Add Kirk McKusick's soft update code. This is an implementation of:

    http://www.ece.cmu.edu/~ganger/papers/CSE-TR-254-95/
This code currently has a restrictive copyright (for the time being)
and so is not enabled by default; "option FFS_SOFTUPDATES" will enable it.
Note that you will need to re-run config(8) as the old softdep stubs
have moved to ffs_softdep_stub.c.

1 files changed, 546 insertions, 0 deletions

diff --git a/sys/ufs/ffs/softdep.h b/sys/ufs/ffs/softdep.h
new file mode 100644
index 00000000000..3435f630417
--- /dev/null
+++ b/sys/ufs/ffs/softdep.h
@@ -0,0 +1,546 @@
+/*
+ * Copyright 1998 Marshall Kirk McKusick. All Rights Reserved.
+ *
+ * The soft updates code is derived from the appendix of a University
+ * of Michigan technical report (Gregory R. Ganger and Yale N. Patt,
+ * "Soft Updates: A Solution to the Metadata Update Problem in File
+ * Systems", CSE-TR-254-95, August 1995).
+ *
+ * The following are the copyrights and redistribution conditions that
+ * apply to this copy of the soft update software. For a license
+ * to use, redistribute or sell the soft update software under
+ * conditions other than those described here, please contact the
+ * author at one of the following addresses:
+ *
+ *	Marshall Kirk McKusick		mckusick@mckusick.com
+ *	1614 Oxford Street		+1-510-843-9542
+ *	Berkeley, CA 94709-1608
+ *	USA
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. None of the names of McKusick, Ganger, Patt, or the University of
+ *    Michigan may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ * 4. Redistributions in any form must be accompanied by information on
+ *    how to obtain complete source code for any accompanying software
+ *    that uses this software. This source code must either be included
+ *    in the distribution or be available for no more than the cost of
+ *    distribution plus a nominal fee, and must be freely redistributable
+ *    under reasonable conditions. For an executable file, complete
+ *    source code means the source code for all modules it contains.
+ *    It does not mean source code for modules or files that typically
+ *    accompany the operating system on which the executable file runs,
+ *    e.g., standard library modules or system header files.
+ *
+ * THIS SOFTWARE IS PROVIDED BY MARSHALL KIRK MCKUSICK ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED.  IN NO EVENT SHALL MARSHALL KIRK MCKUSICK BE LIABLE FOR
+ * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ *	@(#)softdep.h	9.5 (McKusick) 2/11/98
+ */
+
+#include <sys/queue.h>
+
+/*
+ * Allocation dependencies are handled with undo/redo on the in-memory
+ * copy of the data. A particular data dependency is eliminated when
+ * it is ALLCOMPLETE: that is ATTACHED, DEPCOMPLETE, and COMPLETE.
+ * 
+ * ATTACHED means that the data is not currently being written to
+ * disk. UNDONE means that the data has been rolled back to a safe
+ * state for writing to the disk. When the I/O completes, the data is
+ * restored to its current form and the state reverts to ATTACHED.
+ * The data must be locked throughout the rollback, I/O, and roll
+ * forward so that the rolled back information is never visible to
+ * user processes. The COMPLETE flag indicates that the item has been
+ * written. For example, a dependency that requires that an inode be
+ * written will be marked COMPLETE after the inode has been written
+ * to disk. The DEPCOMPLETE flag indicates the completion of any other
+ * dependencies such as the writing of a cylinder group map has been
+ * completed. A dependency structure may be freed only when both it
+ * and its dependencies have completed and any rollbacks that are in
+ * progress have finished as indicated by the set of ALLCOMPLETE flags
+ * all being set. The two MKDIR flags indicate additional dependencies
+ * that must be done when creating a new directory. MKDIR_BODY is
+ * cleared when the directory data block containing the "." and ".."
+ * entries has been written. MKDIR_PARENT is cleared when the parent
+ * inode with the increased link count for ".." has been written. When
+ * both MKDIR flags have been cleared, the DEPCOMPLETE flag is set to
+ * indicate that the directory dependencies have been completed. The
+ * writing of the directory inode itself sets the COMPLETE flag which
+ * then allows the directory entry for the new directory to be written
+ * to disk. The RMDIR flag marks a dirrem structure as representing
+ * the removal of a directory rather than a file. When the removal
+ * dependencies are completed, additional work needs to be done
+ * (truncation of the "." and ".." entries, an additional decrement
+ * of the associated inode, and a decrement of the parent inode). The
+ * DIRCHG flag marks a diradd structure as representing the changing
+ * of an existing entry rather than the addition of a new one. When
+ * the update is complete the dirrem associated with the inode for
+ * the old name must be added to the worklist to do the necessary
+ * reference count decrement. The GOINGAWAY flag indicates that the
+ * data structure is frozen from further change until its dependencies
+ * have been completed and its resources freed after which it will be
+ * discarded. The IOSTARTED flag prevents multiple calls to the I/O
+ * start routine from doing multiple rollbacks. The ONWORKLIST flag
+ * shows whether the structure is currently linked onto a worklist.
+ */
+#define	ATTACHED	0x0001
+#define	UNDONE		0x0002
+#define	COMPLETE	0x0004
+#define	DEPCOMPLETE	0x0008
+#define MKDIR_PARENT	0x0010
+#define MKDIR_BODY	0x0020
+#define RMDIR		0x0040
+#define DIRCHG		0x0080
+#define GOINGAWAY	0x0100
+#define IOSTARTED	0x0200
+#define ONWORKLIST	0x8000
+
+#define	ALLCOMPLETE	(ATTACHED | COMPLETE | DEPCOMPLETE)
+
+/*
+ * The workitem queue.
+ * 
+ * It is sometimes useful and/or necessary to clean up certain dependencies
+ * in the background rather than during execution of an application process
+ * or interrupt service routine. To realize this, we append dependency
+ * structures corresponding to such tasks to a "workitem" queue. In a soft
+ * updates implementation, most pending workitems should not wait for more
+ * than a couple of seconds, so the filesystem syncer process awakens once
+ * per second to process the items on the queue.
+ */
+
+/* LIST_HEAD(workhead, worklist);	-- declared in buf.h */
+
+/*
+ * Each request can be linked onto a work queue through its worklist structure.
+ * To avoid the need for a pointer to the structure itself, this structure
+ * MUST be declared FIRST in each type in which it appears! If more than one
+ * worklist is needed in the structure, then a wk_data field must be added
+ * and the macros below changed to use it.
+ */
+struct worklist {
+	LIST_ENTRY(worklist)	wk_list;	/* list of work requests */
+	unsigned short		wk_type;	/* type of request */
+	unsigned short		wk_state;	/* state flags */
+};
+#define WK_DATA(wk) ((void *)(wk))
+#define WK_PAGEDEP(wk) ((struct pagedep *)(wk))
+#define WK_INODEDEP(wk) ((struct inodedep *)(wk))
+#define WK_NEWBLK(wk) ((struct newblk *)(wk))
+#define WK_BMSAFEMAP(wk) ((struct bmsafemap *)(wk))
+#define WK_ALLOCDIRECT(wk) ((struct allocdirect *)(wk))
+#define WK_INDIRDEP(wk) ((struct indirdep *)(wk))
+#define WK_ALLOCINDIR(wk) ((struct allocindir *)(wk))
+#define WK_FREEFRAG(wk) ((struct freefrag *)(wk))
+#define WK_FREEBLKS(wk) ((struct freeblks *)(wk))
+#define WK_FREEFILE(wk) ((struct freefile *)(wk))
+#define WK_DIRADD(wk) ((struct diradd *)(wk))
+#define WK_MKDIR(wk) ((struct mkdir *)(wk))
+#define WK_DIRREM(wk) ((struct dirrem *)(wk))
+
+/*
+ * Various types of lists
+ */
+LIST_HEAD(dirremhd, dirrem);
+LIST_HEAD(diraddhd, diradd);
+LIST_HEAD(newblkhd, newblk);
+LIST_HEAD(inodedephd, inodedep);
+LIST_HEAD(allocindirhd, allocindir);
+LIST_HEAD(allocdirecthd, allocdirect);
+TAILQ_HEAD(allocdirectlst, allocdirect);
+
+/*
+ * The "pagedep" structure tracks the various dependencies related to
+ * a particular directory page. If a directory page has any dependencies,
+ * it will have a pagedep linked to its associated buffer. The
+ * pd_dirremhd list holds the list of dirrem requests which decrement
+ * inode reference counts. These requests are processed after the
+ * directory page with the corresponding zero'ed entries has been
+ * written. The pd_diraddhd list maintains the list of diradd requests
+ * which cannot be committed until their corresponding inode has been
+ * written to disk. Because a directory may have many new entries
+ * being created, several lists are maintained hashed on bits of the
+ * offset of the entry into the directory page to keep the lists from
+ * getting too long. Once a new directory entry has been cleared to
+ * be written, it is moved to the pd_pendinghd list. After the new
+ * entry has been written to disk it is removed from the pd_pendinghd
+ * list, any removed operations are done, and the dependency structure
+ * is freed.
+ */
+#define DAHASHSZ 6
+#define DIRADDHASH(offset) (((offset) >> 2) % DAHASHSZ)
+struct pagedep {
+	struct	worklist pd_list;	/* page buffer */
+#	define	pd_state pd_list.wk_state /* check for multiple I/O starts */
+	LIST_ENTRY(pagedep) pd_hash;	/* hashed lookup */
+	struct	mount *pd_mnt;		/* associated mount point */
+	ino_t	pd_ino;			/* associated file */
+	ufs_lbn_t pd_lbn;		/* block within file */
+	struct	dirremhd pd_dirremhd;	/* dirrem's waiting for page */
+	struct	diraddhd pd_diraddhd[DAHASHSZ]; /* diradd dir entry updates */
+	struct	diraddhd pd_pendinghd;	/* directory entries awaiting write */
+};
+
+/*
+ * The "inodedep" structure tracks the set of dependencies associated
+ * with an inode. One task that it must manage is delayed operations
+ * (i.e., work requests that must be held until the inodedep's associated
+ * inode has been written to disk). Getting an inode from its incore 
+ * state to the disk requires two steps to be taken by the filesystem
+ * in this order: first the inode must be copied to its disk buffer by
+ * the VOP_UPDATE operation; second the inode's buffer must be written
+ * to disk. To ensure that both operations have happened in the required
+ * order, the inodedep maintains two lists. Delayed operations are
+ * placed on the id_inowait list. When the VOP_UPDATE is done, all
+ * operations on the id_inowait list are moved to the id_bufwait list.
+ * When the buffer is written, the items on the id_bufwait list can be
+ * safely moved to the work queue to be processed. A second task of the
+ * inodedep structure is to track the status of block allocation within
+ * the inode.  Each block that is allocated is represented by an
+ * "allocdirect" structure (see below). It is linked onto the id_newinoupdt
+ * list until both its contents and its allocation in the cylinder
+ * group map have been written to disk. Once these dependencies have been
+ * satisfied, it is removed from the id_newinoupdt list and any followup
+ * actions such as releasing the previous block or fragment are placed
+ * on the id_inowait list. When an inode is updated (a VOP_UPDATE is
+ * done), the "inodedep" structure is linked onto the buffer through
+ * its worklist. Thus, it will be notified when the buffer is about
+ * to be written and when it is done. At the update time, all the
+ * elements on the id_newinoupdt list are moved to the id_inoupdt list
+ * since those changes are now relevant to the copy of the inode in the
+ * buffer. Also at update time, the tasks on the id_inowait list are
+ * moved to the id_bufwait list so that they will be executed when
+ * the updated inode has been written to disk. When the buffer containing
+ * the inode is written to disk, any updates listed on the id_inoupdt
+ * list are rolled back as they are not yet safe. Following the write,
+ * the changes are once again rolled forward and any actions on the
+ * id_bufwait list are processed (since those actions are now safe).
+ * The entries on the id_inoupdt and id_newinoupdt lists must be kept
+ * sorted by logical block number to speed the calculation of the size
+ * of the rolled back inode (see explanation in initiate_write_inodeblock).
+ * When a directory entry is created, it is represented by a diradd.
+ * The diradd is added to the id_inowait list as it cannot be safely
+ * written to disk until the inode that it represents is on disk. After
+ * the inode is written, the id_bufwait list is processed and the diradd
+ * entries are moved to the id_pendinghd list where they remain until
+ * the directory block containing the name has been written to disk.
+ * The purpose of keeping the entries on the id_pendinghd list is so that
+ * the softdep_fsync function can find and push the inode's directory
+ * name(s) as part of the fsync operation for that file.
+ */
+struct inodedep {
+	struct	worklist id_list;	/* buffer holding inode block */
+#	define	id_state id_list.wk_state /* inode dependency state */
+	LIST_ENTRY(inodedep) id_hash;	/* hashed lookup */
+	struct	fs *id_fs;		/* associated filesystem */
+	ino_t	id_ino;			/* dependent inode */
+	nlink_t	id_nlinkdelta;		/* saved effective link count */
+	struct	dinode *id_savedino;	/* saved dinode contents */
+	LIST_ENTRY(inodedep) id_deps;	/* bmsafemap's list of inodedep's */
+	struct	buf *id_buf;		/* related bmsafemap (if pending) */
+	off_t	id_savedsize;		/* file size saved during rollback */
+	struct	workhead id_pendinghd;	/* entries awaiting directory write */
+	struct	workhead id_bufwait;	/* operations after inode written */
+	struct	workhead id_inowait;	/* operations waiting inode update */
+	struct	allocdirectlst id_inoupdt; /* updates before inode written */
+	struct	allocdirectlst id_newinoupdt; /* updates when inode written */
+};
+
+/*
+ * A "newblk" structure is attached to a bmsafemap structure when a block
+ * or fragment is allocated from a cylinder group. Its state is set to
+ * DEPCOMPLETE when its cylinder group map is written. It is consumed by
+ * an associated allocdirect or allocindir allocation which will attach
+ * themselves to the bmsafemap structure if the newblk's DEPCOMPLETE flag
+ * is not set (i.e., its cylinder group map has not been written).
+ */ 
+struct newblk {
+	LIST_ENTRY(newblk) nb_hash;	/* hashed lookup */
+	struct	fs *nb_fs;		/* associated filesystem */
+	ufs_daddr_t nb_newblkno;	/* allocated block number */
+	int	nb_state;		/* state of bitmap dependency */
+	LIST_ENTRY(newblk) nb_deps;	/* bmsafemap's list of newblk's */
+	struct	bmsafemap *nb_bmsafemap; /* associated bmsafemap */
+};
+
+/*
+ * A "bmsafemap" structure maintains a list of dependency structures
+ * that depend on the update of a particular cylinder group map.
+ * It has lists for newblks, allocdirects, allocindirs, and inodedeps.
+ * It is attached to the buffer of a cylinder group block when any of
+ * these things are allocated from the cylinder group. It is freed
+ * after the cylinder group map is written and the state of its
+ * dependencies are updated with DEPCOMPLETE to indicate that it has
+ * been processed.
+ */
+struct bmsafemap {
+	struct	worklist sm_list;	/* cylgrp buffer */
+	struct	buf *sm_buf;		/* associated buffer */
+	struct	allocdirecthd sm_allocdirecthd; /* allocdirect deps */
+	struct	allocindirhd sm_allocindirhd; /* allocindir deps */
+	struct	inodedephd sm_inodedephd; /* inodedep deps */
+	struct	newblkhd sm_newblkhd;	/* newblk deps */
+};
+
+/*
+ * An "allocdirect" structure is attached to an "inodedep" when a new block
+ * or fragment is allocated and pointed to by the inode described by
+ * "inodedep". The worklist is linked to the buffer that holds the block.
+ * When the block is first allocated, it is linked to the bmsafemap
+ * structure associated with the buffer holding the cylinder group map
+ * from which it was allocated. When the cylinder group map is written
+ * to disk, ad_state has the DEPCOMPLETE flag set. When the block itself
+ * is written, the COMPLETE flag is set. Once both the cylinder group map
+ * and the data itself have been written, it is safe to write the inode
+ * that claims the block. If there was a previous fragment that had been
+ * allocated before the file was increased in size, the old fragment may
+ * be freed once the inode claiming the new block is written to disk.
+ * This ad_fragfree request is attached to the id_inowait list of the
+ * associated inodedep (pointed to by ad_inodedep) for processing after
+ * the inode is written.
+ */
+struct allocdirect {
+	struct	worklist ad_list;	/* buffer holding block */
+#	define	ad_state ad_list.wk_state /* block pointer state */
+	TAILQ_ENTRY(allocdirect) ad_next; /* inodedep's list of allocdirect's */
+	ufs_lbn_t ad_lbn;		/* block within file */
+	ufs_daddr_t ad_newblkno;	/* new value of block pointer */
+	ufs_daddr_t ad_oldblkno;	/* old value of block pointer */
+	long	ad_newsize;		/* size of new block */
+	long	ad_oldsize;		/* size of old block */
+	LIST_ENTRY(allocdirect) ad_deps; /* bmsafemap's list of allocdirect's */
+	struct	buf *ad_buf;		/* cylgrp buffer (if pending) */
+	struct	inodedep *ad_inodedep;	/* associated inodedep */
+	struct	freefrag *ad_freefrag;	/* fragment to be freed (if any) */
+};
+
+/*
+ * A single "indirdep" structure manages all allocation dependencies for
+ * pointers in an indirect block. The up-to-date state of the indirect
+ * block is stored in ir_savedata. The set of pointers that may be safely
+ * written to the disk is stored in ir_safecopy. The state field is used
+ * only to track whether the buffer is currently being written (in which
+ * case it is not safe to update ir_safecopy). Ir_deplisthd contains the
+ * list of allocindir structures, one for each block that needs to be
+ * written to disk. Once the block and its bitmap allocation have been
+ * written the safecopy can be updated to reflect the allocation and the
+ * allocindir structure freed. If ir_state indicates that an I/O on the
+ * indirect block is in progress when ir_safecopy is to be updated, the
+ * update is deferred by placing the allocindir on the ir_donehd list.
+ * When the I/O on the indirect block completes, the entries on the
+ * ir_donehd list are processed by updating their corresponding ir_safecopy
+ * pointers and then freeing the allocindir structure.
+ */
+struct indirdep {
+	struct	worklist ir_list;	/* buffer holding indirect block */
+#	define	ir_state ir_list.wk_state /* indirect block pointer state */
+	ufs_daddr_t *ir_saveddata;	/* buffer cache contents */
+	struct	buf *ir_savebp;		/* buffer holding safe copy */
+	struct	allocindirhd ir_donehd;	/* done waiting to update safecopy */
+	struct	allocindirhd ir_deplisthd; /* allocindir deps for this block */
+};
+
+/*
+ * An "allocindir" structure is attached to an "indirdep" when a new block
+ * is allocated and pointed to by the indirect block described by the
+ * "indirdep". The worklist is linked to the buffer that holds the new block.
+ * When the block is first allocated, it is linked to the bmsafemap
+ * structure associated with the buffer holding the cylinder group map
+ * from which it was allocated. When the cylinder group map is written
+ * to disk, ai_state has the DEPCOMPLETE flag set. When the block itself
+ * is written, the COMPLETE flag is set. Once both the cylinder group map
+ * and the data itself have been written, it is safe to write the entry in
+ * the indirect block that claims the block; the "allocindir" dependency 
+ * can then be freed as it is no longer applicable.
+ */
+struct allocindir {
+	struct	worklist ai_list;	/* buffer holding indirect block */
+#	define	ai_state ai_list.wk_state /* indirect block pointer state */
+	LIST_ENTRY(allocindir) ai_next;	/* indirdep's list of allocindir's */
+	int	ai_offset;		/* pointer offset in indirect block */
+	ufs_daddr_t ai_newblkno;	/* new block pointer value */
+	ufs_daddr_t ai_oldblkno;	/* old block pointer value */
+	struct	freefrag *ai_freefrag;	/* block to be freed when complete */
+	struct	indirdep *ai_indirdep;	/* address of associated indirdep */
+	LIST_ENTRY(allocindir) ai_deps;	/* bmsafemap's list of allocindir's */
+	struct	buf *ai_buf;		/* cylgrp buffer (if pending) */
+};
+
+/*
+ * A "freefrag" structure is attached to an "inodedep" when a previously
+ * allocated fragment is replaced with a larger fragment, rather than extended.
+ * The "freefrag" structure is constructed and attached when the replacement
+ * block is first allocated. It is processed after the inode claiming the
+ * bigger block that replaces it has been written to disk. Note that the
+ * ff_state field is is used to store the uid, so may lose data. However,
+ * the uid is used only in printing an error message, so is not critical.
+ * Keeping it in a short keeps the data structure down to 32 bytes.
+ */
+struct freefrag {
+	struct	worklist ff_list;	/* id_inowait or delayed worklist */
+#	define	ff_state ff_list.wk_state /* owning user; should be uid_t */
+	struct	vnode *ff_devvp;	/* filesystem device vnode */
+	struct	fs *ff_fs;		/* addr of superblock */
+	ufs_daddr_t ff_blkno;		/* fragment physical block number */
+	long	ff_fragsize;		/* size of fragment being deleted */
+	ino_t	ff_inum;		/* owning inode number */
+};
+
+/*
+ * A "freeblks" structure is attached to an "inodedep" when the
+ * corresponding file's length is reduced to zero. It records all
+ * the information needed to free the blocks of a file after its
+ * zero'ed inode has been written to disk.
+ */
+struct freeblks {
+	struct	worklist fb_list;	/* id_inowait or delayed worklist */
+	ino_t	fb_previousinum;	/* inode of previous owner of blocks */
+	struct	vnode *fb_devvp;	/* filesystem device vnode */
+	struct	fs *fb_fs;		/* addr of superblock */
+	off_t	fb_oldsize;		/* previous file size */
+	off_t	fb_newsize;		/* new file size */
+	int	fb_chkcnt;		/* used to check cnt of blks released */
+	uid_t	fb_uid;			/* uid of previous owner of blocks */
+	ufs_daddr_t fb_dblks[NDADDR];	/* direct blk ptrs to deallocate */
+	ufs_daddr_t fb_iblks[NIADDR];	/* indirect blk ptrs to deallocate */
+};
+
+/*
+ * A "freefile" structure is attached to an inode when its
+ * link count is reduced to zero. It marks the inode as free in
+ * the cylinder group map after the zero'ed inode has been written
+ * to disk and any associated blocks and fragments have been freed.
+ */
+struct freefile {
+	struct	worklist fx_list;	/* id_inowait or delayed worklist */
+	mode_t	fx_mode;		/* mode of inode */
+	ino_t	fx_oldinum;		/* inum of the unlinked file */
+	struct	vnode *fx_devvp;	/* filesystem device vnode */
+	struct	fs *fx_fs;		/* addr of superblock */
+};
+
+/*
+ * A "diradd" structure is linked to an "inodedep" id_inowait list when a
+ * new directory entry is allocated that references the inode described
+ * by "inodedep". When the inode itself is written (either the initial
+ * allocation for new inodes or with the increased link count for
+ * existing inodes), the COMPLETE flag is set in da_state. If the entry
+ * is for a newly allocated inode, the "inodedep" structure is associated
+ * with a bmsafemap which prevents the inode from being written to disk
+ * until the cylinder group has been updated. Thus the da_state COMPLETE
+ * flag cannot be set until the inode bitmap dependency has been removed.
+ * When creating a new file, it is safe to write the directory entry that
+ * claims the inode once the referenced inode has been written. Since
+ * writing the inode clears the bitmap dependencies, the DEPCOMPLETE flag
+ * in the diradd can be set unconditionally when creating a file. When
+ * creating a directory, there are two additional dependencies described by
+ * mkdir structures (see their description below). When these dependencies
+ * are resolved the DEPCOMPLETE flag is set in the diradd structure.
+ * If there are multiple links created to the same inode, there will be
+ * a separate diradd structure created for each link. The diradd is
+ * linked onto the pg_diraddhd list of the pagedep for the directory
+ * page that contains the entry. When a directory page is written,
+ * the pg_diraddhd list is traversed to rollback any entries that are
+ * not yet ready to be written to disk. If a directory entry is being
+ * changed (by rename) rather than added, the DIRCHG flag is set and
+ * the da_previous entry points to the entry that will be "removed"
+ * once the new entry has been committed. During rollback, entries
+ * with da_previous are replaced with the previous inode number rather
+ * than zero.
+ *
+ * The overlaying of da_pagedep and da_previous is done to keep the
+ * structure down to 32 bytes in size on a 32-bit machine. If a
+ * da_previous entry is present, the pointer to its pagedep is available
+ * in the associated dirrem entry. If the DIRCHG flag is set, the
+ * da_previous entry is valid; if not set the da_pagedep entry is valid.
+ * The DIRCHG flag never changes; it is set when the structure is created
+ * if appropriate and is never cleared.
+ */
+struct diradd {
+	struct	worklist da_list;	/* id_inowait or id_pendinghd list */
+#	define	da_state da_list.wk_state /* state of the new directory entry */
+	LIST_ENTRY(diradd) da_pdlist;	/* pagedep holding directory block */
+	doff_t	da_offset;		/* offset of new dir entry in dir blk */
+	ino_t	da_newinum;		/* inode number for the new dir entry */
+	union {
+	struct	dirrem *dau_previous;	/* entry being replaced in dir change */
+	struct	pagedep *dau_pagedep;	/* pagedep dependency for addition */
+	} da_un;
+};
+#define da_previous da_un.dau_previous
+#define da_pagedep da_un.dau_pagedep
+
+/*
+ * Two "mkdir" structures are needed to track the additional dependencies
+ * associated with creating a new directory entry. Normally a directory
+ * addition can be committed as soon as the newly referenced inode has been
+ * written to disk with its increased link count. When a directory is
+ * created there are two additional dependencies: writing the directory
+ * data block containing the "." and ".." entries (MKDIR_BODY) and writing
+ * the parent inode with the increased link count for ".." (MKDIR_PARENT).
+ * These additional dependencies are tracked by two mkdir structures that
+ * reference the associated "diradd" structure. When they have completed,
+ * they set the DEPCOMPLETE flag on the diradd so that it knows that its
+ * extra dependencies have been completed. The md_state field is used only
+ * to identify which type of dependency the mkdir structure is tracking.
+ * It is not used in the mainline code for any purpose other than consistency
+ * checking. All the mkdir structures in the system are linked together on
+ * a list. This list is needed so that a diradd can find its associated
+ * mkdir structures and deallocate them if it is prematurely freed (as for
+ * example if a mkdir is immediately followed by a rmdir of the same directory).
+ * Here, the free of the diradd must traverse the list to find the associated
+ * mkdir structures that reference it. The deletion would be faster if the
+ * diradd structure were simply augmented to have two pointers that referenced
+ * the associated mkdir's. However, this would increase the size of the diradd
+ * structure from 32 to 64-bits to speed a very infrequent operation.
+ */
+struct mkdir {
+	struct	worklist md_list;	/* id_inowait or buffer holding dir */
+#	define	md_state md_list.wk_state /* type: MKDIR_PARENT or MKDIR_BODY */
+	struct	diradd *md_diradd;	/* associated diradd */
+	LIST_ENTRY(mkdir) md_mkdirs;	/* list of all mkdirs */
+};
+LIST_HEAD(mkdirlist, mkdir) mkdirlisthd;
+
+/*
+ * A "dirrem" structure describes an operation to decrement the link
+ * count on an inode. The dirrem structure is attached to the pg_dirremhd
+ * list of the pagedep for the directory page that contains the entry.
+ * It is processed after the directory page with the deleted entry has
+ * been written to disk.
+ *
+ * The overlaying of dm_pagedep and dm_dirinum is done to keep the
+ * structure down to 32 bytes in size on a 32-bit machine. It works
+ * because they are never used concurrently.
+ */
+struct dirrem {
+	struct	worklist dm_list;	/* delayed worklist */
+#	define	dm_state dm_list.wk_state /* state of the old directory entry */
+	LIST_ENTRY(dirrem) dm_next;	/* pagedep's list of dirrem's */
+	struct	mount *dm_mnt;		/* associated mount point */
+	ino_t	dm_oldinum;		/* inum of the removed dir entry */
+	union {
+	struct	pagedep *dmu_pagedep;	/* pagedep dependency for remove */
+	ino_t	dmu_dirinum;		/* parent inode number (for rmdir) */
+	} dm_un;
+};
+#define dm_pagedep dm_un.dmu_pagedep
+#define dm_dirinum dm_un.dmu_dirinum