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

Amit K. Arora
2007-06-28T18:19:20Z

On Wed, Jun 27, 2007 at 09:18:04AM +1000, David Chinner wrote:
> 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.
>
> No, XFs does not free preallocated space on error. it is up to the
> application to clean up.

Since XFS also does not free preallocated space on error and this
behavior is inline with dd, posix_fallocate() and the current ext4
implementation, do we still need FA_FL_FREE_ENOSPC flag ?

> > 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.
>
> Correct, but for swap files that's not an issue - no user should be able
> too read them, and FA_MKSWAP would really need root privileges to execute.

Will the FA_MKSWAP mode still be required with your suggested change of
teaching do_mpage_readpage() about unwritten extents being in place ?
Or, will you still like to have FA_MKSWAP mode ?

--
Regards,
Amit Arora