FreeBSD/Linux Kernel Cross Reference
sys/ufs/ffs/softdep.h

     /*
      * Copyright 1998, 2000 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).
      *
      * Further information about soft updates can be obtained from:
     *
     *      Marshall Kirk McKusick          http://www.mckusick.com/softdep/
     *      1614 Oxford Street              mckusick@mckusick.com
     *      Berkeley, CA 94709-1608         +1-510-843-9542
     *      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.
     *
     * 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   9.7 (McKusick) 6/21/00
     * $FreeBSD: releng/5.2/sys/ufs/ffs/softdep.h 100344 2002-07-19 07:29:39Z mckusick $
     */
    
    #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 SPACECOUNTED flag
     * says that the files space has been accounted to the pending free
     * space count. The NEWBLOCK flag marks pagedep structures that have
     * just been allocated, so must be claimed by the inode before all
     * dependencies are complete. The INPROGRESS flag marks worklist
     * structures that are still on the worklist, but are being considered
     * for action by some process. The UFS1FMT flag indicates that the
     * inode being processed is a ufs1 format. The EXTDATA flag indicates
     * that the allocdirect describes an extended-attributes dependency.
     * 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  /* diradd & mkdir only */
   #define MKDIR_BODY      0x0020  /* diradd & mkdir only */
   #define RMDIR           0x0040  /* dirrem only */
   #define DIRCHG          0x0080  /* diradd & dirrem only */
   #define GOINGAWAY       0x0100  /* indirdep only */
   #define IOSTARTED       0x0200  /* inodedep & pagedep only */
   #define SPACECOUNTED    0x0400  /* inodedep only */
   #define NEWBLOCK        0x0800  /* pagedep only */
   #define INPROGRESS      0x1000  /* dirrem, freeblks, freefrag, freefile only */
   #define UFS1FMT         0x2000  /* indirdep only */
   #define EXTDATA         0x4000  /* allocdirect only */
   #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))
   #define WK_NEWDIRBLK(wk) ((struct newdirblk *)(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 5
   #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 */
           LIST_ENTRY(inodedep) id_deps;   /* bmsafemap's list of inodedep's */
           struct  buf *id_buf;            /* related bmsafemap (if pending) */
           long    id_savedextsize;        /* ext size saved during rollback */
           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 */
           struct  allocdirectlst id_extupdt; /* extdata updates pre-inode write */
           struct  allocdirectlst id_newextupdt; /* extdata updates at ino write */
           union {
           struct  ufs1_dinode *idu_savedino1; /* saved ufs1_dinode contents */
           struct  ufs2_dinode *idu_savedino2; /* saved ufs2_dinode contents */
           } id_un;
   };
   #define id_savedino1 id_un.idu_savedino1
   #define id_savedino2 id_un.idu_savedino2
   
   /*
    * 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 */
           int     nb_state;               /* state of bitmap dependency */
           ufs2_daddr_t nb_newblkno;       /* allocated block number */
           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. When a block is allocated to a directory, an
    * fsync of a file whose name is within that block must ensure not only
    * that the block containing the file name has been written, but also
    * that the on-disk inode references that block. When a new directory
    * block is created, we allocate a newdirblk structure which is linked
    * to the associated allocdirect (on its ad_newdirblk list). When the
    * allocdirect has been satisfied, the newdirblk structure is moved to
    * the inodedep id_bufwait list of its directory to await the inode
    * being written. When the inode is written, the directory entries are
    * fully committed and can be deleted from their pagedep->id_pendinghd
    * and inodedep->id_pendinghd lists.
    */
   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 */
           ufs2_daddr_t ad_newblkno;       /* new value of block pointer */
           ufs2_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) */
           struct  workhead ad_newdirblk;  /* dir block to notify when written */
   };
   
   /*
    * 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 */
           caddr_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 */
           ufs2_daddr_t ai_newblkno;       /* new block pointer value */
           ufs2_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  mount *ff_mnt;          /* associated mount point */
           ufs2_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 */
           uid_t   fb_uid;                 /* uid of previous owner of blocks */
           struct  vnode *fb_devvp;        /* filesystem device vnode */
           struct  mount *fb_mnt;          /* associated mount point */
           long    fb_oldextsize;          /* previous ext data size */
           off_t   fb_oldsize;             /* previous file size */
           ufs2_daddr_t fb_chkcnt;         /* used to check cnt of blks released */
           ufs2_daddr_t fb_dblks[NDADDR];  /* direct blk ptrs to deallocate */
           ufs2_daddr_t fb_iblks[NIADDR];  /* indirect blk ptrs to deallocate */
           ufs2_daddr_t fb_eblks[NXADDR];  /* 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  mount *fx_mnt;          /* associated mount point */
   };
   
   /*
    * 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 */
           struct  buf *md_buf;            /* MKDIR_BODY: buffer holding dir */
           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
   
   /*
    * A "newdirblk" structure tracks the progress of a newly allocated
    * directory block from its creation until it is claimed by its on-disk
    * inode. When a block is allocated to a directory, an fsync of a file
    * whose name is within that block must ensure not only that the block
    * containing the file name has been written, but also that the on-disk
    * inode references that block. When a new directory block is created,
    * we allocate a newdirblk structure which is linked to the associated
    * allocdirect (on its ad_newdirblk list). When the allocdirect has been
    * satisfied, the newdirblk structure is moved to the inodedep id_bufwait
    * list of its directory to await the inode being written. When the inode
    * is written, the directory entries are fully committed and can be
    * deleted from their pagedep->id_pendinghd and inodedep->id_pendinghd
    * lists. Note that we could track directory blocks allocated to indirect
    * blocks using a similar scheme with the allocindir structures. Rather
    * than adding this level of complexity, we simply write those newly 
    * allocated indirect blocks synchronously as such allocations are rare.
    */
   struct newdirblk {
           struct  worklist db_list;       /* id_inowait or pg_newdirblk */
   #       define  db_state db_list.wk_state /* unused */
           struct  pagedep *db_pagedep;    /* associated pagedep */
   };

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