[Cluster-devel] [PATCH v2 2/3] GFS2: Implement iomap for block_map

[Dave Chinner <david@fromorbit.com>]

On Wed, Nov 02, 2016 at 09:37:00AM +0000, Steven Whitehouse wrote:
> Hi,
> 
> On 31/10/16 20:07, Dave Chinner wrote:
> >On Sat, Oct 29, 2016 at 10:24:45AM +0100, Steven Whitehouse wrote:
> >>On 28/10/16 20:29, Bob Peterson wrote:
> >>>+	if (create)
> >>>+		flags |= IOMAP_WRITE;
> >>Hmm, I wonder why IOMAP_WRITE and IOMAP_ZERO are separate flags from
> >>the iomap.flags field... Christoph, was there a specific reason for
> >>that?
> >They are different actions. IOMAP_WRITE requires allocation over
> >holes and conversion of unwritten extents to allow writing of
> >user data into the range. IOMAP_ZERO is for zeroing a range of a
> >file via iomap_zero_range() and it does not require allocation - it
> >skips holes and unwritten regions as they are already guaranteed to
> >contain zeros.
> >
> >One *could* allocate blocks with IOMAP_ZERO if desired (i.e.
> >implement IOMAP_ZERO as though it implied IOMAP_WRITE) and
> >iomap_zero_range_actor() will zero the allocated regions
> >appropriately, but it's not necessary to do if it is already known
> >what ranges of the file contain zeros...
> >
> >Cheers,
> >
> >Dave.
> That wasn't quite what I was getting at... we have two sets of
> flags. IOMAP_ZERO, IOMAP_WRITE and IOMAP_REPORT form one set that
> are passed as an argument to ->iomap_begin() and ->iomap_end()

Yes, those are mapping operation control flags that determine the
mapping operation to be done.

> whereas we also have IOMAP_F_* which are set in the iomap.flags

And those are per-map state flags that apply to the extent being
returned.

> field. I guess perhaps the former is intended as the input flags to
> the functions, where as the latter are output flags from
> ->iomap_begin()?

Yes, I thought that was obvious - it didn't occur to me that there'd
be any confusion there.  What can we add to the iomap.h definitions
to make this clearer?

> In that case I would expect that ->iomap_end()
> would view the iomap.flags as read only.

iomap.flags, like the rest of the struct iomap that is returned from
->iomap_begin(), is readonly for all users. Only the filesystem
itself can change extent mappings or state.....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com