[PATCH] GOP_ALLOC and fallocate for puffs (round 2)

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

Hi

Here is the latest iteration of my patch to ensure we avoid sending data
to the page cache we cannot flush later.
http://ftp.espci.fr/shadow/manu/puffs-alloc.patch

- When writing, we make sure backend storage is allocated in two cases:
  1) the file is sparse, hence we never know if backend storage is here.
  2) the file grows: we know we need to allocated for the new area.
- sparse file detection is done at when updating metadata cache: if
  size is bigger than allocated blocks, it is sparse.
- I add fallocate FUSE operation (userland support is in another patch)
  for efficient backend storage allocation.
- If fallocate is unavailable, I fallback to explicit zero-filling. 
- puffs_vnop_write() is split into two functions, puffs_vnop_write_cache()
  that writes in cache, puffs_vnop_write_fs that writes to the FS

The change lets NetBSD pass GlusterFS quota test. Without it, process
writing over quota hang in DE state (exitting, disk I/O ongoing: it cannot
flush cached data overquota and therefore cannot exit).

Opinions?

One possible improvement: if PUFFS fallocate returne EOPNOTSUP / ENOSYS, 
we remove it from the operation array? This is dirty, but since we have 
a fallocate systemcalls that has no implementation for FFS, we are very
likely to encounter that case.


-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost