Re: [PATCH 3/5] vfs: add a zero-initialization mode to fallocate

Darrick J. Wong
2021-09-22T02:38:03Z

On Tue, Sep 21, 2021 at 07:16:26PM -0700, Dan Williams wrote:
> On Tue, Sep 21, 2021 at 1:32 AM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > On Tue, Sep 21, 2021 at 10:44:31AM +1000, Dave Chinner wrote:
> > > I think this wants to be a behavioural modifier for existing
> > > operations rather than an operation unto itself. i.e. similar to how
> > > KEEP_SIZE modifies ALLOC behaviour but doesn't fundamentally alter
> > > the guarantees ALLOC provides userspace.
> > >
> > > In this case, the change of behaviour over ZERO_RANGE is that we
> > > want physical zeros to be written instead of the filesystem
> > > optimising away the physical zeros by manipulating the layout
> > > of the file.
> >
> > Yes.
> >
> > > Then we have and API that looks like:
> > >
> > > ALLOC - allocate space efficiently
> > > ALLOC | INIT - allocate space by writing zeros to it
> > > ZERO - zero data and preallocate space efficiently
> > > ZERO | INIT - zero range by writing zeros to it
> > >
> > > Which seems to cater for all the cases I know of where physically
> > > writing zeros instead of allocating unwritten extents is the
> > > preferred behaviour of fallocate()....
> >
> > Agreed. I'm not sure INIT is really the right name, but I can't come
> > up with a better idea offhand.
>
> FUA? As in, this is a forced-unit-access zeroing all the way to media
> bypassing any mechanisms to emulate zero-filled payloads on future
> reads.

FALLOC_FL_ZERO_EXISTING, because you want to zero the storage that
already exists at that file range?

--D