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989 Commits

Author SHA1 Message Date
Andriy Gapon
ecaf3257be ffs_mount: remove redundant assignment of geom consumer to devvp.v_bufobj
The assignment is already done in g_vfs_open.
Redundant assignment is harmless, but can become a problem if g_vfs_open
logic is changed.

MFC after:	1 week
2010-04-03 08:25:04 +00:00
Konstantin Belousov
2950ff259c When ffs_realloccg() failed to allocate bigger fragment and, because
pending blocks are scheduled for removal, goes to retry the (re)allocation,
clear the bp pointer. It might happen that meantime free space is really
exhausted and we are entering nospace: label without bread()ing buffer,
causing stale bp value to be brelse()d again.

Tested by:	pho
    (Producing a scenario to reliably reproduce the
     race appeared to be much harder then fixing the bug)
MFC after:	1 week
2010-02-13 10:34:50 +00:00
Kirk McKusick
81479e688b One last pass to get all the unsigned comparisons correct. 2010-02-11 18:14:53 +00:00
Kirk McKusick
e870d1e6f9 This fix corrects a problem in the file system that treats large
inode numbers as negative rather than unsigned. For a default
(16K block) file system, this bug began to show up at a file system
size above about 16Tb.

To fully handle this problem, newfs must be updated to ensure that
it will never create a filesystem with more than 2^32 inodes. That
patch will be forthcoming soon.

Reported by: Scott Burns, John Kilburg, Bruce Evans
Followup by: Jeff Roberson
PR:          133980
MFC after:   2 weeks
2010-02-10 20:10:35 +00:00
Edward Tomasz Napierala
619961810c Remove unused variable. 2010-02-10 18:56:49 +00:00
Kirk McKusick
53298164b8 Cast 64-bit quantity to intptr_t rather than int so as to work properly
with 64-bit architectures (such as amd64).

Reported by:	bz
2010-01-11 22:42:06 +00:00
Kirk McKusick
e268f54cb4 Background:
When renaming a directory it passes through several intermediate
states. First its new name will be created causing it to have two
names (from possibly different parents). Next, if it has different
parents, its value of ".." will be changed from pointing to the old
parent to pointing to the new parent. Concurrently, its old name
will be removed bringing it back into a consistent state. When fsck
encounters an extra name for a directory, it offers to remove the
"extraneous hard link"; when it finds that the names have been
changed but the update to ".." has not happened, it offers to rewrite
".." to point at the correct parent. Both of these changes were
considered unexpected so would cause fsck in preen mode or fsck in
background mode to fail with the need to run fsck manually to fix
these problems. Fsck running in preen mode or background mode now
corrects these expected inconsistencies that arise during directory
rename. The functionality added with this update is used by fsck
running in background mode to make these fixes.

Solution:

This update adds three new fsck sysctl commands to support background
fsck in correcting expected inconsistencies that arise from incomplete
directory rename operations. They are:

setcwd(dirinode) - set the current directory to dirinode in the
    filesystem associated with the snapshot.
setdotdot(oldvalue, newvalue) - Verify that the inode number for ".."
    in the current directory is oldvalue then change it to newvalue.
unlink(nameptr, oldvalue) - Verify that the inode number associated
    with nameptr in the current directory is oldvalue then unlink it.

As with all other fsck sysctls, these new ones may only be used by
processes with appropriate priviledge.

Reported by:    	jeff
Security issues:	rwatson
2010-01-11 20:44:05 +00:00
Martin Blapp
c2ede4b379 Remove extraneous semicolons, no functional changes.
Submitted by:	Marc Balmer <marc@msys.ch>
MFC after:	1 week
2010-01-07 21:01:37 +00:00
Edward Tomasz Napierala
9340fc72e6 Implement NFSv4 ACL support for UFS.
Reviewed by:	rwatson
2009-12-21 19:39:10 +00:00
Konstantin Belousov
49e3050e6c VI_OBJDIRTY vnode flag mirrors the state of OBJ_MIGHTBEDIRTY vm object
flag. Besides providing the redundand information, need to update both
vnode and object flags causes more acquisition of vnode interlock.
OBJ_MIGHTBEDIRTY is only checked for vnode-backed vm objects.

Remove VI_OBJDIRTY and make sure that OBJ_MIGHTBEDIRTY is set only for
vnode-backed vm objects.

Suggested and reviewed by:	alc
Tested by:	pho
MFC after:	3 weeks
2009-12-21 12:29:38 +00:00
Konstantin Belousov
6cc745d2d7 insmntque_stddtr() clears vp->v_data and resets vp->v_op to
dead_vnodeops before calling vgone(). Revert r189706 and corresponding
part of the r186560.

Noted and reviewed by:	tegge
Approved by:	des (pseudofs part)
MFC after:	3 days
2009-09-07 11:55:34 +00:00
Konstantin Belousov
5c61c646a3 The clear_remove() and clear_inodedeps() call vn_start_write(NULL, &mp,
V_NOWAIT) on the non-busied mount point. Unmount might free ufs-specific
mp data, causing ffs_vgetf() to access freed memory.

Busy mountpoint before dropping softdep lk.

Noted and reviewed by:	tegge
Tested by:	pho
MFC after:	1 week
2009-09-06 11:46:51 +00:00
Konstantin Belousov
165a3b418f When a UFS node is truncated to the zero length, e.g. by explicit
truncate(2) call, or by being removed or truncated on open, either
new softupdate freeblks structure is allocated to track the freed
blocks of the node, or truncation is done syncronously when too many SU
dependencies are accumulated. The decision does not take into account
the allocated freeblks dependencies, allowing workloads that do huge
amount of truncations to exhaust the kernel memory.

Take the number of allocated freeblks into consideration for
softdep_slowdown().

Reported by:	pluknet gmail com
Diagnosed and tested by:	pho
Approved by:	re (rwatson)
MFC after:	1 month
2009-08-14 11:00:38 +00:00
Konstantin Belousov
f1eccd05ec In vn_vget_ino() and their inline equivalents, mnt_ref() the mount point
around the sequence that drop vnode lock and then busies the mount point.
Not having vlocked node or direct reference to the mp allows for the
forced unmount to proceed, making mp unmounted or reused.

Tested by:	pho
Reviewed by:	jeff
Approved by:	re (kensmith)
MFC after:	2 weeks
2009-07-02 18:02:55 +00:00
Edward Tomasz Napierala
4bc61fd4ec Don't panic on attempt to set ACL on a block device file.
This is just a part of kern/125613.

PR:		kern/125613
Submitted by:	Jaakko Heinonen <jh at saunalahti dot fi>
Reviewed by:	rwatson
Approved by:	re (kib)
2009-07-01 22:30:36 +00:00
Konstantin Belousov
cfba50c070 For SU mounts, softdep_fsync() might drop vnode lock, allowing other
threads to put dirty buffers on the vnode bufobj list. For regular files
and synchronous fsync requests, check for the condition and restart the
fsync vop if a new dirty buffer arrived.

Tested by:	pho
Approved by:	re (kensmith)
MFC after:	1 month
2009-06-30 10:07:33 +00:00
Konstantin Belousov
a50d1b2a66 Softdep_fsync() may need to lock parent directory of the synced vnode.
Use inlined (due to FFSV_FORCEINSMQ) version of vn_vget_ino() to prevent
mountpoint from being unmounted and freed while no vnodes are locked.

Tested by:	pho
Approved by:	re (kensmith)
MFC after:	1 month
2009-06-30 10:07:00 +00:00
Robert Watson
bcf11e8d00 Move "options MAC" from opt_mac.h to opt_global.h, as it's now in GENERIC
and used in a large number of files, but also because an increasing number
of incorrect uses of MAC calls were sneaking in due to copy-and-paste of
MAC-aware code without the associated opt_mac.h include.

Discussed with:	pjd
2009-06-05 14:55:22 +00:00
Attilio Rao
f083018223 Handle lock recursion differenty by always checking against LO_RECURSABLE
instead the lock own flag itself.

Tested by:	pho
2009-06-02 13:03:35 +00:00
Alan Cox
6e5982caf7 Introduce vfs_bio_set_valid() and use it from ffs_realloccg(). This
eliminates the misuse of vfs_bio_clrbuf() by ffs_realloccg().

In collaboration with:	tegge
2009-05-17 20:26:00 +00:00
Attilio Rao
dfd233edd5 Remove the thread argument from the FSD (File-System Dependent) parts of
the VFS.  Now all the VFS_* functions and relating parts don't want the
context as long as it always refers to curthread.

In some points, in particular when dealing with VOPs and functions living
in the same namespace (eg. vflush) which still need to be converted,
pass curthread explicitly in order to retain the old behaviour.
Such loose ends will be fixed ASAP.

While here fix a bug: now, UFS_EXTATTR can be compiled alone without the
UFS_EXTATTR_AUTOSTART option.

VFS KPI is heavilly changed by this commit so thirdy parts modules needs
to be recompiled.  Bump __FreeBSD_version in order to signal such
situation.
2009-05-11 15:33:26 +00:00
Robert Watson
885868cd8f Remove VOP_LEASE and supporting functions. This hasn't been used since
the removal of NQNFS, but was left in in case it was required for NFSv4.
Since our new NFSv4 client and server can't use it for their
requirements, GC the old mechanism, as well as other unused lease-
related code and interfaces.

Due to its impact on kernel programming and binary interfaces, this
change should not be MFC'd.

Proposed by:    jeff
Reviewed by:    jeff
Discussed with: rmacklem, zach loafman @ isilon
2009-04-10 10:52:19 +00:00
Konstantin Belousov
bc364c4e99 When removing or renaming snaphost, do not delve into request_cleanup().
The later may need blocks from the underlying device that belongs
to normal files, that should not be locked while snap lock is held.

Reported and tested by:	pho
MFC after:	1 month
2009-04-04 12:19:52 +00:00
Konstantin Belousov
02e06d99e6 Correct typo.
Noted by:	kensmith
2009-03-27 15:46:02 +00:00
Konstantin Belousov
c1d8b5e82c Fix two issues with bufdaemon, often causing the processes to hang in
the "nbufkv" sleep.

First, ffs background cg group block write requests a new buffer for
the shadow copy. When ffs_bufwrite() is called from the bufdaemon due
to buffers shortage, requesting the buffer deadlock bufdaemon.
Introduce a new flag for getnewbuf(), GB_NOWAIT_BD, to request getblk
to not block while allocating the buffer, and return failure
instead. Add a flag argument to the geteblk to allow to pass the flags
to getblk(). Do not repeat the getnewbuf() call from geteblk if buffer
allocation failed and either GB_NOWAIT_BD is specified, or geteblk()
is called from bufdaemon (or its helper, see below). In
ffs_bufwrite(), fall back to synchronous cg block write if shadow
block allocation failed.

Since r107847, buffer write assumes that vnode owning the buffer is
locked. The second problem is that buffer cache may accumulate many
buffers belonging to limited number of vnodes. With such workload,
quite often threads that own the mentioned vnodes locks are trying to
read another block from the vnodes, and, due to buffer cache
exhaustion, are asking bufdaemon for help. Bufdaemon is unable to make
any substantial progress because the vnodes are locked.

Allow the threads owning vnode locks to help the bufdaemon by doing
the flush pass over the buffer cache before getnewbuf() is going to
uninterruptible sleep. Move the flushing code from buf_daemon() to new
helper function buf_do_flush(), that is called from getnewbuf().  The
number of buffers flushed by single call to buf_do_flush() from
getnewbuf() is limited by new sysctl vfs.flushbufqtarget.  Prevent
recursive calls to buf_do_flush() by marking the bufdaemon and threads
that temporarily help bufdaemon by TDP_BUFNEED flag.

In collaboration with:	pho
Reviewed by:	 tegge (previous version)
Tested by:	 glebius, yandex ...
MFC after:	 3 weeks
2009-03-16 15:39:46 +00:00
Konstantin Belousov
e65f5a4ead The non-modifying EA VOPs are executed with only shared vnode lock taken.
Provide a custom lock around initializing and tearing down EA area,
to prevent both memory leaks and double-free of it. Count the number
of EA area accessors.

Lock protocol requires either holding exclusive vnode lock to modify
i_ea_area, or shared vnode lock and owning IN_EA_LOCKED flag in i_flag.

Noted by:	YAMAMOTO, Taku <taku tackymt homeip net>
Tested by:	pho (previous version)
MFC after:	2 weeks
2009-03-12 12:43:56 +00:00
Konstantin Belousov
a9d9537110 Do not double-free the struct inode when insmntque failed. Default
insmntque destructor reclaims the vnode, and ufs_reclaim frees the memory.

Reviewed by:	tegge
MFC after:	3 days
2009-03-11 19:45:52 +00:00
John Baldwin
33fc362512 Add a new internal mount flag (MNTK_EXTENDED_SHARED) to indicate that a
filesystem supports additional operations using shared vnode locks.
Currently this is used to enable shared locks for open() and close() of
read-only file descriptors.
- When an ISOPEN namei() request is performed with LOCKSHARED, use a
  shared vnode lock for the leaf vnode only if the mount point has the
  extended shared flag set.
- Set LOCKSHARED in vn_open_cred() for requests that specify O_RDONLY but
  not O_CREAT.
- Use a shared vnode lock around VOP_CLOSE() if the file was opened with
  O_RDONLY and the mountpoint has the extended shared flag set.
- Adjust md(4) to upgrade the vnode lock on the vnode it gets back from
  vn_open() since it now may only have a shared vnode lock.
- Don't enable shared vnode locks on FIFO vnodes in ZFS and UFS since
  FIFO's require exclusive vnode locks for their open() and close()
  routines.  (My recent MPSAFE patches for UDF and cd9660 already included
  this change.)
- Enable extended shared operations on UFS, cd9660, and UDF.

Submitted by:	ups
Reviewed by:	pjd (ZFS bits)
MFC after:	1 month
2009-03-11 14:13:47 +00:00
John Baldwin
5bd65606f4 Adjust some variables (mostly related to the buffer cache) that hold
address space sizes to be longs instead of ints.  Specifically, the follow
values are now longs: runningbufspace, bufspace, maxbufspace,
bufmallocspace, maxbufmallocspace, lobufspace, hibufspace, lorunningspace,
hirunningspace, maxswzone, maxbcache, and maxpipekva.  Previously, a
relatively small number (~ 44000) of buffers set in kern.nbuf would result
in integer overflows resulting either in hangs or bogus values of
hidirtybuffers and lodirtybuffers.  Now one has to overflow a long to see
such problems.  There was a check for a nbuf setting that would cause
overflows in the auto-tuning of nbuf.  I've changed it to always check and
cap nbuf but warn if a user-supplied tunable would cause overflow.

Note that this changes the ABI of several sysctls that are used by things
like top(1), etc., so any MFC would probably require a some gross shims
to allow for that.

MFC after:	1 month
2009-03-09 19:35:20 +00:00
Edward Tomasz Napierala
4f560d7595 Right now, when trying to unmount a device that's already gone,
msdosfs_unmount() and ffs_unmount() exit early after getting ENXIO.
However, dounmount() treats ENXIO as a success and proceeds with
unmounting.  In effect, the filesystem gets unmounted without closing
GEOM provider etc.

Reviewed by:	kib
Approved by:	rwatson (mentor)
Tested by:	dho
Sponsored by:	FreeBSD Foundation
2009-02-23 21:09:28 +00:00
Edward Tomasz Napierala
3c140b2df4 Refactor, moving error checking outside of the
'if (mp->mnt_flag & MNT_SOFTDEP)' conditional.  No functional
changes.

Reviewed by:	kib
Approved by:	rwatson (mentor)
Tested by:	pho
Sponsored by:	FreeBSD Foundation
2009-02-23 20:56:27 +00:00
John Baldwin
ee445a69c5 - If the g_access() call for the initial root mount fails, then fully
cleanup.  Before the GEOM consumer would not have been closed.
- Bump the reference on the character device being mounted while the
  associated devfs vnode is locked.

Reviewed by:	kib
2009-02-11 22:19:54 +00:00
Edward Tomasz Napierala
8a3f2c376a When a device containing mounted UFS filesystem disappears, the type
of devvp becomes VBAD, which UFS incorrectly interprets as snapshot
vnode, which in turns causes panic.  Fix it by replacing '!= VCHR'
with '== VREG'.

With this fix in place, you should no longer be able to panic the system
by removing a device with an UFS filesystem mounted from it - assuming
you don't use softupdates.

Reviewed by:	kib
Tested by:	pho
Approved by:	rwatson (mentor)
Sponsored by:	FreeBSD Foundation
2009-02-06 17:14:07 +00:00
Edward Tomasz Napierala
49c4791ccc Make sure the cdev doesn't go away while the filesystem is still mounted.
Otherwise dev2udev() could return garbage.

Reviewed by:	kib
Approved by:	rwatson (mentor)
Sponsored by:	FreeBSD Foundation
2009-01-29 16:47:15 +00:00
Robert Watson
ec7e66e84c Following a fair amount of real world experience with ACLs and
extended attributes since FreeBSD 5, make the following semantic
changes:

- Don't update the inode modification time (mtime) when extended
  attributes (and hence also ACLs) are added, modified, or removed.
- Don't update the inode access tie (atime) when extended attributes
  (and hence also ACLs) are queried.

This means that rsync (and related tools) won't improperly think
that the data in the file has changed when only the ACL has changed.

Note that ffs_reallocblks() has not been changed to not update on an
IO_EXT transaction, but currently EAs don't use the cluster write
routines so this shouldn't be a problem.  If EAs grow support for
clustering, then VOP_REALLOCBLKS() will need to grow a flag argument
to carry down IO_EXT to UFS.

MFC after:	1 week
PR:             ports/125739
Reported by:    Alexander Zagrebin <alexz@visp.ru>
Tested by:      pluknet <pluknet@gmail.com>,
                Greg Byshenk <freebsd@byshenk.net>
Discussed with: kib, kientzle, timur, Alexander Bokovoy <ab@samba.org>
2009-01-27 21:48:47 +00:00
Konstantin Belousov
b51b07be87 The r187467 should remove all pages for V_NORMAL case too, because
indirect block pages are not removed by the mentioned invocation of
the vnode_pager_setsize().

Put a common code into the helper function ffs_pages_remove().

Reported and tested by:	dchagin
Reviewed by:	ups
MFC after:	3 weeks
2009-01-20 22:00:19 +00:00
Konstantin Belousov
b1a4c8e522 When extending inode size, we call vnode_pager_setsize(), to have a
address space where to put vnode pages, and then call UFS_BALLOC(),
to actually allocate new block and map it. When UFS_BALLOC() returns
error, sometimes we forget to revert the vm object size increase,
allowing for the pages that are not backed by the logical disk blocks.

Revert vnode_pager_setsize() back when UFS_BALLOC() failed, for
ffs_truncate() and ffs_write().

PR:	129956
Reviewed by:	ups
MFC after:	3 weeks
2009-01-20 11:30:22 +00:00
Konstantin Belousov
9316467d05 FFS puts the extended attributes blocks at the negative blocks for the
vnode, from -1 down. When vinvalbuf(vp, V_ALT) is done for the vnode, it
incorrectly does vm_object_page_remove(0, 0), removing all pages from
the underlying vm object, not only the pages that back the extended
attributes data.

Change vinvalbuf() to not remove any pages from the object when
V_NORMAL or V_ALT are specified. Instead, the only in-tree caller
in ffs_inode.c:ffs_truncate() that specifies V_ALT explicitely
removes the corresponding page range. The V_NORMAL caller
does vnode_pager_setsize(vp, 0) immediately after the call to
vinvalbuf(V_NORMAL) already.

Reported by:	csjp
Reviewed by:	ups
MFC after:	3 weeks
2009-01-20 11:27:45 +00:00
Konstantin Belousov
df86ccf642 If unmount of the ffs mp failed, reinitialize the extended attributes
for the mp, and restart them if autostart is enabled.

Reported and tested by:	pho
Reviewed by:	rwatson
MFC after:	3 weeks
2009-01-08 12:48:27 +00:00
Doug Ambrisko
f1c1cdbb9c For now on every 10 cyclinder groups flush the buffer cache to free
up space.  If the buffer cache fills up then the disk systems can
grind to a halt.  Better tuning can be figured out later.

Tested by:	Tim, others and work
Reviewed by:	Kostik Belousov
PR:		128832
2008-11-13 17:40:21 +00:00
Attilio Rao
83b3bdbc8a Improve VFS locking:
- Implement real draining for vfs consumers by not relying on the
  mnt_lock and using instead a refcount in order to keep track of lock
  requesters.
- Due to the change above, remove the mnt_lock lockmgr because it is now
  useless.
- Due to the change above, vfs_busy() is no more linked to a lockmgr.
  Change so its KPI by removing the interlock argument and defining 2 new
  flags for it: MBF_NOWAIT which basically replaces the LK_NOWAIT of the
  old version (which was unlinked from the lockmgr alredy) and
  MBF_MNTLSTLOCK which provides the ability to drop the mountlist_mtx
  once the mnt interlock is held (ability still desired by most consumers).
- The stub used into vfs_mount_destroy(), that allows to override the
  mnt_ref if running for more than 3 seconds, make it totally useless.
  Remove it as it was thought to work into older versions.
  If a problem of "refcount held never going away" should appear, we will
  need to fix properly instead than trust on such hackish solution.
- Fix a bug where returning (with an error) from dounmount() was still
  leaving the MNTK_MWAIT flag on even if it the waiters were actually
  woken up. Just a place in vfs_mount_destroy() is left because it is
  going to recycle the structure in any case, so it doesn't matter.
- Remove the markercnt refcount as it is useless.

This patch modifies VFS ABI and breaks KPI for vfs_busy() so manpages and
__FreeBSD_version will be modified accordingly.

Discussed with:	kib
Tested by:	pho
2008-11-02 10:15:42 +00:00
Edward Tomasz Napierala
15bc6b2bd8 Introduce accmode_t. This is required for NFSv4 ACLs - it will be neccessary
to add more V* constants, and the variables changed by this patch were often
being assigned to mode_t variables, which is 16 bit.

Approved by:	rwatson (mentor)
2008-10-28 13:44:11 +00:00
Dag-Erling Smørgrav
e11e3f187d Fix a number of style issues in the MALLOC / FREE commit. I've tried to
be careful not to fix anything that was already broken; the NFSv4 code is
particularly bad in this respect.
2008-10-23 20:26:15 +00:00
Dag-Erling Smørgrav
1ede983cc9 Retire the MALLOC and FREE macros. They are an abomination unto style(9).
MFC after:	3 months
2008-10-23 15:53:51 +00:00
Konstantin Belousov
016f98f947 Assert that v_holdcnt is non-zero before entering lockmgr in vn_lock
and ffs_lock. This cannot catch situations where holdcnt is incremented
not by curthread, but I think it is useful.

Reviewed by:	tegge, attilio
Tested by:	pho
MFC after:	2 weeks
2008-10-20 10:11:33 +00:00
Konstantin Belousov
4560452f01 Sync up summary information for cylinder groups while data is already
in memory during snapshot creation. This improves the results of the
background fsck.

Submitted by: tegge
MFC after: 1 week
2008-10-13 14:05:01 +00:00
Attilio Rao
0d7935fd01 Remove the struct thread unuseful argument from bufobj interface.
In particular following functions KPI results modified:
- bufobj_invalbuf()
- bufsync()

and BO_SYNC() "virtual method" of the buffer objects set.
Main consumers of bufobj functions are affected by this change too and,
in particular, functions which changed their KPI are:
- vinvalbuf()
- g_vfs_close()

Due to the KPI breakage, __FreeBSD_version will be bumped in a later
commit.

As a side note, please consider just temporary the 'curthread' argument
passing to VOP_SYNC() (in bufsync()) as it will be axed out ASAP

Reviewed by:	kib
Tested by:	Giovanni Trematerra <giovanni dot trematerra at gmail dot com>
2008-10-10 21:23:50 +00:00
John Baldwin
f888634792 Enable shared lookups on UFS. There are some remaining issues with forced
unmounts, but those are in the VFS lookup code are not UFS specific.

Tested by:	pho, kris
2008-09-24 18:53:04 +00:00
Konstantin Belousov
6fecb4e41e Suspend the write operations on the UFS filesystem being unmounted or
remounted from rw to ro.

Proposed and reviewed by:  tegge
In collaboration with:	pho
MFC after:	 1 month
2008-09-16 11:55:53 +00:00
Konstantin Belousov
2814d5ba5f When attempt is made to suspend a filesystem that is already syspended,
wait until the current suspension is lifted instead of silently returning
success immediately. The consequences of calling vfs_write() resume when
not owning the suspension are not well-defined at best.

Add the vfs_susp_clean() mount method to be called from
vfs_write_resume(). Set it to process_deferred_inactive() for ffs, and
stop calling it manually.

Add the thread flag TDP_IGNSUSP that allows to bypass the suspension
point in the vn_start_write. It is intended for use by VFS in the
situations where the suspender want to do some i/o requiring calls to
vn_start_write(), and this i/o cannot be done later.

Reviewed by:	tegge
In collaboration with:	pho
MFC after:	 1 month
2008-09-16 11:51:06 +00:00