fs/xfs/xfs_extent_busy.c - kernel/msm-4.9 - Gitiles

 /*
  * Copyright (c) 2000-2002,2005 Silicon Graphics, Inc.
  * Copyright (c) 2010 David Chinner.
  * Copyright (c) 2011 Christoph Hellwig.
  * All Rights Reserved.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
  * published by the Free Software Foundation.
  *
  * This program is distributed in the hope that it would be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with this program; if not, write the Free Software Foundation,
  * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  */
 #include "xfs.h"
 #include "xfs_fs.h"
 #include "xfs_format.h"
 #include "xfs_log_format.h"
 #include "xfs_shared.h"
 #include "xfs_trans_resv.h"
 #include "xfs_sb.h"
 #include "xfs_ag.h"
 #include "xfs_mount.h"
 #include "xfs_alloc.h"
 #include "xfs_extent_busy.h"
 #include "xfs_trace.h"
 #include "xfs_trans.h"
 #include "xfs_log.h"

 void
 xfs_extent_busy_insert(
 	struct xfs_trans	*tp,
 	xfs_agnumber_t		agno,
 	xfs_agblock_t		bno,
 	xfs_extlen_t		len,
 	unsigned int		flags)
 {
 	struct xfs_extent_busy	*new;
 	struct xfs_extent_busy	*busyp;
 	struct xfs_perag	*pag;
 	struct rb_node		**rbp;
 	struct rb_node		*parent = NULL;

 	new = kmem_zalloc(sizeof(struct xfs_extent_busy), KM_MAYFAIL);
 	if (!new) {
 		/*
 		 * No Memory!  Since it is now not possible to track the free
 		 * block, make this a synchronous transaction to insure that
 		 * the block is not reused before this transaction commits.
 		 */
 		trace_xfs_extent_busy_enomem(tp->t_mountp, agno, bno, len);
 		xfs_trans_set_sync(tp);
 		return;
 	}

 	new->agno = agno;
 	new->bno = bno;
 	new->length = len;
 	INIT_LIST_HEAD(&new->list);
 	new->flags = flags;

 	/* trace before insert to be able to see failed inserts */
 	trace_xfs_extent_busy(tp->t_mountp, agno, bno, len);

 	pag = xfs_perag_get(tp->t_mountp, new->agno);
 	spin_lock(&pag->pagb_lock);
 	rbp = &pag->pagb_tree.rb_node;
 	while (*rbp) {
 		parent = *rbp;
 		busyp = rb_entry(parent, struct xfs_extent_busy, rb_node);

 		if (new->bno < busyp->bno) {
 			rbp = &(*rbp)->rb_left;
 			ASSERT(new->bno + new->length <= busyp->bno);
 		} else if (new->bno > busyp->bno) {
 			rbp = &(*rbp)->rb_right;
 			ASSERT(bno >= busyp->bno + busyp->length);
 		} else {
 			ASSERT(0);
 		}
 	}

 	rb_link_node(&new->rb_node, parent, rbp);
 	rb_insert_color(&new->rb_node, &pag->pagb_tree);

 	list_add(&new->list, &tp->t_busy);
 	spin_unlock(&pag->pagb_lock);
 	xfs_perag_put(pag);
 }

 /*
  * Search for a busy extent within the range of the extent we are about to
  * allocate.  You need to be holding the busy extent tree lock when calling
  * xfs_extent_busy_search(). This function returns 0 for no overlapping busy
  * extent, -1 for an overlapping but not exact busy extent, and 1 for an exact
  * match. This is done so that a non-zero return indicates an overlap that
  * will require a synchronous transaction, but it can still be
  * used to distinguish between a partial or exact match.
  */
 int
 xfs_extent_busy_search(
 	struct xfs_mount	*mp,
 	xfs_agnumber_t		agno,
 	xfs_agblock_t		bno,
 	xfs_extlen_t		len)
 {
 	struct xfs_perag	*pag;
 	struct rb_node		*rbp;
 	struct xfs_extent_busy	*busyp;
 	int			match = 0;

 	pag = xfs_perag_get(mp, agno);
 	spin_lock(&pag->pagb_lock);

 	rbp = pag->pagb_tree.rb_node;

 	/* find closest start bno overlap */
 	while (rbp) {
 		busyp = rb_entry(rbp, struct xfs_extent_busy, rb_node);
 		if (bno < busyp->bno) {
 			/* may overlap, but exact start block is lower */
 			if (bno + len > busyp->bno)
 				match = -1;
 			rbp = rbp->rb_left;
 		} else if (bno > busyp->bno) {
 			/* may overlap, but exact start block is higher */
 			if (bno < busyp->bno + busyp->length)
 				match = -1;
 			rbp = rbp->rb_right;
 		} else {
 			/* bno matches busyp, length determines exact match */
 			match = (busyp->length == len) ? 1 : -1;
 			break;
 		}
 	}
 	spin_unlock(&pag->pagb_lock);
 	xfs_perag_put(pag);
 	return match;
 }

 /*
  * The found free extent [fbno, fend] overlaps part or all of the given busy
  * extent.  If the overlap covers the beginning, the end, or all of the busy
  * extent, the overlapping portion can be made unbusy and used for the
  * allocation.  We can't split a busy extent because we can't modify a
  * transaction/CIL context busy list, but we can update an entry's block
  * number or length.
  *
  * Returns true if the extent can safely be reused, or false if the search
  * needs to be restarted.
  */
 STATIC bool
 xfs_extent_busy_update_extent(
 	struct xfs_mount	*mp,
 	struct xfs_perag	*pag,
 	struct xfs_extent_busy	*busyp,
 	xfs_agblock_t		fbno,
 	xfs_extlen_t		flen,
 	bool			userdata) __releases(&pag->pagb_lock)
 					  __acquires(&pag->pagb_lock)
 {
 	xfs_agblock_t		fend = fbno + flen;
 	xfs_agblock_t		bbno = busyp->bno;
 	xfs_agblock_t		bend = bbno + busyp->length;

 	/*
 	 * This extent is currently being discarded.  Give the thread
 	 * performing the discard a chance to mark the extent unbusy
 	 * and retry.
 	 */
 	if (busyp->flags & XFS_EXTENT_BUSY_DISCARDED) {
 		spin_unlock(&pag->pagb_lock);
 		delay(1);
 		spin_lock(&pag->pagb_lock);
 		return false;
 	}

 	/*
 	 * If there is a busy extent overlapping a user allocation, we have
 	 * no choice but to force the log and retry the search.
 	 *
 	 * Fortunately this does not happen during normal operation, but
 	 * only if the filesystem is very low on space and has to dip into
 	 * the AGFL for normal allocations.
 	 */
 	if (userdata)
 		goto out_force_log;

 	if (bbno < fbno && bend > fend) {
 		/*
 		 * Case 1:
 		 *    bbno           bend
 		 *    +BBBBBBBBBBBBBBBBB+
 		 *        +---------+
 		 *        fbno   fend
 		 */

 		/*
 		 * We would have to split the busy extent to be able to track
 		 * it correct, which we cannot do because we would have to
 		 * modify the list of busy extents attached to the transaction
 		 * or CIL context, which is immutable.
 		 *
 		 * Force out the log to clear the busy extent and retry the
 		 * search.
 		 */
 		goto out_force_log;
 	} else if (bbno >= fbno && bend <= fend) {
 		/*
 		 * Case 2:
 		 *    bbno           bend
 		 *    +BBBBBBBBBBBBBBBBB+
 		 *    +-----------------+
 		 *    fbno           fend
 		 *
 		 * Case 3:
 		 *    bbno           bend
 		 *    +BBBBBBBBBBBBBBBBB+
 		 *    +--------------------------+
 		 *    fbno                    fend
 		 *
 		 * Case 4:
 		 *             bbno           bend
 		 *             +BBBBBBBBBBBBBBBBB+
 		 *    +--------------------------+
 		 *    fbno                    fend
 		 *
 		 * Case 5:
 		 *             bbno           bend
 		 *             +BBBBBBBBBBBBBBBBB+
 		 *    +-----------------------------------+
 		 *    fbno                             fend
 		 *
 		 */

 		/*
 		 * The busy extent is fully covered by the extent we are
 		 * allocating, and can simply be removed from the rbtree.
 		 * However we cannot remove it from the immutable list
 		 * tracking busy extents in the transaction or CIL context,
 		 * so set the length to zero to mark it invalid.
 		 *
 		 * We also need to restart the busy extent search from the
 		 * tree root, because erasing the node can rearrange the
 		 * tree topology.
 		 */
 		rb_erase(&busyp->rb_node, &pag->pagb_tree);
 		busyp->length = 0;
 		return false;
 	} else if (fend < bend) {
 		/*
 		 * Case 6:
 		 *              bbno           bend
 		 *             +BBBBBBBBBBBBBBBBB+
 		 *             +---------+
 		 *             fbno   fend
 		 *
 		 * Case 7:
 		 *             bbno           bend
 		 *             +BBBBBBBBBBBBBBBBB+
 		 *    +------------------+
 		 *    fbno            fend
 		 *
 		 */
 		busyp->bno = fend;
 	} else if (bbno < fbno) {
 		/*
 		 * Case 8:
 		 *    bbno           bend
 		 *    +BBBBBBBBBBBBBBBBB+
 		 *        +-------------+
 		 *        fbno       fend
 		 *
 		 * Case 9:
 		 *    bbno           bend
 		 *    +BBBBBBBBBBBBBBBBB+
 		 *        +----------------------+
 		 *        fbno                fend
 		 */
 		busyp->length = fbno - busyp->bno;
 	} else {
 		ASSERT(0);
 	}

 	trace_xfs_extent_busy_reuse(mp, pag->pag_agno, fbno, flen);
 	return true;

 out_force_log:
 	spin_unlock(&pag->pagb_lock);
 	xfs_log_force(mp, XFS_LOG_SYNC);
 	trace_xfs_extent_busy_force(mp, pag->pag_agno, fbno, flen);
 	spin_lock(&pag->pagb_lock);
 	return false;
 }


 /*
  * For a given extent [fbno, flen], make sure we can reuse it safely.
  */
 void
 xfs_extent_busy_reuse(
 	struct xfs_mount	*mp,
 	xfs_agnumber_t		agno,
 	xfs_agblock_t		fbno,
 	xfs_extlen_t		flen,
 	bool			userdata)
 {
 	struct xfs_perag	*pag;
 	struct rb_node		*rbp;

 	ASSERT(flen > 0);

 	pag = xfs_perag_get(mp, agno);
 	spin_lock(&pag->pagb_lock);
 restart:
 	rbp = pag->pagb_tree.rb_node;
 	while (rbp) {
 		struct xfs_extent_busy *busyp =
 			rb_entry(rbp, struct xfs_extent_busy, rb_node);
 		xfs_agblock_t	bbno = busyp->bno;
 		xfs_agblock_t	bend = bbno + busyp->length;

 		if (fbno + flen <= bbno) {
 			rbp = rbp->rb_left;
 			continue;
 		} else if (fbno >= bend) {
 			rbp = rbp->rb_right;
 			continue;
 		}

 		if (!xfs_extent_busy_update_extent(mp, pag, busyp, fbno, flen,
 						  userdata))
 			goto restart;
 	}
 	spin_unlock(&pag->pagb_lock);
 	xfs_perag_put(pag);
 }

 /*
  * For a given extent [fbno, flen], search the busy extent list to find a
  * subset of the extent that is not busy.  If *rlen is smaller than
  * args->minlen no suitable extent could be found, and the higher level
  * code needs to force out the log and retry the allocation.
  */
 void
 xfs_extent_busy_trim(
 	struct xfs_alloc_arg	*args,
 	xfs_agblock_t		bno,
 	xfs_extlen_t		len,
 	xfs_agblock_t		*rbno,
 	xfs_extlen_t		*rlen)
 {
 	xfs_agblock_t		fbno;
 	xfs_extlen_t		flen;
 	struct rb_node		*rbp;

 	ASSERT(len > 0);

 	spin_lock(&args->pag->pagb_lock);
 restart:
 	fbno = bno;
 	flen = len;
 	rbp = args->pag->pagb_tree.rb_node;
 	while (rbp && flen >= args->minlen) {
 		struct xfs_extent_busy *busyp =
 			rb_entry(rbp, struct xfs_extent_busy, rb_node);
 		xfs_agblock_t	fend = fbno + flen;
 		xfs_agblock_t	bbno = busyp->bno;
 		xfs_agblock_t	bend = bbno + busyp->length;

 		if (fend <= bbno) {
 			rbp = rbp->rb_left;
 			continue;
 		} else if (fbno >= bend) {
 			rbp = rbp->rb_right;
 			continue;
 		}

 		/*
 		 * If this is a metadata allocation, try to reuse the busy
 		 * extent instead of trimming the allocation.
 		 */
 		if (!args->userdata &&
 		    !(busyp->flags & XFS_EXTENT_BUSY_DISCARDED)) {
 			if (!xfs_extent_busy_update_extent(args->mp, args->pag,
 							  busyp, fbno, flen,
 							  false))
 				goto restart;
 			continue;
 		}

 		if (bbno <= fbno) {
 			/* start overlap */

 			/*
 			 * Case 1:
 			 *    bbno           bend
 			 *    +BBBBBBBBBBBBBBBBB+
 			 *        +---------+
 			 *        fbno   fend
 			 *
 			 * Case 2:
 			 *    bbno           bend
 			 *    +BBBBBBBBBBBBBBBBB+
 			 *    +-------------+
 			 *    fbno       fend
 			 *
 			 * Case 3:
 			 *    bbno           bend
 			 *    +BBBBBBBBBBBBBBBBB+
 			 *        +-------------+
 			 *        fbno       fend
 			 *
 			 * Case 4:
 			 *    bbno           bend
 			 *    +BBBBBBBBBBBBBBBBB+
 			 *    +-----------------+
 			 *    fbno           fend
 			 *
 			 * No unbusy region in extent, return failure.
 			 */
 			if (fend <= bend)
 				goto fail;

 			/*
 			 * Case 5:
 			 *    bbno           bend
 			 *    +BBBBBBBBBBBBBBBBB+
 			 *        +----------------------+
 			 *        fbno                fend
 			 *
 			 * Case 6:
 			 *    bbno           bend
 			 *    +BBBBBBBBBBBBBBBBB+
 			 *    +--------------------------+
 			 *    fbno                    fend
 			 *
 			 * Needs to be trimmed to:
 			 *                       +-------+
 			 *                       fbno fend
 			 */
 			fbno = bend;
 		} else if (bend >= fend) {
 			/* end overlap */

 			/*
 			 * Case 7:
 			 *             bbno           bend
 			 *             +BBBBBBBBBBBBBBBBB+
 			 *    +------------------+
 			 *    fbno            fend
 			 *
 			 * Case 8:
 			 *             bbno           bend
 			 *             +BBBBBBBBBBBBBBBBB+
 			 *    +--------------------------+
 			 *    fbno                    fend
 			 *
 			 * Needs to be trimmed to:
 			 *    +-------+
 			 *    fbno fend
 			 */
 			fend = bbno;
 		} else {
 			/* middle overlap */

 			/*
 			 * Case 9:
 			 *             bbno           bend
 			 *             +BBBBBBBBBBBBBBBBB+
 			 *    +-----------------------------------+
 			 *    fbno                             fend
 			 *
 			 * Can be trimmed to:
 			 *    +-------+        OR         +-------+
 			 *    fbno fend                   fbno fend
 			 *
 			 * Backward allocation leads to significant
 			 * fragmentation of directories, which degrades
 			 * directory performance, therefore we always want to
 			 * choose the option that produces forward allocation
 			 * patterns.
 			 * Preferring the lower bno extent will make the next
 			 * request use "fend" as the start of the next
 			 * allocation;  if the segment is no longer busy at
 			 * that point, we'll get a contiguous allocation, but
 			 * even if it is still busy, we will get a forward
 			 * allocation.
 			 * We try to avoid choosing the segment at "bend",
 			 * because that can lead to the next allocation
 			 * taking the segment at "fbno", which would be a
 			 * backward allocation.  We only use the segment at
 			 * "fbno" if it is much larger than the current
 			 * requested size, because in that case there's a
 			 * good chance subsequent allocations will be
 			 * contiguous.
 			 */
 			if (bbno - fbno >= args->maxlen) {
 				/* left candidate fits perfect */
 				fend = bbno;
 			} else if (fend - bend >= args->maxlen * 4) {
 				/* right candidate has enough free space */
 				fbno = bend;
 			} else if (bbno - fbno >= args->minlen) {
 				/* left candidate fits minimum requirement */
 				fend = bbno;
 			} else {
 				goto fail;
 			}
 		}

 		flen = fend - fbno;
 	}
 	spin_unlock(&args->pag->pagb_lock);

 	if (fbno != bno || flen != len) {
 		trace_xfs_extent_busy_trim(args->mp, args->agno, bno, len,
 					  fbno, flen);
 	}
 	*rbno = fbno;
 	*rlen = flen;
 	return;
 fail:
 	/*
 	 * Return a zero extent length as failure indications.  All callers
 	 * re-check if the trimmed extent satisfies the minlen requirement.
 	 */
 	spin_unlock(&args->pag->pagb_lock);
 	trace_xfs_extent_busy_trim(args->mp, args->agno, bno, len, fbno, 0);
 	*rbno = fbno;
 	*rlen = 0;
 }

 STATIC void
 xfs_extent_busy_clear_one(
 	struct xfs_mount	*mp,
 	struct xfs_perag	*pag,
 	struct xfs_extent_busy	*busyp)
 {
 	if (busyp->length) {
 		trace_xfs_extent_busy_clear(mp, busyp->agno, busyp->bno,
 						busyp->length);
 		rb_erase(&busyp->rb_node, &pag->pagb_tree);
 	}

 	list_del_init(&busyp->list);
 	kmem_free(busyp);
 }

 /*
  * Remove all extents on the passed in list from the busy extents tree.
  * If do_discard is set skip extents that need to be discarded, and mark
  * these as undergoing a discard operation instead.
  */
 void
 xfs_extent_busy_clear(
 	struct xfs_mount	*mp,
 	struct list_head	*list,
 	bool			do_discard)
 {
 	struct xfs_extent_busy	*busyp, *n;
 	struct xfs_perag	*pag = NULL;
 	xfs_agnumber_t		agno = NULLAGNUMBER;

 	list_for_each_entry_safe(busyp, n, list, list) {
 		if (busyp->agno != agno) {
 			if (pag) {
 				spin_unlock(&pag->pagb_lock);
 				xfs_perag_put(pag);
 			}
 			pag = xfs_perag_get(mp, busyp->agno);
 			spin_lock(&pag->pagb_lock);
 			agno = busyp->agno;
 		}

 		if (do_discard && busyp->length &&
 		    !(busyp->flags & XFS_EXTENT_BUSY_SKIP_DISCARD))
 			busyp->flags = XFS_EXTENT_BUSY_DISCARDED;
 		else
 			xfs_extent_busy_clear_one(mp, pag, busyp);
 	}

 	if (pag) {
 		spin_unlock(&pag->pagb_lock);
 		xfs_perag_put(pag);
 	}
 }

 /*
  * Callback for list_sort to sort busy extents by the AG they reside in.
  */
 int
 xfs_extent_busy_ag_cmp(
 	void			*priv,
 	struct list_head	*a,
 	struct list_head	*b)
 {
 	return container_of(a, struct xfs_extent_busy, list)->agno -
 		container_of(b, struct xfs_extent_busy, list)->agno;
 }
	/*
	* Copyright (c) 2000-2002,2005 Silicon Graphics, Inc.
	* Copyright (c) 2010 David Chinner.
	* Copyright (c) 2011 Christoph Hellwig.
	* All Rights Reserved.
	*
	* This program is free software; you can redistribute it and/or
	* modify it under the terms of the GNU General Public License as
	* published by the Free Software Foundation.
	*
	* This program is distributed in the hope that it would be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program; if not, write the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
	*/
	#include "xfs.h"
	#include "xfs_fs.h"
	#include "xfs_format.h"
	#include "xfs_log_format.h"
	#include "xfs_shared.h"
	#include "xfs_trans_resv.h"
	#include "xfs_sb.h"
	#include "xfs_ag.h"
	#include "xfs_mount.h"
	#include "xfs_alloc.h"
	#include "xfs_extent_busy.h"
	#include "xfs_trace.h"
	#include "xfs_trans.h"
	#include "xfs_log.h"

	void
	xfs_extent_busy_insert(
	struct xfs_trans *tp,
	xfs_agnumber_t agno,
	xfs_agblock_t bno,
	xfs_extlen_t len,
	unsigned int flags)
	{
	struct xfs_extent_busy *new;
	struct xfs_extent_busy *busyp;
	struct xfs_perag *pag;
	struct rb_node **rbp;
	struct rb_node *parent = NULL;

	new = kmem_zalloc(sizeof(struct xfs_extent_busy), KM_MAYFAIL);
	if (!new) {
	/*
	* No Memory! Since it is now not possible to track the free
	* block, make this a synchronous transaction to insure that
	* the block is not reused before this transaction commits.
	*/
	trace_xfs_extent_busy_enomem(tp->t_mountp, agno, bno, len);
	xfs_trans_set_sync(tp);
	return;
	}

	new->agno = agno;
	new->bno = bno;
	new->length = len;
	INIT_LIST_HEAD(&new->list);
	new->flags = flags;

	/* trace before insert to be able to see failed inserts */
	trace_xfs_extent_busy(tp->t_mountp, agno, bno, len);

	pag = xfs_perag_get(tp->t_mountp, new->agno);
	spin_lock(&pag->pagb_lock);
	rbp = &pag->pagb_tree.rb_node;
	while (*rbp) {
	parent = *rbp;
	busyp = rb_entry(parent, struct xfs_extent_busy, rb_node);

	if (new->bno < busyp->bno) {
	rbp = &(*rbp)->rb_left;
	ASSERT(new->bno + new->length <= busyp->bno);
	} else if (new->bno > busyp->bno) {
	rbp = &(*rbp)->rb_right;
	ASSERT(bno >= busyp->bno + busyp->length);
	} else {
	ASSERT(0);
	}
	}

	rb_link_node(&new->rb_node, parent, rbp);
	rb_insert_color(&new->rb_node, &pag->pagb_tree);

	list_add(&new->list, &tp->t_busy);
	spin_unlock(&pag->pagb_lock);
	xfs_perag_put(pag);
	}

	/*
	* Search for a busy extent within the range of the extent we are about to
	* allocate. You need to be holding the busy extent tree lock when calling
	* xfs_extent_busy_search(). This function returns 0 for no overlapping busy
	* extent, -1 for an overlapping but not exact busy extent, and 1 for an exact
	* match. This is done so that a non-zero return indicates an overlap that
	* will require a synchronous transaction, but it can still be
	* used to distinguish between a partial or exact match.
	*/
	int
	xfs_extent_busy_search(
	struct xfs_mount *mp,
	xfs_agnumber_t agno,
	xfs_agblock_t bno,
	xfs_extlen_t len)
	{
	struct xfs_perag *pag;
	struct rb_node *rbp;
	struct xfs_extent_busy *busyp;
	int match = 0;

	pag = xfs_perag_get(mp, agno);
	spin_lock(&pag->pagb_lock);

	rbp = pag->pagb_tree.rb_node;

	/* find closest start bno overlap */
	while (rbp) {
	busyp = rb_entry(rbp, struct xfs_extent_busy, rb_node);
	if (bno < busyp->bno) {
	/* may overlap, but exact start block is lower */
	if (bno + len > busyp->bno)
	match = -1;
	rbp = rbp->rb_left;
	} else if (bno > busyp->bno) {
	/* may overlap, but exact start block is higher */
	if (bno < busyp->bno + busyp->length)
	match = -1;
	rbp = rbp->rb_right;
	} else {
	/* bno matches busyp, length determines exact match */
	match = (busyp->length == len) ? 1 : -1;
	break;
	}
	}
	spin_unlock(&pag->pagb_lock);
	xfs_perag_put(pag);
	return match;
	}

	/*
	* The found free extent [fbno, fend] overlaps part or all of the given busy
	* extent. If the overlap covers the beginning, the end, or all of the busy
	* extent, the overlapping portion can be made unbusy and used for the
	* allocation. We can't split a busy extent because we can't modify a
	* transaction/CIL context busy list, but we can update an entry's block
	* number or length.
	*
	* Returns true if the extent can safely be reused, or false if the search
	* needs to be restarted.
	*/
	STATIC bool
	xfs_extent_busy_update_extent(
	struct xfs_mount *mp,
	struct xfs_perag *pag,
	struct xfs_extent_busy *busyp,
	xfs_agblock_t fbno,
	xfs_extlen_t flen,
	bool userdata) __releases(&pag->pagb_lock)
	__acquires(&pag->pagb_lock)
	{
	xfs_agblock_t fend = fbno + flen;
	xfs_agblock_t bbno = busyp->bno;
	xfs_agblock_t bend = bbno + busyp->length;

	/*
	* This extent is currently being discarded. Give the thread
	* performing the discard a chance to mark the extent unbusy
	* and retry.
	*/
	if (busyp->flags & XFS_EXTENT_BUSY_DISCARDED) {
	spin_unlock(&pag->pagb_lock);
	delay(1);
	spin_lock(&pag->pagb_lock);
	return false;
	}

	/*
	* If there is a busy extent overlapping a user allocation, we have
	* no choice but to force the log and retry the search.
	*
	* Fortunately this does not happen during normal operation, but
	* only if the filesystem is very low on space and has to dip into
	* the AGFL for normal allocations.
	*/
	if (userdata)
	goto out_force_log;

	if (bbno < fbno && bend > fend) {
	/*
	* Case 1:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +---------+
	* fbno fend
	*/

	/*
	* We would have to split the busy extent to be able to track
	* it correct, which we cannot do because we would have to
	* modify the list of busy extents attached to the transaction
	* or CIL context, which is immutable.
	*
	* Force out the log to clear the busy extent and retry the
	* search.
	*/
	goto out_force_log;
	} else if (bbno >= fbno && bend <= fend) {
	/*
	* Case 2:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +-----------------+
	* fbno fend
	*
	* Case 3:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +--------------------------+
	* fbno fend
	*
	* Case 4:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +--------------------------+
	* fbno fend
	*
	* Case 5:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +-----------------------------------+
	* fbno fend
	*
	*/

	/*
	* The busy extent is fully covered by the extent we are
	* allocating, and can simply be removed from the rbtree.
	* However we cannot remove it from the immutable list
	* tracking busy extents in the transaction or CIL context,
	* so set the length to zero to mark it invalid.
	*
	* We also need to restart the busy extent search from the
	* tree root, because erasing the node can rearrange the
	* tree topology.
	*/
	rb_erase(&busyp->rb_node, &pag->pagb_tree);
	busyp->length = 0;
	return false;
	} else if (fend < bend) {
	/*
	* Case 6:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +---------+
	* fbno fend
	*
	* Case 7:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +------------------+
	* fbno fend
	*
	*/
	busyp->bno = fend;
	} else if (bbno < fbno) {
	/*
	* Case 8:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +-------------+
	* fbno fend
	*
	* Case 9:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +----------------------+
	* fbno fend
	*/
	busyp->length = fbno - busyp->bno;
	} else {
	ASSERT(0);
	}

	trace_xfs_extent_busy_reuse(mp, pag->pag_agno, fbno, flen);
	return true;

	out_force_log:
	spin_unlock(&pag->pagb_lock);
	xfs_log_force(mp, XFS_LOG_SYNC);
	trace_xfs_extent_busy_force(mp, pag->pag_agno, fbno, flen);
	spin_lock(&pag->pagb_lock);
	return false;
	}


	/*
	* For a given extent [fbno, flen], make sure we can reuse it safely.
	*/
	void
	xfs_extent_busy_reuse(
	struct xfs_mount *mp,
	xfs_agnumber_t agno,
	xfs_agblock_t fbno,
	xfs_extlen_t flen,
	bool userdata)
	{
	struct xfs_perag *pag;
	struct rb_node *rbp;

	ASSERT(flen > 0);

	pag = xfs_perag_get(mp, agno);
	spin_lock(&pag->pagb_lock);
	restart:
	rbp = pag->pagb_tree.rb_node;
	while (rbp) {
	struct xfs_extent_busy *busyp =
	rb_entry(rbp, struct xfs_extent_busy, rb_node);
	xfs_agblock_t bbno = busyp->bno;
	xfs_agblock_t bend = bbno + busyp->length;

	if (fbno + flen <= bbno) {
	rbp = rbp->rb_left;
	continue;
	} else if (fbno >= bend) {
	rbp = rbp->rb_right;
	continue;
	}

	if (!xfs_extent_busy_update_extent(mp, pag, busyp, fbno, flen,
	userdata))
	goto restart;
	}
	spin_unlock(&pag->pagb_lock);
	xfs_perag_put(pag);
	}

	/*
	* For a given extent [fbno, flen], search the busy extent list to find a
	* subset of the extent that is not busy. If *rlen is smaller than
	* args->minlen no suitable extent could be found, and the higher level
	* code needs to force out the log and retry the allocation.
	*/
	void
	xfs_extent_busy_trim(
	struct xfs_alloc_arg *args,
	xfs_agblock_t bno,
	xfs_extlen_t len,
	xfs_agblock_t *rbno,
	xfs_extlen_t *rlen)
	{
	xfs_agblock_t fbno;
	xfs_extlen_t flen;
	struct rb_node *rbp;

	ASSERT(len > 0);

	spin_lock(&args->pag->pagb_lock);
	restart:
	fbno = bno;
	flen = len;
	rbp = args->pag->pagb_tree.rb_node;
	while (rbp && flen >= args->minlen) {
	struct xfs_extent_busy *busyp =
	rb_entry(rbp, struct xfs_extent_busy, rb_node);
	xfs_agblock_t fend = fbno + flen;
	xfs_agblock_t bbno = busyp->bno;
	xfs_agblock_t bend = bbno + busyp->length;

	if (fend <= bbno) {
	rbp = rbp->rb_left;
	continue;
	} else if (fbno >= bend) {
	rbp = rbp->rb_right;
	continue;
	}

	/*
	* If this is a metadata allocation, try to reuse the busy
	* extent instead of trimming the allocation.
	*/
	if (!args->userdata &&
	!(busyp->flags & XFS_EXTENT_BUSY_DISCARDED)) {
	if (!xfs_extent_busy_update_extent(args->mp, args->pag,
	busyp, fbno, flen,
	false))
	goto restart;
	continue;
	}

	if (bbno <= fbno) {
	/* start overlap */

	/*
	* Case 1:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +---------+
	* fbno fend
	*
	* Case 2:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +-------------+
	* fbno fend
	*
	* Case 3:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +-------------+
	* fbno fend
	*
	* Case 4:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +-----------------+
	* fbno fend
	*
	* No unbusy region in extent, return failure.
	*/
	if (fend <= bend)
	goto fail;

	/*
	* Case 5:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +----------------------+
	* fbno fend
	*
	* Case 6:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +--------------------------+
	* fbno fend
	*
	* Needs to be trimmed to:
	* +-------+
	* fbno fend
	*/
	fbno = bend;
	} else if (bend >= fend) {
	/* end overlap */

	/*
	* Case 7:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +------------------+
	* fbno fend
	*
	* Case 8:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +--------------------------+
	* fbno fend
	*
	* Needs to be trimmed to:
	* +-------+
	* fbno fend
	*/
	fend = bbno;
	} else {
	/* middle overlap */

	/*
	* Case 9:
	* bbno bend
	* +BBBBBBBBBBBBBBBBB+
	* +-----------------------------------+
	* fbno fend
	*
	* Can be trimmed to:
	* +-------+ OR +-------+
	* fbno fend fbno fend
	*
	* Backward allocation leads to significant
	* fragmentation of directories, which degrades
	* directory performance, therefore we always want to
	* choose the option that produces forward allocation
	* patterns.
	* Preferring the lower bno extent will make the next
	* request use "fend" as the start of the next
	* allocation; if the segment is no longer busy at
	* that point, we'll get a contiguous allocation, but
	* even if it is still busy, we will get a forward
	* allocation.
	* We try to avoid choosing the segment at "bend",
	* because that can lead to the next allocation
	* taking the segment at "fbno", which would be a
	* backward allocation. We only use the segment at
	* "fbno" if it is much larger than the current
	* requested size, because in that case there's a
	* good chance subsequent allocations will be
	* contiguous.
	*/
	if (bbno - fbno >= args->maxlen) {
	/* left candidate fits perfect */
	fend = bbno;
	} else if (fend - bend >= args->maxlen * 4) {
	/* right candidate has enough free space */
	fbno = bend;
	} else if (bbno - fbno >= args->minlen) {
	/* left candidate fits minimum requirement */
	fend = bbno;
	} else {
	goto fail;
	}
	}

	flen = fend - fbno;
	}
	spin_unlock(&args->pag->pagb_lock);

	if (fbno != bno \|\| flen != len) {
	trace_xfs_extent_busy_trim(args->mp, args->agno, bno, len,
	fbno, flen);
	}
	*rbno = fbno;
	*rlen = flen;
	return;
	fail:
	/*
	* Return a zero extent length as failure indications. All callers
	* re-check if the trimmed extent satisfies the minlen requirement.
	*/
	spin_unlock(&args->pag->pagb_lock);
	trace_xfs_extent_busy_trim(args->mp, args->agno, bno, len, fbno, 0);
	*rbno = fbno;
	*rlen = 0;
	}

	STATIC void
	xfs_extent_busy_clear_one(
	struct xfs_mount *mp,
	struct xfs_perag *pag,
	struct xfs_extent_busy *busyp)
	{
	if (busyp->length) {
	trace_xfs_extent_busy_clear(mp, busyp->agno, busyp->bno,
	busyp->length);
	rb_erase(&busyp->rb_node, &pag->pagb_tree);
	}

	list_del_init(&busyp->list);
	kmem_free(busyp);
	}

	/*
	* Remove all extents on the passed in list from the busy extents tree.
	* If do_discard is set skip extents that need to be discarded, and mark
	* these as undergoing a discard operation instead.
	*/
	void
	xfs_extent_busy_clear(
	struct xfs_mount *mp,
	struct list_head *list,
	bool do_discard)
	{
	struct xfs_extent_busy busyp, n;
	struct xfs_perag *pag = NULL;
	xfs_agnumber_t agno = NULLAGNUMBER;

	list_for_each_entry_safe(busyp, n, list, list) {
	if (busyp->agno != agno) {
	if (pag) {
	spin_unlock(&pag->pagb_lock);
	xfs_perag_put(pag);
	}
	pag = xfs_perag_get(mp, busyp->agno);
	spin_lock(&pag->pagb_lock);
	agno = busyp->agno;
	}

	if (do_discard && busyp->length &&
	!(busyp->flags & XFS_EXTENT_BUSY_SKIP_DISCARD))
	busyp->flags = XFS_EXTENT_BUSY_DISCARDED;
	else
	xfs_extent_busy_clear_one(mp, pag, busyp);
	}

	if (pag) {
	spin_unlock(&pag->pagb_lock);
	xfs_perag_put(pag);
	}
	}

	/*
	* Callback for list_sort to sort busy extents by the AG they reside in.
	*/
	int
	xfs_extent_busy_ag_cmp(
	void *priv,
	struct list_head *a,
	struct list_head *b)
	{
	return container_of(a, struct xfs_extent_busy, list)->agno -
	container_of(b, struct xfs_extent_busy, list)->agno;
	}