Re: [RFC v5 PATCH 6/9] xfs: automatically relog the quotaoff start intent

[Brian Foster <bfoster@redhat.com>]

On Thu, Feb 27, 2020 at 04:19:42PM -0700, Allison Collins wrote:
> 
> 
> On 2/27/20 6:43 AM, Brian Foster wrote:
> > The quotaoff operation has a rare but longstanding deadlock vector
> > in terms of how the operation is logged. A quotaoff start intent is
> > logged (synchronously) at the onset to ensure recovery can handle
> > the operation if interrupted before in-core changes are made. This
> > quotaoff intent pins the log tail while the quotaoff sequence scans
> > and purges dquots from all in-core inodes. While this operation
> > generally doesn't generate much log traffic on its own, it can be
> > time consuming. If unrelated, concurrent filesystem activity
> > consumes remaining log space before quotaoff is able to acquire log
> > reservation for the quotaoff end intent, the filesystem locks up
> > indefinitely.
> > 
> > quotaoff cannot allocate the end intent before the scan because the
> > latter can result in transaction allocation itself in certain
> > indirect cases (releasing an inode, for example). Further, rolling
> > the original transaction is difficult because the scanning work
> > occurs multiple layers down where caller context is lost and not
> > much information is available to determine how often to roll the
> > transaction.
> > 
> > To address this problem, enable automatic relogging of the quotaoff
> > start intent. This automatically relogs the intent whenever AIL
> > pushing finds the item at the tail of the log. When quotaoff
> > completes, wait for relogging to complete as the end intent expects
> > to be able to permanently remove the start intent from the log
> > subsystem. This ensures that the log tail is kept moving during a
> > particularly long quotaoff operation and avoids the log reservation
> > deadlock.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> >   fs/xfs/libxfs/xfs_trans_resv.c |  3 ++-
> >   fs/xfs/xfs_dquot_item.c        |  7 +++++++
> >   fs/xfs/xfs_qm_syscalls.c       | 12 +++++++++++-
> >   3 files changed, 20 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_trans_resv.c b/fs/xfs/libxfs/xfs_trans_resv.c
> > index 1f5c9e6e1afc..f49b20c9ca33 100644
> > --- a/fs/xfs/libxfs/xfs_trans_resv.c
> > +++ b/fs/xfs/libxfs/xfs_trans_resv.c
> > @@ -935,7 +935,8 @@ xfs_trans_resv_calc(
> >   	resp->tr_qm_setqlim.tr_logcount = XFS_DEFAULT_LOG_COUNT;
> >   	resp->tr_qm_quotaoff.tr_logres = xfs_calc_qm_quotaoff_reservation(mp);
> > -	resp->tr_qm_quotaoff.tr_logcount = XFS_DEFAULT_LOG_COUNT;
> > +	resp->tr_qm_quotaoff.tr_logcount = XFS_DEFAULT_PERM_LOG_COUNT;
> > +	resp->tr_qm_quotaoff.tr_logflags |= XFS_TRANS_PERM_LOG_RES;
> What's the reason for the log count change here?  Otherwise looks ok.
> 

Permanent transactions have a separate default log count (2 instead of
1) because they are intended to be rolled (at least once). This
basically means the initial allocation will acquire enough log
reservation for the initial transaction and a subsequent roll, similar
to how all other permanant transactions are initialized in this file.
This is required for quotaoff because the transaction now uses
XFS_TRANS_RELOG, which requires a roll up front.

Brian

> Allison
> >   	resp->tr_qm_equotaoff.tr_logres =
> >   		xfs_calc_qm_quotaoff_end_reservation();
> > diff --git a/fs/xfs/xfs_dquot_item.c b/fs/xfs/xfs_dquot_item.c
> > index d60647d7197b..ea5123678466 100644
> > --- a/fs/xfs/xfs_dquot_item.c
> > +++ b/fs/xfs/xfs_dquot_item.c
> > @@ -297,6 +297,13 @@ xfs_qm_qoff_logitem_push(
> >   	struct xfs_log_item	*lip,
> >   	struct list_head	*buffer_list)
> >   {
> > +	struct xfs_log_item	*mlip = xfs_ail_min(lip->li_ailp);
> > +
> > +	if (test_bit(XFS_LI_RELOG, &lip->li_flags) &&
> > +	    !test_bit(XFS_LI_RELOGGED, &lip->li_flags) &&
> > +	    !XFS_LSN_CMP(lip->li_lsn, mlip->li_lsn))
> > +		return XFS_ITEM_RELOG;
> > +
> >   	return XFS_ITEM_LOCKED;
> >   }
> > diff --git a/fs/xfs/xfs_qm_syscalls.c b/fs/xfs/xfs_qm_syscalls.c
> > index 1ea82764bf89..7b48d34da0f4 100644
> > --- a/fs/xfs/xfs_qm_syscalls.c
> > +++ b/fs/xfs/xfs_qm_syscalls.c
> > @@ -18,6 +18,7 @@
> >   #include "xfs_quota.h"
> >   #include "xfs_qm.h"
> >   #include "xfs_icache.h"
> > +#include "xfs_trans_priv.h"
> >   STATIC int
> >   xfs_qm_log_quotaoff(
> > @@ -31,12 +32,14 @@ xfs_qm_log_quotaoff(
> >   	*qoffstartp = NULL;
> > -	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_RELOG, &tp);
> >   	if (error)
> >   		goto out;
> >   	qoffi = xfs_trans_get_qoff_item(tp, NULL, flags & XFS_ALL_QUOTA_ACCT);
> >   	xfs_trans_log_quotaoff_item(tp, qoffi);
> > +	xfs_trans_relog_item(&qoffi->qql_item);
> >   	spin_lock(&mp->m_sb_lock);
> >   	mp->m_sb.sb_qflags = (mp->m_qflags & ~(flags)) & XFS_MOUNT_QUOTA_ALL;
> > @@ -69,6 +72,13 @@ xfs_qm_log_quotaoff_end(
> >   	int			error;
> >   	struct xfs_qoff_logitem	*qoffi;
> > +	/*
> > +	 * startqoff must be in the AIL and not the CIL when the end intent
> > +	 * commits to ensure it is not readded to the AIL out of order. Wait on
> > +	 * relog activity to drain to isolate startqoff to the AIL.
> > +	 */
> > +	xfs_trans_relog_item_cancel(&startqoff->qql_item, true);
> > +
> >   	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_qm_equotaoff, 0, 0, 0, &tp);
> >   	if (error)
> >   		return error;
> > 
>