Merge tag 'xfs-alloc-perag-conversion' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs into xfs-6.3-merge-A

xfs: per-ag centric allocation alogrithms

This series continues the work towards making shrinking a filesystem
possible.  We need to be able to stop operations from taking place
on AGs that need to be removed by a shrink, so before shrink can be
implemented we need to have the infrastructure in place to prevent
incursion into AGs that are going to be, or are in the process, of
being removed from active duty.

The focus of this is making operations that depend on access to AGs
use the perag to access and pin the AG in active use, thereby
creating a barrier we can use to delay shrink until all active uses
of an AG have been drained and new uses are prevented.

This series starts by fixing some existing issues that are exposed
by changes later in the series. They stand alone, so can be picked
up independently of the rest of this patchset.

The most complex of these fixes is cleaning up the mess that is the
AGF deadlock avoidance algorithm. This algorithm stores the first
block that is allocated in a transaction in tp->t_firstblock, then
uses this to try to limit future allocations within the transaction
to AGs at or higher than the filesystem block stored in
tp->t_firstblock. This depends on one of the initial bug fixes in
the series to move the deadlock avoidance checks to
xfs_alloc_vextent(), and then builds on it to relax the constraints
of the avoidance algorithm to only be active when a deadlock is
possible.

We also update the algorithm to record allocations from higher AGs
that are allocated from, because we when we need to lock more than
two AGs we still have to ensure lock order is correct. Therefore we
can't lock AGs in the order 1, 3, 2, even though tp->t_firstblock
indicates that we've allocated from AG 1 and so AG is valid to lock.
It's not valid, because we already hold AG 3 locked, and so
tp->t-first_block should actually point at AG 3, not AG 1 in this
situation.

It should now be obvious that the deadlock avoidance algorithm
should record AGs, not filesystem blocks. So the series then changes
the transaction to store the highest AG we've allocated in rather
than a filesystem block we allocated.  This makes it obvious what
the constraints are, and trivial to update as we lock and allocate
from various AGs.

With all the bug fixes out of the way, the series then starts
converting the code to use active references. Active reference
counts are used by high level code that needs to prevent the AG from
being taken out from under it by a shrink operation. The high level
code needs to be able to handle not getting an active reference
gracefully, and the shrink code will need to wait for active
references to drain before continuing.

Active references are implemented just as reference counts right now
- an active reference is taken at perag init during mount, and all
other active references are dependent on the active reference count
being greater than zero. This gives us an initial method of stopping
new active references without needing other infrastructure; just
drop the reference taken at filesystem mount time and when the
refcount then falls to zero no new references can be taken.

In future, this will need to take into account AG control state
(e.g. offline, no alloc, etc) as well as the reference count, but
right now we can implement a basic barrier for shrink with just
reference count manipulations. As such, patches to convert the perag
state to atomic opstate fields similar to the xfs_mount and xlog
opstate fields follow the initial active perag reference counting
patches.

The first target for active reference conversion is the
for_each_perag*() iterators. This captures a lot of high level code
that should skip offline AGs, and introduces the ability to
differentiate between a lookup that didn't have an online AG and the
end of the AG iteration range.

From there, the inode allocation AG selection is converted to active
references, and the perag is driven deeper into the inode allocation
and btree code to replace the xfs_mount. Most of the inode
allocation code operates on a single AG once it is selected, hence
it should pass the perag as the primary referenced object around for
allocation, not the xfs_mount. There is a bit of churn here, but it
emphasises that inode allocation is inherently an allocation group
based operation.

Next the bmap/alloc interface undergoes a major untangling,
reworking xfs_bmap_btalloc() into separate allocation operations for
different contexts and failure handling behaviours. This then allows
us to completely remove the xfs_alloc_vextent() layer via
restructuring the xfs_alloc_vextent/xfs_alloc_ag_vextent() into a
set of realtively simple helper function that describe the
allocation that they are doing. e.g.  xfs_alloc_vextent_exact_bno().

This allows the requirements for accessing AGs to be allocation
context dependent. The allocations that require operation on a
single AG generally can't tolerate failure after the allocation
method and AG has been decided on, and hence the caller needs to
manage the active references to ensure the allocation does not race
with shrink removing the selected AG for the duration of the
operation that requires access to that allocation group.

Other allocations iterate AGs and so the first AG is just a hint -
these do not need to pin a perag first as they can tolerate not
being able to access an AG by simply skipping over it. These require
new perag iteration functions that can start at arbitrary AGs and
wrap around at arbitrary AGs, hence a new set for
for_each_perag_wrap*() helpers to do this.

Next is the rework of the filestreams allocator. This doesn't change
any functionality, but gets rid of the unnecessary multi-pass
selection algorithm when the selected AG is not available. It
currently does a lookup pass which might iterate all AGs to select
an AG, then checks if the AG is acceptible and if not does a "new
AG" pass that is essentially identical to the lookup pass. Both of
these scans also do the same "longest extent in AG" check before
selecting an AG as is done after the AG is selected.

IOWs, the filestreams algorithm can be greatly simplified into a
single new AG selection pass if the there is no current association
or the currently associated AG doesn't have enough contiguous free
space for the allocation to proceed.  With this simplification of
the filestreams allocator, it's then trivial to convert it to use
for_each_perag_wrap() for the AG scan algorithm.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

* tag 'xfs-alloc-perag-conversion' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs: (42 commits)
  xfs: refactor the filestreams allocator pick functions
  xfs: return a referenced perag from filestreams allocator
  xfs: pass perag to filestreams tracing
  xfs: use for_each_perag_wrap in xfs_filestream_pick_ag
  xfs: track an active perag reference in filestreams
  xfs: factor out MRU hit case in xfs_filestream_select_ag
  xfs: remove xfs_filestream_select_ag() longest extent check
  xfs: merge new filestream AG selection into xfs_filestream_select_ag()
  xfs: merge filestream AG lookup into xfs_filestream_select_ag()
  xfs: move xfs_bmap_btalloc_filestreams() to xfs_filestreams.c
  xfs: use xfs_bmap_longest_free_extent() in filestreams
  xfs: get rid of notinit from xfs_bmap_longest_free_extent
  xfs: factor out filestreams from xfs_bmap_btalloc_nullfb
  xfs: convert trim to use for_each_perag_range
  xfs: convert xfs_alloc_vextent_iterate_ags() to use perag walker
  xfs: move the minimum agno checks into xfs_alloc_vextent_check_args
  xfs: fold xfs_alloc_ag_vextent() into callers
  xfs: move allocation accounting to xfs_alloc_vextent_set_fsbno()
  xfs: introduce xfs_alloc_vextent_prepare()
  xfs: introduce xfs_alloc_vextent_exact_bno()
  ...

1 files changed, 429 insertions, 250 deletions

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index f8ff81c3de76..6a037173d20d 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -36,10 +36,6 @@ struct workqueue_struct *xfs_alloc_wq;
 #define	XFSA_FIXUP_BNO_OK	1
 #define	XFSA_FIXUP_CNT_OK	2
 
-STATIC int xfs_alloc_ag_vextent_exact(xfs_alloc_arg_t *);
-STATIC int xfs_alloc_ag_vextent_near(xfs_alloc_arg_t *);
-STATIC int xfs_alloc_ag_vextent_size(xfs_alloc_arg_t *);
-
 /*
  * Size of the AGFL.  For CRC-enabled filesystes we steal a couple of slots in
  * the beginning of the block for a proper header with the location information
@@ -772,8 +768,6 @@ xfs_alloc_cur_setup(
 	int			error;
 	int			i;
 
-	ASSERT(args->alignment == 1 || args->type != XFS_ALLOCTYPE_THIS_BNO);
-
 	acur->cur_len = args->maxlen;
 	acur->rec_bno = 0;
 	acur->rec_len = 0;
@@ -887,7 +881,6 @@ xfs_alloc_cur_check(
 	 * We have an aligned record that satisfies minlen and beats or matches
 	 * the candidate extent size. Compare locality for near allocation mode.
 	 */
-	ASSERT(args->type == XFS_ALLOCTYPE_NEAR_BNO);
 	diff = xfs_alloc_compute_diff(args->agbno, args->len,
 				      args->alignment, args->datatype,
 				      bnoa, lena, &bnew);
@@ -1133,78 +1126,6 @@ error:
 }
 
 /*
- * Allocate a variable extent in the allocation group agno.
- * Type and bno are used to determine where in the allocation group the
- * extent will start.
- * Extent's length (returned in *len) will be between minlen and maxlen,
- * and of the form k * prod + mod unless there's nothing that large.
- * Return the starting a.g. block, or NULLAGBLOCK if we can't do it.
- */
-STATIC int			/* error */
-xfs_alloc_ag_vextent(
-	xfs_alloc_arg_t	*args)	/* argument structure for allocation */
-{
-	int		error=0;
-
-	ASSERT(args->minlen > 0);
-	ASSERT(args->maxlen > 0);
-	ASSERT(args->minlen <= args->maxlen);
-	ASSERT(args->mod < args->prod);
-	ASSERT(args->alignment > 0);
-
-	/*
-	 * Branch to correct routine based on the type.
-	 */
-	args->wasfromfl = 0;
-	switch (args->type) {
-	case XFS_ALLOCTYPE_THIS_AG:
-		error = xfs_alloc_ag_vextent_size(args);
-		break;
-	case XFS_ALLOCTYPE_NEAR_BNO:
-		error = xfs_alloc_ag_vextent_near(args);
-		break;
-	case XFS_ALLOCTYPE_THIS_BNO:
-		error = xfs_alloc_ag_vextent_exact(args);
-		break;
-	default:
-		ASSERT(0);
-		/* NOTREACHED */
-	}
-
-	if (error || args->agbno == NULLAGBLOCK)
-		return error;
-
-	ASSERT(args->len >= args->minlen);
-	ASSERT(args->len <= args->maxlen);
-	ASSERT(!args->wasfromfl || args->resv != XFS_AG_RESV_AGFL);
-	ASSERT(args->agbno % args->alignment == 0);
-
-	/* if not file data, insert new block into the reverse map btree */
-	if (!xfs_rmap_should_skip_owner_update(&args->oinfo)) {
-		error = xfs_rmap_alloc(args->tp, args->agbp, args->pag,
-				       args->agbno, args->len, &args->oinfo);
-		if (error)
-			return error;
-	}
-
-	if (!args->wasfromfl) {
-		error = xfs_alloc_update_counters(args->tp, args->agbp,
-						  -((long)(args->len)));
-		if (error)
-			return error;
-
-		ASSERT(!xfs_extent_busy_search(args->mp, args->pag,
-					      args->agbno, args->len));
-	}
-
-	xfs_ag_resv_alloc_extent(args->pag, args->resv, args);
-
-	XFS_STATS_INC(args->mp, xs_allocx);
-	XFS_STATS_ADD(args->mp, xs_allocb, args->len);
-	return error;
-}
-
-/*
  * Allocate a variable extent at exactly agno/bno.
  * Extent's length (returned in *len) will be between minlen and maxlen,
  * and of the form k * prod + mod unless there's nothing that large.
@@ -1389,7 +1310,6 @@ xfs_alloc_ag_vextent_locality(
 	bool			fbinc;
 
 	ASSERT(acur->len == 0);
-	ASSERT(args->type == XFS_ALLOCTYPE_NEAR_BNO);
 
 	*stat = 0;
 
@@ -2435,7 +2355,7 @@ xfs_agfl_reset(
 	struct xfs_mount	*mp = tp->t_mountp;
 	struct xfs_agf		*agf = agbp->b_addr;
 
-	ASSERT(pag->pagf_agflreset);
+	ASSERT(xfs_perag_agfl_needs_reset(pag));
 	trace_xfs_agfl_reset(mp, agf, 0, _RET_IP_);
 
 	xfs_warn(mp,
@@ -2450,7 +2370,7 @@ xfs_agfl_reset(
 				    XFS_AGF_FLCOUNT);
 
 	pag->pagf_flcount = 0;
-	pag->pagf_agflreset = false;
+	clear_bit(XFS_AGSTATE_AGFL_NEEDS_RESET, &pag->pag_opstate);
 }
 
 /*
@@ -2605,7 +2525,7 @@ xfs_alloc_fix_freelist(
 	/* deferred ops (AGFL block frees) require permanent transactions */
 	ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
 
-	if (!pag->pagf_init) {
+	if (!xfs_perag_initialised_agf(pag)) {
 		error = xfs_alloc_read_agf(pag, tp, flags, &agbp);
 		if (error) {
 			/* Couldn't lock the AGF so skip this AG. */
@@ -2620,7 +2540,8 @@ xfs_alloc_fix_freelist(
 	 * somewhere else if we are not being asked to try harder at this
 	 * point
 	 */
-	if (pag->pagf_metadata && (args->datatype & XFS_ALLOC_USERDATA) &&
+	if (xfs_perag_prefers_metadata(pag) &&
+	    (args->datatype & XFS_ALLOC_USERDATA) &&
 	    (flags & XFS_ALLOC_FLAG_TRYLOCK)) {
 		ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
 		goto out_agbp_relse;
@@ -2646,7 +2567,7 @@ xfs_alloc_fix_freelist(
 	}
 
 	/* reset a padding mismatched agfl before final free space check */
-	if (pag->pagf_agflreset)
+	if (xfs_perag_agfl_needs_reset(pag))
 		xfs_agfl_reset(tp, agbp, pag);
 
 	/* If there isn't enough total space or single-extent, reject it. */
@@ -2707,7 +2628,6 @@ xfs_alloc_fix_freelist(
 	targs.agbp = agbp;
 	targs.agno = args->agno;
 	targs.alignment = targs.minlen = targs.prod = 1;
-	targs.type = XFS_ALLOCTYPE_THIS_AG;
 	targs.pag = pag;
 	error = xfs_alloc_read_agfl(pag, tp, &agflbp);
 	if (error)
@@ -2720,7 +2640,7 @@ xfs_alloc_fix_freelist(
 		targs.resv = XFS_AG_RESV_AGFL;
 
 		/* Allocate as many blocks as possible at once. */
-		error = xfs_alloc_ag_vextent(&targs);
+		error = xfs_alloc_ag_vextent_size(&targs);
 		if (error)
 			goto out_agflbp_relse;
 
@@ -2734,6 +2654,18 @@ xfs_alloc_fix_freelist(
 				break;
 			goto out_agflbp_relse;
 		}
+
+		if (!xfs_rmap_should_skip_owner_update(&targs.oinfo)) {
+			error = xfs_rmap_alloc(tp, agbp, pag,
+				       targs.agbno, targs.len, &targs.oinfo);
+			if (error)
+				goto out_agflbp_relse;
+		}
+		error = xfs_alloc_update_counters(tp, agbp,
+						  -((long)(targs.len)));
+		if (error)
+			goto out_agflbp_relse;
+
 		/*
 		 * Put each allocated block on the list.
 		 */
@@ -2803,7 +2735,7 @@ xfs_alloc_get_freelist(
 	if (be32_to_cpu(agf->agf_flfirst) == xfs_agfl_size(mp))
 		agf->agf_flfirst = 0;
 
-	ASSERT(!pag->pagf_agflreset);
+	ASSERT(!xfs_perag_agfl_needs_reset(pag));
 	be32_add_cpu(&agf->agf_flcount, -1);
 	pag->pagf_flcount--;
 
@@ -2892,7 +2824,7 @@ xfs_alloc_put_freelist(
 	if (be32_to_cpu(agf->agf_fllast) == xfs_agfl_size(mp))
 		agf->agf_fllast = 0;
 
-	ASSERT(!pag->pagf_agflreset);
+	ASSERT(!xfs_perag_agfl_needs_reset(pag));
 	be32_add_cpu(&agf->agf_flcount, 1);
 	pag->pagf_flcount++;
 
@@ -3099,7 +3031,7 @@ xfs_alloc_read_agf(
 		return error;
 
 	agf = agfbp->b_addr;
-	if (!pag->pagf_init) {
+	if (!xfs_perag_initialised_agf(pag)) {
 		pag->pagf_freeblks = be32_to_cpu(agf->agf_freeblks);
 		pag->pagf_btreeblks = be32_to_cpu(agf->agf_btreeblks);
 		pag->pagf_flcount = be32_to_cpu(agf->agf_flcount);
@@ -3111,8 +3043,8 @@ xfs_alloc_read_agf(
 		pag->pagf_levels[XFS_BTNUM_RMAPi] =
 			be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAPi]);
 		pag->pagf_refcount_level = be32_to_cpu(agf->agf_refcount_level);
-		pag->pagf_init = 1;
-		pag->pagf_agflreset = xfs_agfl_needs_reset(pag->pag_mount, agf);
+		if (xfs_agfl_needs_reset(pag->pag_mount, agf))
+			set_bit(XFS_AGSTATE_AGFL_NEEDS_RESET, &pag->pag_opstate);
 
 		/*
 		 * Update the in-core allocbt counter. Filter out the rmapbt
@@ -3127,6 +3059,8 @@ xfs_alloc_read_agf(
 		if (allocbt_blks > 0)
 			atomic64_add(allocbt_blks,
 					&pag->pag_mount->m_allocbt_blks);
+
+		set_bit(XFS_AGSTATE_AGF_INIT, &pag->pag_opstate);
 	}
 #ifdef DEBUG
 	else if (!xfs_is_shutdown(pag->pag_mount)) {
@@ -3148,26 +3082,25 @@ xfs_alloc_read_agf(
 }
 
 /*
- * Allocate an extent (variable-size).
- * Depending on the allocation type, we either look in a single allocation
- * group or loop over the allocation groups to find the result.
+ * Pre-proces allocation arguments to set initial state that we don't require
+ * callers to set up correctly, as well as bounds check the allocation args
+ * that are set up.
  */
-int				/* error */
-xfs_alloc_vextent(
-	struct xfs_alloc_arg	*args)	/* allocation argument structure */
+static int
+xfs_alloc_vextent_check_args(
+	struct xfs_alloc_arg	*args,
+	xfs_fsblock_t		target,
+	xfs_agnumber_t		*minimum_agno)
 {
-	xfs_agblock_t		agsize;	/* allocation group size */
-	int			error;
-	int			flags;	/* XFS_ALLOC_FLAG_... locking flags */
-	struct xfs_mount	*mp;	/* mount structure pointer */
-	xfs_agnumber_t		sagno;	/* starting allocation group number */
-	xfs_alloctype_t		type;	/* input allocation type */
-	int			bump_rotor = 0;
-	xfs_agnumber_t		rotorstep = xfs_rotorstep; /* inode32 agf stepper */
+	struct xfs_mount	*mp = args->mp;
+	xfs_agblock_t		agsize;
+
+	args->fsbno = NULLFSBLOCK;
+
+	*minimum_agno = 0;
+	if (args->tp->t_highest_agno != NULLAGNUMBER)
+		*minimum_agno = args->tp->t_highest_agno;
 
-	mp = args->mp;
-	type = args->otype = args->type;
-	args->agbno = NULLAGBLOCK;
 	/*
 	 * Just fix this up, for the case where the last a.g. is shorter
 	 * (or there's only one a.g.) and the caller couldn't easily figure
@@ -3178,168 +3111,414 @@ xfs_alloc_vextent(
 		args->maxlen = agsize;
 	if (args->alignment == 0)
 		args->alignment = 1;
-	ASSERT(XFS_FSB_TO_AGNO(mp, args->fsbno) < mp->m_sb.sb_agcount);
-	ASSERT(XFS_FSB_TO_AGBNO(mp, args->fsbno) < agsize);
+
+	ASSERT(args->minlen > 0);
+	ASSERT(args->maxlen > 0);
+	ASSERT(args->alignment > 0);
+	ASSERT(args->resv != XFS_AG_RESV_AGFL);
+
+	ASSERT(XFS_FSB_TO_AGNO(mp, target) < mp->m_sb.sb_agcount);
+	ASSERT(XFS_FSB_TO_AGBNO(mp, target) < agsize);
 	ASSERT(args->minlen <= args->maxlen);
 	ASSERT(args->minlen <= agsize);
 	ASSERT(args->mod < args->prod);
-	if (XFS_FSB_TO_AGNO(mp, args->fsbno) >= mp->m_sb.sb_agcount ||
-	    XFS_FSB_TO_AGBNO(mp, args->fsbno) >= agsize ||
+
+	if (XFS_FSB_TO_AGNO(mp, target) >= mp->m_sb.sb_agcount ||
+	    XFS_FSB_TO_AGBNO(mp, target) >= agsize ||
 	    args->minlen > args->maxlen || args->minlen > agsize ||
 	    args->mod >= args->prod) {
-		args->fsbno = NULLFSBLOCK;
 		trace_xfs_alloc_vextent_badargs(args);
+		return -ENOSPC;
+	}
+
+	if (args->agno != NULLAGNUMBER && *minimum_agno > args->agno) {
+		trace_xfs_alloc_vextent_skip_deadlock(args);
+		return -ENOSPC;
+	}
+	return 0;
+
+}
+
+/*
+ * Prepare an AG for allocation. If the AG is not prepared to accept the
+ * allocation, return failure.
+ *
+ * XXX(dgc): The complexity of "need_pag" will go away as all caller paths are
+ * modified to hold their own perag references.
+ */
+static int
+xfs_alloc_vextent_prepare_ag(
+	struct xfs_alloc_arg	*args)
+{
+	bool			need_pag = !args->pag;
+	int			error;
+
+	if (need_pag)
+		args->pag = xfs_perag_get(args->mp, args->agno);
+
+	args->agbp = NULL;
+	error = xfs_alloc_fix_freelist(args, 0);
+	if (error) {
+		trace_xfs_alloc_vextent_nofix(args);
+		if (need_pag)
+			xfs_perag_put(args->pag);
+		args->agbno = NULLAGBLOCK;
+		return error;
+	}
+	if (!args->agbp) {
+		/* cannot allocate in this AG at all */
+		trace_xfs_alloc_vextent_noagbp(args);
+		args->agbno = NULLAGBLOCK;
 		return 0;
 	}
+	args->wasfromfl = 0;
+	return 0;
+}
 
-	switch (type) {
-	case XFS_ALLOCTYPE_THIS_AG:
-	case XFS_ALLOCTYPE_NEAR_BNO:
-	case XFS_ALLOCTYPE_THIS_BNO:
-		/*
-		 * These three force us into a single a.g.
-		 */
-		args->agno = XFS_FSB_TO_AGNO(mp, args->fsbno);
-		args->pag = xfs_perag_get(mp, args->agno);
-		error = xfs_alloc_fix_freelist(args, 0);
-		if (error) {
-			trace_xfs_alloc_vextent_nofix(args);
-			goto error0;
-		}
-		if (!args->agbp) {
-			trace_xfs_alloc_vextent_noagbp(args);
+/*
+ * Post-process allocation results to account for the allocation if it succeed
+ * and set the allocated block number correctly for the caller.
+ *
+ * XXX: we should really be returning ENOSPC for ENOSPC, not
+ * hiding it behind a "successful" NULLFSBLOCK allocation.
+ */
+static int
+xfs_alloc_vextent_finish(
+	struct xfs_alloc_arg	*args,
+	xfs_agnumber_t		minimum_agno,
+	int			alloc_error,
+	bool			drop_perag)
+{
+	struct xfs_mount	*mp = args->mp;
+	int			error = 0;
+
+	/*
+	 * We can end up here with a locked AGF. If we failed, the caller is
+	 * likely going to try to allocate again with different parameters, and
+	 * that can widen the AGs that are searched for free space. If we have
+	 * to do BMBT block allocation, we have to do a new allocation.
+	 *
+	 * Hence leaving this function with the AGF locked opens up potential
+	 * ABBA AGF deadlocks because a future allocation attempt in this
+	 * transaction may attempt to lock a lower number AGF.
+	 *
+	 * We can't release the AGF until the transaction is commited, so at
+	 * this point we must update the "first allocation" tracker to point at
+	 * this AG if the tracker is empty or points to a lower AG. This allows
+	 * the next allocation attempt to be modified appropriately to avoid
+	 * deadlocks.
+	 */
+	if (args->agbp &&
+	    (args->tp->t_highest_agno == NULLAGNUMBER ||
+	     args->agno > minimum_agno))
+		args->tp->t_highest_agno = args->agno;
+
+	/*
+	 * If the allocation failed with an error or we had an ENOSPC result,
+	 * preserve the returned error whilst also marking the allocation result
+	 * as "no extent allocated". This ensures that callers that fail to
+	 * capture the error will still treat it as a failed allocation.
+	 */
+	if (alloc_error || args->agbno == NULLAGBLOCK) {
+		args->fsbno = NULLFSBLOCK;
+		error = alloc_error;
+		goto out_drop_perag;
+	}
+
+	args->fsbno = XFS_AGB_TO_FSB(mp, args->agno, args->agbno);
+
+	ASSERT(args->len >= args->minlen);
+	ASSERT(args->len <= args->maxlen);
+	ASSERT(args->agbno % args->alignment == 0);
+	XFS_AG_CHECK_DADDR(mp, XFS_FSB_TO_DADDR(mp, args->fsbno), args->len);
+
+	/* if not file data, insert new block into the reverse map btree */
+	if (!xfs_rmap_should_skip_owner_update(&args->oinfo)) {
+		error = xfs_rmap_alloc(args->tp, args->agbp, args->pag,
+				       args->agbno, args->len, &args->oinfo);
+		if (error)
+			goto out_drop_perag;
+	}
+
+	if (!args->wasfromfl) {
+		error = xfs_alloc_update_counters(args->tp, args->agbp,
+						  -((long)(args->len)));
+		if (error)
+			goto out_drop_perag;
+
+		ASSERT(!xfs_extent_busy_search(mp, args->pag, args->agbno,
+				args->len));
+	}
+
+	xfs_ag_resv_alloc_extent(args->pag, args->resv, args);
+
+	XFS_STATS_INC(mp, xs_allocx);
+	XFS_STATS_ADD(mp, xs_allocb, args->len);
+
+out_drop_perag:
+	if (drop_perag && args->pag) {
+		xfs_perag_rele(args->pag);
+		args->pag = NULL;
+	}
+	return error;
+}
+
+/*
+ * Allocate within a single AG only. This uses a best-fit length algorithm so if
+ * you need an exact sized allocation without locality constraints, this is the
+ * fastest way to do it.
+ *
+ * Caller is expected to hold a perag reference in args->pag.
+ */
+int
+xfs_alloc_vextent_this_ag(
+	struct xfs_alloc_arg	*args,
+	xfs_agnumber_t		agno)
+{
+	struct xfs_mount	*mp = args->mp;
+	xfs_agnumber_t		minimum_agno;
+	int			error;
+
+	args->agno = agno;
+	args->agbno = 0;
+	error = xfs_alloc_vextent_check_args(args, XFS_AGB_TO_FSB(mp, agno, 0),
+			&minimum_agno);
+	if (error) {
+		if (error == -ENOSPC)
+			return 0;
+		return error;
+	}
+
+	error = xfs_alloc_vextent_prepare_ag(args);
+	if (!error && args->agbp)
+		error = xfs_alloc_ag_vextent_size(args);
+
+	return xfs_alloc_vextent_finish(args, minimum_agno, error, false);
+}
+
+/*
+ * Iterate all AGs trying to allocate an extent starting from @start_ag.
+ *
+ * If the incoming allocation type is XFS_ALLOCTYPE_NEAR_BNO, it means the
+ * allocation attempts in @start_agno have locality information. If we fail to
+ * allocate in that AG, then we revert to anywhere-in-AG for all the other AGs
+ * we attempt to allocation in as there is no locality optimisation possible for
+ * those allocations.
+ *
+ * On return, args->pag may be left referenced if we finish before the "all
+ * failed" return point. The allocation finish still needs the perag, and
+ * so the caller will release it once they've finished the allocation.
+ *
+ * When we wrap the AG iteration at the end of the filesystem, we have to be
+ * careful not to wrap into AGs below ones we already have locked in the
+ * transaction if we are doing a blocking iteration. This will result in an
+ * out-of-order locking of AGFs and hence can cause deadlocks.
+ */
+static int
+xfs_alloc_vextent_iterate_ags(
+	struct xfs_alloc_arg	*args,
+	xfs_agnumber_t		minimum_agno,
+	xfs_agnumber_t		start_agno,
+	xfs_agblock_t		target_agbno,
+	uint32_t		flags)
+{
+	struct xfs_mount	*mp = args->mp;
+	xfs_agnumber_t		agno;
+	int			error = 0;
+
+restart:
+	for_each_perag_wrap_range(mp, start_agno, minimum_agno,
+			mp->m_sb.sb_agcount, agno, args->pag) {
+		args->agno = agno;
+		error = xfs_alloc_vextent_prepare_ag(args);
+		if (error)
 			break;
+		if (!args->agbp) {
+			trace_xfs_alloc_vextent_loopfailed(args);
+			continue;
 		}
-		args->agbno = XFS_FSB_TO_AGBNO(mp, args->fsbno);
-		if ((error = xfs_alloc_ag_vextent(args)))
-			goto error0;
-		break;
-	case XFS_ALLOCTYPE_START_BNO:
-		/*
-		 * Try near allocation first, then anywhere-in-ag after
-		 * the first a.g. fails.
-		 */
-		if ((args->datatype & XFS_ALLOC_INITIAL_USER_DATA) &&
-		    xfs_is_inode32(mp)) {
-			args->fsbno = XFS_AGB_TO_FSB(mp,
-					((mp->m_agfrotor / rotorstep) %
-					mp->m_sb.sb_agcount), 0);
-			bump_rotor = 1;
-		}
-		args->agbno = XFS_FSB_TO_AGBNO(mp, args->fsbno);
-		args->type = XFS_ALLOCTYPE_NEAR_BNO;
-		fallthrough;
-	case XFS_ALLOCTYPE_FIRST_AG:
+
 		/*
-		 * Rotate through the allocation groups looking for a winner.
+		 * Allocation is supposed to succeed now, so break out of the
+		 * loop regardless of whether we succeed or not.
 		 */
-		if (type == XFS_ALLOCTYPE_FIRST_AG) {
-			/*
-			 * Start with allocation group given by bno.
-			 */
-			args->agno = XFS_FSB_TO_AGNO(mp, args->fsbno);
-			args->type = XFS_ALLOCTYPE_THIS_AG;
-			sagno = 0;
-			flags = 0;
+		if (args->agno == start_agno && target_agbno) {
+			args->agbno = target_agbno;
+			error = xfs_alloc_ag_vextent_near(args);
 		} else {
-			/*
-			 * Start with the given allocation group.
-			 */
-			args->agno = sagno = XFS_FSB_TO_AGNO(mp, args->fsbno);
-			flags = XFS_ALLOC_FLAG_TRYLOCK;
+			args->agbno = 0;
+			error = xfs_alloc_ag_vextent_size(args);
 		}
-		/*
-		 * Loop over allocation groups twice; first time with
-		 * trylock set, second time without.
-		 */
-		for (;;) {
-			args->pag = xfs_perag_get(mp, args->agno);
-			error = xfs_alloc_fix_freelist(args, flags);
-			if (error) {
-				trace_xfs_alloc_vextent_nofix(args);
-				goto error0;
-			}
-			/*
-			 * If we get a buffer back then the allocation will fly.
-			 */
-			if (args->agbp) {
-				if ((error = xfs_alloc_ag_vextent(args)))
-					goto error0;
-				break;
-			}
+		break;
+	}
+	if (error) {
+		xfs_perag_rele(args->pag);
+		args->pag = NULL;
+		return error;
+	}
+	if (args->agbp)
+		return 0;
 
-			trace_xfs_alloc_vextent_loopfailed(args);
+	/*
+	 * We didn't find an AG we can alloation from. If we were given
+	 * constraining flags by the caller, drop them and retry the allocation
+	 * without any constraints being set.
+	 */
+	if (flags) {
+		flags = 0;
+		goto restart;
+	}
 
-			/*
-			 * Didn't work, figure out the next iteration.
-			 */
-			if (args->agno == sagno &&
-			    type == XFS_ALLOCTYPE_START_BNO)
-				args->type = XFS_ALLOCTYPE_THIS_AG;
-			/*
-			* For the first allocation, we can try any AG to get
-			* space.  However, if we already have allocated a
-			* block, we don't want to try AGs whose number is below
-			* sagno. Otherwise, we may end up with out-of-order
-			* locking of AGF, which might cause deadlock.
-			*/
-			if (++(args->agno) == mp->m_sb.sb_agcount) {
-				if (args->tp->t_firstblock != NULLFSBLOCK)
-					args->agno = sagno;
-				else
-					args->agno = 0;
-			}
-			/*
-			 * Reached the starting a.g., must either be done
-			 * or switch to non-trylock mode.
-			 */
-			if (args->agno == sagno) {
-				if (flags == 0) {
-					args->agbno = NULLAGBLOCK;
-					trace_xfs_alloc_vextent_allfailed(args);
-					break;
-				}
+	ASSERT(args->pag == NULL);
+	trace_xfs_alloc_vextent_allfailed(args);
+	return 0;
+}
 
-				flags = 0;
-				if (type == XFS_ALLOCTYPE_START_BNO) {
-					args->agbno = XFS_FSB_TO_AGBNO(mp,
-						args->fsbno);
-					args->type = XFS_ALLOCTYPE_NEAR_BNO;
-				}
-			}
-			xfs_perag_put(args->pag);
-		}
-		if (bump_rotor) {
-			if (args->agno == sagno)
-				mp->m_agfrotor = (mp->m_agfrotor + 1) %
-					(mp->m_sb.sb_agcount * rotorstep);
-			else
-				mp->m_agfrotor = (args->agno * rotorstep + 1) %
-					(mp->m_sb.sb_agcount * rotorstep);
-		}
-		break;
-	default:
-		ASSERT(0);
-		/* NOTREACHED */
+/*
+ * Iterate from the AGs from the start AG to the end of the filesystem, trying
+ * to allocate blocks. It starts with a near allocation attempt in the initial
+ * AG, then falls back to anywhere-in-ag after the first AG fails. It will wrap
+ * back to zero if allowed by previous allocations in this transaction,
+ * otherwise will wrap back to the start AG and run a second blocking pass to
+ * the end of the filesystem.
+ */
+int
+xfs_alloc_vextent_start_ag(
+	struct xfs_alloc_arg	*args,
+	xfs_fsblock_t		target)
+{
+	struct xfs_mount	*mp = args->mp;
+	xfs_agnumber_t		minimum_agno;
+	xfs_agnumber_t		start_agno;
+	xfs_agnumber_t		rotorstep = xfs_rotorstep;
+	bool			bump_rotor = false;
+	int			error;
+
+	args->agno = NULLAGNUMBER;
+	args->agbno = NULLAGBLOCK;
+	error = xfs_alloc_vextent_check_args(args, target, &minimum_agno);
+	if (error) {
+		if (error == -ENOSPC)
+			return 0;
+		return error;
 	}
-	if (args->agbno == NULLAGBLOCK)
-		args->fsbno = NULLFSBLOCK;
-	else {
-		args->fsbno = XFS_AGB_TO_FSB(mp, args->agno, args->agbno);
-#ifdef DEBUG
-		ASSERT(args->len >= args->minlen);
-		ASSERT(args->len <= args->maxlen);
-		ASSERT(args->agbno % args->alignment == 0);
-		XFS_AG_CHECK_DADDR(mp, XFS_FSB_TO_DADDR(mp, args->fsbno),
-			args->len);
-#endif
 
+	if ((args->datatype & XFS_ALLOC_INITIAL_USER_DATA) &&
+	    xfs_is_inode32(mp)) {
+		target = XFS_AGB_TO_FSB(mp,
+				((mp->m_agfrotor / rotorstep) %
+				mp->m_sb.sb_agcount), 0);
+		bump_rotor = 1;
 	}
-	xfs_perag_put(args->pag);
-	return 0;
-error0:
-	xfs_perag_put(args->pag);
-	return error;
+
+	start_agno = max(minimum_agno, XFS_FSB_TO_AGNO(mp, target));
+	error = xfs_alloc_vextent_iterate_ags(args, minimum_agno, start_agno,
+			XFS_FSB_TO_AGBNO(mp, target), XFS_ALLOC_FLAG_TRYLOCK);
+
+	if (bump_rotor) {
+		if (args->agno == start_agno)
+			mp->m_agfrotor = (mp->m_agfrotor + 1) %
+				(mp->m_sb.sb_agcount * rotorstep);
+		else
+			mp->m_agfrotor = (args->agno * rotorstep + 1) %
+				(mp->m_sb.sb_agcount * rotorstep);
+	}
+
+	return xfs_alloc_vextent_finish(args, minimum_agno, error, true);
+}
+
+/*
+ * Iterate from the agno indicated via @target through to the end of the
+ * filesystem attempting blocking allocation. This does not wrap or try a second
+ * pass, so will not recurse into AGs lower than indicated by the target.
+ */
+int
+xfs_alloc_vextent_first_ag(
+	struct xfs_alloc_arg	*args,
+	xfs_fsblock_t		target)
+ {
+	struct xfs_mount	*mp = args->mp;
+	xfs_agnumber_t		minimum_agno;
+	xfs_agnumber_t		start_agno;
+	int			error;
+
+	args->agno = NULLAGNUMBER;
+	args->agbno = NULLAGBLOCK;
+	error = xfs_alloc_vextent_check_args(args, target, &minimum_agno);
+	if (error) {
+		if (error == -ENOSPC)
+			return 0;
+		return error;
+	}
+
+	start_agno = max(minimum_agno, XFS_FSB_TO_AGNO(mp, target));
+	error = xfs_alloc_vextent_iterate_ags(args, minimum_agno, start_agno,
+			XFS_FSB_TO_AGBNO(mp, target), 0);
+	return xfs_alloc_vextent_finish(args, minimum_agno, error, true);
+}
+
+/*
+ * Allocate at the exact block target or fail. Caller is expected to hold a
+ * perag reference in args->pag.
+ */
+int
+xfs_alloc_vextent_exact_bno(
+	struct xfs_alloc_arg	*args,
+	xfs_fsblock_t		target)
+{
+	struct xfs_mount	*mp = args->mp;
+	xfs_agnumber_t		minimum_agno;
+	int			error;
+
+	args->agno = XFS_FSB_TO_AGNO(mp, target);
+	args->agbno = XFS_FSB_TO_AGBNO(mp, target);
+	error = xfs_alloc_vextent_check_args(args, target, &minimum_agno);
+	if (error) {
+		if (error == -ENOSPC)
+			return 0;
+		return error;
+	}
+
+	error = xfs_alloc_vextent_prepare_ag(args);
+	if (!error && args->agbp)
+		error = xfs_alloc_ag_vextent_exact(args);
+
+	return xfs_alloc_vextent_finish(args, minimum_agno, error, false);
+}
+
+/*
+ * Allocate an extent as close to the target as possible. If there are not
+ * viable candidates in the AG, then fail the allocation.
+ *
+ * Caller may or may not have a per-ag reference in args->pag.
+ */
+int
+xfs_alloc_vextent_near_bno(
+	struct xfs_alloc_arg	*args,
+	xfs_fsblock_t		target)
+{
+	struct xfs_mount	*mp = args->mp;
+	xfs_agnumber_t		minimum_agno;
+	bool			needs_perag = args->pag == NULL;
+	int			error;
+
+	args->agno = XFS_FSB_TO_AGNO(mp, target);
+	args->agbno = XFS_FSB_TO_AGBNO(mp, target);
+	error = xfs_alloc_vextent_check_args(args, target, &minimum_agno);
+	if (error) {
+		if (error == -ENOSPC)
+			return 0;
+		return error;
+	}
+
+	if (needs_perag)
+		args->pag = xfs_perag_grab(mp, args->agno);
+
+	error = xfs_alloc_vextent_prepare_ag(args);
+	if (!error && args->agbp)
+		error = xfs_alloc_ag_vextent_near(args);
+
+	return xfs_alloc_vextent_finish(args, minimum_agno, error, needs_perag);
 }
 
 /* Ensure that the freelist is at full capacity. */