Re: Review: Fix sub-page zeroing for buffered writes into unwritten extents

[David Chinner <dgc@sgi.com>]

On Wed, Jan 24, 2007 at 04:44:04PM +0000, Lachlan McIlroy wrote:
> Dave,
> 
> I'm trying to understand what the sequence of events is here.
> 
> If we write to an unwritten extent then will __xfs_get_blocks()
> be called with create=1 and flags=BMAPI_WRITE?

Yup.

> And calling
> bhv_vop_bmap() with flags set to BMAPI_WRITE will cause xfs_iomap()
> to set iomap_flags to IOMAP_NEW?

Only if we allocate an extent in xfs_iomap:

In xfs_iomap:

    258 phase2:
    259         switch (flags & (BMAPI_WRITE|BMAPI_ALLOCATE|BMAPI_UNWRITTEN)) {
    260         case BMAPI_WRITE:
    261                 /* If we found an extent, return it */
    262                 if (nimaps &&
    263                     (imap.br_startblock != HOLESTARTBLOCK) &&
    264                     (imap.br_startblock != DELAYSTARTBLOCK)) {
    265                         xfs_iomap_map_trace(XFS_IOMAP_WRITE_MAP, io,
    266                                         offset, count, iomapp, &imap, flags);
    267                         break;
    268                 }


We found an extent - an unwritten extent - which means we have a map
and the startblock is a real number (i.e. not a hole or delalloc region).
Hence we break here and never set the IOMAP_NEW flag which is correct
because we didn't just do an allocation.

> The combination of create=1 and
> iomap_flags=IOMAP_NEW in __xfs_get_blocks() should result in calling
> set_buffer_new(), right?

Yes, it would, but unwritten extents are not new extents.....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group