On Thu, 2007-06-28 at 10:39 +1000, David Chinner wrote:

I don't think it does - swapfile I/O looks like it goes direct to
bio without passing through the filesystem.  When the swapfile is
mapped, it scans and records the extent map of the entire swapfile
in a separate structure and AFAICT the swap code uses that built map
without touching the filesystem at all.

If that is true then the written/unwritten state of the extents is
irrelevant; all we need is allocated disk space for the file and
swapping should work. And it's not like anyone should be reading
the contents of that swapfile through the filesystem, either. ;)

Ah, yes, good point - thats true.  Unwritten extents are ideal for
this then, as attempts to read swap via the regular interfaces will
return zeros instead of random swapped out memory contents.

cheers.