This is precisely the problem which needs to be solved for delayed
allocation on ext2/3/4. This is because it is infeasible to work out how
much disk space an ext2 pagecache page will take to write out (it will
require zero to three indirect blocks as well).
When I did delalloc-for-ext2, umm, six years ago I did
maximally-pessimistic in-memory space accounting and I think I just ran a
superblock-wide sync operation when ENOSPC was about to happen. That
caused all the pessimistic reservations to be collapsed into real ones,
releasing space. So as the disk neared a real ENOSPC, the syncs becaome
more frequent. But the overhead was small.
I expect that a similar thing was done in the ext4 delayed allocation
patches - you should take a look at that and see what can be
shared/generalised/etc.
ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/ext4-patches/LATEST/broken-out/
Although, judging by the comment in here:
ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/ext4-patches/LATEST/broken-out/ext4-delayed-allocation.patch
+ * TODO:
+ * MUST:
+ * - flush dirty pages in -ENOSPC case in order to free reserved blocks
things need a bit more work. Hopefully that's a dead comment.
<looks>
omigod, that thing has gone and done a clone-and-own on half the VFS.
Anyway, I doubt if you'll be able to find a design description anyway
but you should spend some time picking it apart. It is the same problem..