[PATCH v2 3/3] xfs: rework quotaoff logging to avoid log deadlock on active fs

[Brian Foster <bfoster@redhat.com>]

The quotaoff operation logs two log items. The start item is
committed first, followed by the bulk of the in-core quotaoff
processing, and then the quotaoff end item is committed to release
the start item from the log. The problem with this mechanism is that
quite a bit of processing can be required to release dquots from all
in-core inodes and subsequently flush/purge all dquots in the
system. This processing work doesn't generally generate much log
traffic itself, but the start item pins the tail of the log. If an
external workload consumes the remaining log space before the
transaction for the end item is allocated, a log deadlock can occur.

The purpose of the separate start and end log items is primarily to
ensure that log recovery does not incorrectly recover dquot data
after an fs crash where a quotaoff was in progress. If we only
logged a single quotaoff item, for example, it could fall behind the
tail of the log before the last dquot modification was made and
incorrectly replay dquot changes that might have occurred after the
start item committed but before quotaoff deactivated the quota.

With that context, we can make some small changes to the quotaoff
algorithm to provide the same general log ordering guarantee without
such a large window to create a log deadlock vector. Rather than
place a start item in the log for the duration of quotaoff
processing, we can quiesce the transaction subsystem up front to
guarantee that no further dquots are logged from that point forward.
IOW, we pause the transaction subsystem, commit the quotaoff start
and end items, deactivate the associated quota such that subsequent
transactions no longer modify associated dquots, and resume the
transaction subsystem. The transaction pause is somewhat of a heavy
weight operation, but quotaoff is already a rare, slow and
performance disruptive operation and the quiesce is only required
for two small transactions.

Altogether, this means that the dquot rele/purge sequence occurs
after the quotaoff end item has committed and thus can technically
fall off the end of the log. This is safe because the remaining
processing is in-core work that doesn't involve logging dquots and
we've guaranteed that no further dquots are modified by external
transactions. This allows quotaoff to complete without risking log
deadlock regardless of how much dquot processing is required.

Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/xfs/xfs_qm_syscalls.c | 133 +++++++++++++++++++--------------------
 fs/xfs/xfs_trans_dquot.c |   2 +
 2 files changed, 67 insertions(+), 68 deletions(-)

diff --git a/fs/xfs/xfs_qm_syscalls.c b/fs/xfs/xfs_qm_syscalls.c
index ca1b57d291dc..b8e55f4947bd 100644
--- a/fs/xfs/xfs_qm_syscalls.c
+++ b/fs/xfs/xfs_qm_syscalls.c
@@ -29,7 +29,8 @@ xfs_qm_log_quotaoff(
 	int			error;
 	struct xfs_qoff_logitem	*qoffi;
 
-	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_qm_quotaoff, 0, 0, 0, &tp);
+	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_qm_quotaoff, 0, 0,
+				XFS_TRANS_NO_WRITECOUNT, &tp);
 	if (error)
 		goto out;
 
@@ -67,7 +68,8 @@ xfs_qm_log_quotaoff_end(
 	int			error;
 	struct xfs_qoff_logitem	*qoffi;
 
-	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_qm_equotaoff, 0, 0, 0, &tp);
+	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_qm_equotaoff, 0, 0,
+				XFS_TRANS_NO_WRITECOUNT, &tp);
 	if (error)
 		return error;
 
@@ -106,8 +108,8 @@ xfs_qm_scall_quotaoff(
 
 	/*
 	 * No file system can have quotas enabled on disk but not in core.
-	 * Note that quota utilities (like quotaoff) _expect_
-	 * errno == -EEXIST here.
+	 * Note that quota utilities (like quotaoff) _expect_ errno == -EEXIST
+	 * here.
 	 */
 	if ((mp->m_qflags & flags) == 0)
 		return -EEXIST;
@@ -116,17 +118,14 @@ xfs_qm_scall_quotaoff(
 	flags &= (XFS_ALL_QUOTA_ACCT | XFS_ALL_QUOTA_ENFD);
 
 	/*
-	 * We don't want to deal with two quotaoffs messing up each other,
-	 * so we're going to serialize it. quotaoff isn't exactly a performance
+	 * We don't want to deal with two quotaoffs messing up each other, so
+	 * we're going to serialize it. quotaoff isn't exactly a performance
 	 * critical thing.
-	 * If quotaoff, then we must be dealing with the root filesystem.
 	 */
 	ASSERT(q);
 	mutex_lock(&q->qi_quotaofflock);
 
-	/*
-	 * If we're just turning off quota enforcement, change mp and go.
-	 */
+	/* if we're just turning off quota enforcement, change mp and go */
 	if ((flags & XFS_ALL_QUOTA_ACCT) == 0) {
 		mp->m_qflags &= ~(flags);
 
@@ -142,9 +141,9 @@ xfs_qm_scall_quotaoff(
 	dqtype = 0;
 	inactivate_flags = 0;
 	/*
-	 * If accounting is off, we must turn enforcement off, clear the
-	 * quota 'CHKD' certificate to make it known that we have to
-	 * do a quotacheck the next time this quota is turned on.
+	 * If accounting is off, we must turn enforcement off, clear the quota
+	 * 'CHKD' certificate to make it known that we have to do a quotacheck
+	 * the next time this quota is turned on.
 	 */
 	if (flags & XFS_UQUOTA_ACCT) {
 		dqtype |= XFS_QMOPT_UQUOTA;
@@ -163,89 +162,87 @@ xfs_qm_scall_quotaoff(
 	}
 
 	/*
-	 * Nothing to do?  Don't complain. This happens when we're just
-	 * turning off quota enforcement.
+	 * Nothing to do? Don't complain. This happens when we're just turning
+	 * off quota enforcement.
 	 */
 	if ((mp->m_qflags & flags) == 0)
 		goto out_unlock;
 
 	/*
-	 * Write the LI_QUOTAOFF log record, and do SB changes atomically,
-	 * and synchronously. If we fail to write, we should abort the
-	 * operation as it cannot be recovered safely if we crash.
+	 * Quotaoff must deactivate the associated quota mode(s), release dquots
+	 * from inodes and purge them from the system all while the filesystem
+	 * remains active. We have two quotaoff log records that traditionally
+	 * bound the start and end of this sequence. This guarantees that no
+	 * dquots are modified after the end item hits the log, but quotaoff can
+	 * be time consuming and thus prone to deadlock because the start item
+	 * pins the tail the of log in the meantime (and we can't hold the end
+	 * transaction open across the dqrele scan).
+	 *
+	 * The critical aspect of correctly logging quotaoff is that no dquots
+	 * are modified after the quotaoff end item hits the on-disk log.
+	 * Otherwise the quotaoff can fall off the tail and log recovery can
+	 * replay incorrect data. Instead of letting the start item sit in the
+	 * log while quotaoff completes, we can provide the same guarantee via a
+	 * runtime barrier for dquot modifications. Specifically, we pause all
+	 * transactions on the system via the transaction subsystem lock, log
+	 * both start and end items (via sync transactions, which drains the
+	 * CIL), deactivate the quota, and then resume the transaction subsystem
+	 * while quotaoff completes.
+	 *
+	 * This is safe because the remaining quotaoff work is in-core cleanup
+	 * and all subsequent transactions should see the updated quota state
+	 * due to memory ordering provided by the lock. We also avoid deadlock
+	 * by committing both items sequentially with near exclusive access to
+	 * the transaction subsystem.
 	 */
+	percpu_down_write(&mp->m_trans_rwsem);
+
 	error = xfs_qm_log_quotaoff(mp, &qoffstart, flags);
-	if (error)
+	if (error) {
+		percpu_up_write(&mp->m_trans_rwsem);
 		goto out_unlock;
+	}
 
-	/*
-	 * Next we clear the XFS_MOUNT_*DQ_ACTIVE bit(s) in the mount struct
-	 * to take care of the race between dqget and quotaoff. We don't take
-	 * any special locks to reset these bits. All processes need to check
-	 * these bits *after* taking inode lock(s) to see if the particular
-	 * quota type is in the process of being turned off. If *ACTIVE, it is
-	 * guaranteed that all dquot structures and all quotainode ptrs will all
-	 * stay valid as long as that inode is kept locked.
-	 *
-	 * There is no turning back after this.
-	 */
 	mp->m_qflags &= ~inactivate_flags;
 
+	error = xfs_qm_log_quotaoff_end(mp, &qoffstart, flags);
+	if (error) {
+		percpu_up_write(&mp->m_trans_rwsem);
+		/* We're screwed now. Shutdown is the only option. */
+		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
+		goto out_unlock;
+	}
+
+	percpu_up_write(&mp->m_trans_rwsem);
+
 	/*
-	 * Give back all the dquot reference(s) held by inodes.
-	 * Here we go thru every single incore inode in this file system, and
-	 * do a dqrele on the i_udquot/i_gdquot that it may have.
-	 * Essentially, as long as somebody has an inode locked, this guarantees
-	 * that quotas will not be turned off. This is handy because in a
-	 * transaction once we lock the inode(s) and check for quotaon, we can
-	 * depend on the quota inodes (and other things) being valid as long as
-	 * we keep the lock(s).
+	 * Release dquot references held by inodes. Technically some contexts
+	 * might not pick up the quota state change until the inode lock is
+	 * cycled if there is no transaction. We don't care about that above
+	 * because a dquot can't be logged without a transaction and we can't
+	 * release/purge a dquot here until we've cycled the locks of all inodes
+	 * that reference it.
 	 */
 	xfs_qm_dqrele_all_inodes(mp, flags);
 
 	/*
 	 * Next we make the changes in the quota flag in the mount struct.
-	 * This isn't protected by a particular lock directly, because we
-	 * don't want to take a mrlock every time we depend on quotas being on.
+	 * This isn't protected by a particular lock directly, because we don't
+	 * want to take a mrlock every time we depend on quotas being on.
 	 */
 	mp->m_qflags &= ~flags;
 
-	/*
-	 * Go through all the dquots of this file system and purge them,
-	 * according to what was turned off.
-	 */
+	/* purge all deactivated dquots from the filesystem */
 	xfs_qm_dqpurge_all(mp, dqtype);
 
-	/*
-	 * Transactions that had started before ACTIVE state bit was cleared
-	 * could have logged many dquots, so they'd have higher LSNs than
-	 * the first QUOTAOFF log record does. If we happen to crash when
-	 * the tail of the log has gone past the QUOTAOFF record, but
-	 * before the last dquot modification, those dquots __will__
-	 * recover, and that's not good.
-	 *
-	 * So, we have QUOTAOFF start and end logitems; the start
-	 * logitem won't get overwritten until the end logitem appears...
-	 */
-	error = xfs_qm_log_quotaoff_end(mp, &qoffstart, flags);
-	if (error) {
-		/* We're screwed now. Shutdown is the only option. */
-		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
-		goto out_unlock;
-	}
-
-	/*
-	 * If all quotas are completely turned off, close shop.
-	 */
+	/* if all quotas are completely turned off, close shop */
 	if (mp->m_qflags == 0) {
 		mutex_unlock(&q->qi_quotaofflock);
 		xfs_qm_destroy_quotainfo(mp);
 		return 0;
 	}
 
-	/*
-	 * Release our quotainode references if we don't need them anymore.
-	 */
+	/* release our quotainode references if we don't need them anymore */
 	if ((dqtype & XFS_QMOPT_UQUOTA) && q->qi_uquotaip) {
 		xfs_irele(q->qi_uquotaip);
 		q->qi_uquotaip = NULL;
diff --git a/fs/xfs/xfs_trans_dquot.c b/fs/xfs/xfs_trans_dquot.c
index eec640999148..e484747d83ba 100644
--- a/fs/xfs/xfs_trans_dquot.c
+++ b/fs/xfs/xfs_trans_dquot.c
@@ -53,6 +53,8 @@ xfs_trans_log_dquot(
 	struct xfs_dquot	*dqp)
 {
 	ASSERT(XFS_DQ_IS_LOCKED(dqp));
+	/* quotaoff expects no dquots logged after deactivation */
+	ASSERT(xfs_this_quota_on(tp->t_mountp, xfs_dquot_type(dqp)));
 
 	/* Upgrade the dquot to bigtime format if possible. */
 	if (dqp->q_id != 0 &&
-- 
2.26.3