tdb/common/freelist.c

説明を見る。

 /* 
   Unix SMB/CIFS implementation.

   trivial database library

   Copyright (C) Andrew Tridgell              1999-2005
   Copyright (C) Paul `Rusty' Russell              2000
   Copyright (C) Jeremy Allison                    2000-2003
   
     ** NOTE! The following LGPL license applies to the tdb
     ** library. This does NOT imply that all of Samba is released
     ** under the LGPL
   
   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2 of the License, or (at your option) any later version.

   This library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with this library; if not, write to the Free Software
   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*/

#include "tdb_private.h"

/* read a freelist record and check for simple errors */
int rec_free_read(struct tdb_context *tdb, tdb_off_t off, struct list_struct *rec)
{
        if (tdb->methods->tdb_read(tdb, off, rec, sizeof(*rec),DOCONV()) == -1)
                return -1;

        if (rec->magic == TDB_MAGIC) {
                /* this happens when a app is showdown while deleting a record - we should
                   not completely fail when this happens */
                TDB_LOG((tdb, TDB_DEBUG_WARNING, "rec_free_read non-free magic 0x%x at offset=%d - fixing\n", 
                         rec->magic, off));
                rec->magic = TDB_FREE_MAGIC;
                if (tdb->methods->tdb_write(tdb, off, rec, sizeof(*rec)) == -1)
                        return -1;
        }

        if (rec->magic != TDB_FREE_MAGIC) {
                /* Ensure ecode is set for log fn. */
                tdb->ecode = TDB_ERR_CORRUPT;
                TDB_LOG((tdb, TDB_DEBUG_WARNING, "rec_free_read bad magic 0x%x at offset=%d\n", 
                           rec->magic, off));
                return TDB_ERRCODE(TDB_ERR_CORRUPT, -1);
        }
        if (tdb->methods->tdb_oob(tdb, rec->next+sizeof(*rec), 0) != 0)
                return -1;
        return 0;
}



/* Remove an element from the freelist.  Must have alloc lock. */
static int remove_from_freelist(struct tdb_context *tdb, tdb_off_t off, tdb_off_t next)
{
        tdb_off_t last_ptr, i;

        /* read in the freelist top */
        last_ptr = FREELIST_TOP;
        while (tdb_ofs_read(tdb, last_ptr, &i) != -1 && i != 0) {
                if (i == off) {
                        /* We've found it! */
                        return tdb_ofs_write(tdb, last_ptr, &next);
                }
                /* Follow chain (next offset is at start of record) */
                last_ptr = i;
        }
        TDB_LOG((tdb, TDB_DEBUG_FATAL,"remove_from_freelist: not on list at off=%d\n", off));
        return TDB_ERRCODE(TDB_ERR_CORRUPT, -1);
}


/* update a record tailer (must hold allocation lock) */
static int update_tailer(struct tdb_context *tdb, tdb_off_t offset,
                         const struct list_struct *rec)
{
        tdb_off_t totalsize;

        /* Offset of tailer from record header */
        totalsize = sizeof(*rec) + rec->rec_len;
        return tdb_ofs_write(tdb, offset + totalsize - sizeof(tdb_off_t),
                         &totalsize);
}

/* Add an element into the freelist. Merge adjacent records if
   neccessary. */
int tdb_free(struct tdb_context *tdb, tdb_off_t offset, struct list_struct *rec)
{
        tdb_off_t right, left;

        /* Allocation and tailer lock */
        if (tdb_lock(tdb, -1, F_WRLCK) != 0)
                return -1;

        /* set an initial tailer, so if we fail we don't leave a bogus record */
        if (update_tailer(tdb, offset, rec) != 0) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: update_tailer failed!\n"));
                goto fail;
        }

        /* Look right first (I'm an Australian, dammit) */
        right = offset + sizeof(*rec) + rec->rec_len;
        if (right + sizeof(*rec) <= tdb->map_size) {
                struct list_struct r;

                if (tdb->methods->tdb_read(tdb, right, &r, sizeof(r), DOCONV()) == -1) {
                        TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: right read failed at %u\n", right));
                        goto left;
                }

                /* If it's free, expand to include it. */
                if (r.magic == TDB_FREE_MAGIC) {
                        if (remove_from_freelist(tdb, right, r.next) == -1) {
                                TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: right free failed at %u\n", right));
                                goto left;
                        }
                        rec->rec_len += sizeof(r) + r.rec_len;
                }
        }

left:
        /* Look left */
        left = offset - sizeof(tdb_off_t);
        if (left > TDB_DATA_START(tdb->header.hash_size)) {
                struct list_struct l;
                tdb_off_t leftsize;
                
                /* Read in tailer and jump back to header */
                if (tdb_ofs_read(tdb, left, &leftsize) == -1) {
                        TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: left offset read failed at %u\n", left));
                        goto update;
                }

                /* it could be uninitialised data */
                if (leftsize == 0 || leftsize == TDB_PAD_U32) {
                        goto update;
                }

                left = offset - leftsize;

                /* Now read in record */
                if (tdb->methods->tdb_read(tdb, left, &l, sizeof(l), DOCONV()) == -1) {
                        TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: left read failed at %u (%u)\n", left, leftsize));
                        goto update;
                }

                /* If it's free, expand to include it. */
                if (l.magic == TDB_FREE_MAGIC) {
                        if (remove_from_freelist(tdb, left, l.next) == -1) {
                                TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: left free failed at %u\n", left));
                                goto update;
                        } else {
                                offset = left;
                                rec->rec_len += leftsize;
                        }
                }
        }

update:
        if (update_tailer(tdb, offset, rec) == -1) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: update_tailer failed at %u\n", offset));
                goto fail;
        }

        /* Now, prepend to free list */
        rec->magic = TDB_FREE_MAGIC;

        if (tdb_ofs_read(tdb, FREELIST_TOP, &rec->next) == -1 ||
            tdb_rec_write(tdb, offset, rec) == -1 ||
            tdb_ofs_write(tdb, FREELIST_TOP, &offset) == -1) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free record write failed at offset=%d\n", offset));
                goto fail;
        }

        /* And we're done. */
        tdb_unlock(tdb, -1, F_WRLCK);
        return 0;

 fail:
        tdb_unlock(tdb, -1, F_WRLCK);
        return -1;
}


/* 
   the core of tdb_allocate - called when we have decided which
   free list entry to use
 */
static tdb_off_t tdb_allocate_ofs(struct tdb_context *tdb, tdb_len_t length, tdb_off_t rec_ptr,
                                struct list_struct *rec, tdb_off_t last_ptr)
{
        struct list_struct newrec;
        tdb_off_t newrec_ptr;

        memset(&newrec, '\0', sizeof(newrec));

        /* found it - now possibly split it up  */
        if (rec->rec_len > length + MIN_REC_SIZE) {
                /* Length of left piece */
                length = TDB_ALIGN(length, TDB_ALIGNMENT);
                
                /* Right piece to go on free list */
                newrec.rec_len = rec->rec_len - (sizeof(*rec) + length);
                newrec_ptr = rec_ptr + sizeof(*rec) + length;
                
                /* And left record is shortened */
                rec->rec_len = length;
        } else {
                newrec_ptr = 0;
        }
        
        /* Remove allocated record from the free list */
        if (tdb_ofs_write(tdb, last_ptr, &rec->next) == -1) {
                return 0;
        }
        
        /* Update header: do this before we drop alloc
           lock, otherwise tdb_free() might try to
           merge with us, thinking we're free.
           (Thanks Jeremy Allison). */
        rec->magic = TDB_MAGIC;
        if (tdb_rec_write(tdb, rec_ptr, rec) == -1) {
                return 0;
        }
        
        /* Did we create new block? */
        if (newrec_ptr) {
                /* Update allocated record tailer (we
                   shortened it). */
                if (update_tailer(tdb, rec_ptr, rec) == -1) {
                        return 0;
                }
                
                /* Free new record */
                if (tdb_free(tdb, newrec_ptr, &newrec) == -1) {
                        return 0;
                }
        }
        
        /* all done - return the new record offset */
        return rec_ptr;
}

/* allocate some space from the free list. The offset returned points
   to a unconnected list_struct within the database with room for at
   least length bytes of total data

   0 is returned if the space could not be allocated
 */
tdb_off_t tdb_allocate(struct tdb_context *tdb, tdb_len_t length, struct list_struct *rec)
{
        tdb_off_t rec_ptr, last_ptr, newrec_ptr;
        struct {
                tdb_off_t rec_ptr, last_ptr;
                tdb_len_t rec_len;
        } bestfit;

        if (tdb_lock(tdb, -1, F_WRLCK) == -1)
                return 0;

        /* Extra bytes required for tailer */
        length += sizeof(tdb_off_t);

 again:
        last_ptr = FREELIST_TOP;

        /* read in the freelist top */
        if (tdb_ofs_read(tdb, FREELIST_TOP, &rec_ptr) == -1)
                goto fail;

        bestfit.rec_ptr = 0;
        bestfit.last_ptr = 0;
        bestfit.rec_len = 0;

        /* 
           this is a best fit allocation strategy. Originally we used
           a first fit strategy, but it suffered from massive fragmentation
           issues when faced with a slowly increasing record size.
         */
        while (rec_ptr) {
                if (rec_free_read(tdb, rec_ptr, rec) == -1) {
                        goto fail;
                }

                if (rec->rec_len >= length) {
                        if (bestfit.rec_ptr == 0 ||
                            rec->rec_len < bestfit.rec_len) {
                                bestfit.rec_len = rec->rec_len;
                                bestfit.rec_ptr = rec_ptr;
                                bestfit.last_ptr = last_ptr;
                                /* consider a fit to be good enough if
                                   we aren't wasting more than half
                                   the space */
                                if (bestfit.rec_len < 2*length) {
                                        break;
                                }
                        }
                }

                /* move to the next record */
                last_ptr = rec_ptr;
                rec_ptr = rec->next;
        }

        if (bestfit.rec_ptr != 0) {
                if (rec_free_read(tdb, bestfit.rec_ptr, rec) == -1) {
                        goto fail;
                }

                newrec_ptr = tdb_allocate_ofs(tdb, length, bestfit.rec_ptr, rec, bestfit.last_ptr);
                tdb_unlock(tdb, -1, F_WRLCK);
                return newrec_ptr;
        }

        /* we didn't find enough space. See if we can expand the
           database and if we can then try again */
        if (tdb_expand(tdb, length + sizeof(*rec)) == 0)
                goto again;
 fail:
        tdb_unlock(tdb, -1, F_WRLCK);
        return 0;
}

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