41Z

On Tue, Jun 26, 2007 at 11:34:13AM -0400, Andreas Dilger wrote:
> On Jun 26, 2007 16:02 +0530, Amit K. Arora wrote:
> > On Mon, Jun 25, 2007 at 03:46:26PM -0600, Andreas Dilger wrote:
> > > Can you clarify - what is the current behaviour when ENOSPC (or some other
> > > error) is hit? Does it keep the current fallocate() or does it free it?
> >
> > Currently it is left on the file system implementation. In ext4, we do
> > not undo preallocation if some error (say, ENOSPC) is hit. Hence it may
> > end up with partial (pre)allocation. This is inline with dd and
> > posix_fallocate, which also do not free the partially allocated space.
>
> Since I believe the XFS allocation ioctls do it the opposite way (free
> preallocated space on error) this should be encoded into the flags.
> Having it "filesystem dependent" just means that nobody will be happy.

Ok, got your point. Maybe we can have a flag for this, as you suggested.
But, default behavior IMHO should be _not_ to undo partial allocation
(thus the file system will have the option of supporting this flag or
not and it will be inline with posix_fallocate; XFS will obviously
like to support this flag, inline with its existing behavior).

> > > For FA_ZERO_SPACE - I'd think this would (IMHO) be the default - we
> > > don't want to expose uninitialized disk blocks to userspace. I'm not
> > > sure if this makes sense at all.
> >
> > I don't think we need to make it default - atleast for filesystems which
> > have a mechanism to distinguish preallocated blocks from "regular" ones.
>
> What I mean is that any data read from the file should have the "appearance"
> of being zeroed (whether zeroes are actually written to disk or not). What
> I _think_ David is proposing is to allow fallocate() to return without
> marking the blocks even "uninitialized" and subsequent reads would return
> the old data from the disk.

I can't think of a good reason for this (i.e. returning stale data from
preallocated blocks). It is infact a security issue to me.
Anyhow, this may though be beneficial for file systems which have
noticable overhead in marking the blocks "uninitialized/preallocated".
Can you or David please throw some light on how this option might really
be helpful ? Thanks!

--
Regards,
Amit Arora