| Message ID | 20201001150310.141467-4-bfoster@redhat.com (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Headers | show |
| Series | xfs: rework quotaoff to avoid log deadlock | expand |
On Thu, Oct 01, 2020 at 11:03:10AM -0400, Brian Foster wrote: > 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); Hmm, so if I read this correctly, you're changing what gets written to the log from: <quotaoff intent> <transactions that maybe log dquots but we don't care> ...now the qflags get cleared... <transactions that maybe log dquots but we don't care> ...run around purging dquots... <transactions that maybe log dquots but we don't care> <quotaoff done> to something that looks more like: <transactions that log dquots> <quotaoff intent, all other transactions locked out> ...now the qflags get cleared; no other transactions... <quotaoff done, turn off the lockout> <transactions that wont log dquots> ...run around purging dquots... Is that right? I guess that makes sense, though it's sort of a pity that we now make every transaction grab a read lock. Though I guess there's not much that can be done about that; it's better than pinning the log tail. I don't even think there's a good way to relog that quotaoff-intent item, is there? Since you'd have to, I dunno, do all the checks that xfs_defer_relog() does to decide if it should relog an intent item? --D > + > /* > - * 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 547ba824542e..9839b83e732a 100644 > --- a/fs/xfs/xfs_trans_dquot.c > +++ b/fs/xfs/xfs_trans_dquot.c > @@ -52,6 +52,8 @@ 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 && > -- > 2.25.4 >
On Wed, Oct 07, 2020 at 03:24:16PM -0700, Darrick J. Wong wrote: > On Thu, Oct 01, 2020 at 11:03:10AM -0400, Brian Foster wrote: > > 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 ... > > @@ -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); > > Hmm, so if I read this correctly, you're changing what gets written to > the log from: > > <quotaoff intent> > <transactions that maybe log dquots but we don't care> > ...now the qflags get cleared... > <transactions that maybe log dquots but we don't care> > ...run around purging dquots... > <transactions that maybe log dquots but we don't care> > <quotaoff done> > > to something that looks more like: > > <transactions that log dquots> > <quotaoff intent, all other transactions locked out> > ...now the qflags get cleared; no other transactions... > <quotaoff done, turn off the lockout> > <transactions that wont log dquots> > ...run around purging dquots... > Yep, pretty much. The idea is to preserve the behavior that no dquots are logged after the quotaoff end item to guarantee the log can never hold a modified dquot after a previous quotaoff falls off the tail. Rather than doing that by bounding the quotaoff operation with the two log items, we introduce a runtime quiesce that deterministically disables dquot activity and log the items after that point. > Is that right? I guess that makes sense, though it's sort of a pity > that we now make every transaction grab a read lock. Though I guess > there's not much that can be done about that; it's better than pinning > the log tail. I don't even think there's a good way to relog that > quotaoff-intent item, is there? Since you'd have to, I dunno, do all > the checks that xfs_defer_relog() does to decide if it should relog an > intent item? > Yeah, well that was a purpose of the previous automatic relog bits. Short of that, I'm not aware of a great way to keep the start intent moving. We can't hold a transaction open across the scan(s), so that also rules out turning quotaoff into a dfop, for example. We could try to relog the intent at random points, but that looks to me like a bunch of busy work to break up the scans for no guarantee in return that the relog transaction wouldn't still deadlock anyways. Brian > --D > > > + > > /* > > - * 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 547ba824542e..9839b83e732a 100644 > > --- a/fs/xfs/xfs_trans_dquot.c > > +++ b/fs/xfs/xfs_trans_dquot.c > > @@ -52,6 +52,8 @@ 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 && > > -- > > 2.25.4 > > >
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 547ba824542e..9839b83e732a 100644 --- a/fs/xfs/xfs_trans_dquot.c +++ b/fs/xfs/xfs_trans_dquot.c @@ -52,6 +52,8 @@ 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 &&
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(-)