Re: [PATCH 2/2] xfs: fix unmount hang and memory leak on shutdown during quotaoff

Brian Foster
2020-03-17T14:57:27Z

On Tue, Mar 17, 2020 at 07:46:07AM -0700, Darrick J. Wong wrote:
> On Tue, Mar 17, 2020 at 07:40:11AM -0400, Brian Foster wrote:
> > On Mon, Mar 16, 2020 at 02:32:23PM -0700, Darrick J. Wong wrote:
> > > On Mon, Mar 16, 2020 at 01:00:32PM -0400, Brian Foster wrote:
> > > > AIL removal of the quotaoff start intent and free of both quotaoff
> > > > intents is currently limited to the ->iop_committed() handler of the
> > > > end intent. This executes when the end intent is committed to the
> > > > on-disk log and marks the completion of the operation. The problem
> > > > with this is it assumes the success of the operation. If a shutdown
> > > > or other error occurs during the quotaoff, it's possible for the
> > > > quotaoff task to exit without removing the start intent from the
> > > > AIL. This results in an unmount hang as the AIL cannot be emptied.
> > > > Further, no other codepath frees the intents and so this is also a
> > > > memory leak vector.
> > >
> > > And I'm guessing that you'd rather we taught the quota items to be
> > > self-releasing under error rather than making the quotaoff code be smart
> > > enough to free the quotaoff-start item?
> > >
> >
> > It's a combination of both because they are separate transactions... If
> > the item is "owned" by a transaction and the transaction aborts, we
> > "release" all attached items as we would for any other transaction/item.
> > Once the start transaction commits, the start item is AIL resident and
> > there's no guarantee we'll ever get to the end transaction (i.e. a
> > shutdown could cause the transaction allocation to fail). The quotaoff
> > code handles cleaning up the start item in that scenario. This is
> > similar to the whole EFD holding a reference to the EFI model, except
> > quotaoff is a bit more hacky..
> >
> > From a development perspective, my approach was to fix up the intent
> > handling such that adheres to common practice wrt to transactions and
> > then address the gap(s) in the quotaoff code (i.e. the separate
> > start/end transactions being an implementation detail of quotaoff).
>
> <nod> Sounds like a reasonable approach for a pretty odd duck.
>
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
>
> > > > First, update the high level quotaoff error path to directly remove
> > > > and free the quotaoff start intent if it still exists in the AIL at
> > > > the time of the error. Next, update both of the start and end
> > > > quotaoff intents with an ->iop_release() callback to properly handle
> > > > transaction abort.
> > >
> > > I wonder, does this mean that we can drop the if (->io_release) check in
> > > xfs_trans_free_items? ISTR we were wondering at one point if there ever
> > > was a real use case for items that don't have a release function.
> > >
> >
> > Hmm.. this was reworked fairly recently IIRC to condense some of the
> > callbacks. This used to be an ->iop_unlock() call. It was renamed to
> > ->iop_release() to primarily handle the abort scenario (yet can also be
> > called via log I/O completion based on a magic flag) and the non-abort
> > ->iop_unlock() call was folded into ->iop_committing(). Clear as mud? :)
> > In any event, I'm not aware of any further effort to remove
> > ->iop_release() from xfs_trans_free_items() as that is the typical abort
> > path that this patch relies on..
>
> Oh, I didn't mean dropping ->iop_release completely, I just wondered
> about removing the conditional, e.g.
>
> if (->iop_release)
> ->iop_release(...);
>
> becomes:
>
> ->iop_release(...);
>
> Now that every log item type (I think?) actually has a release method.
>

Ah, I misread. -ENOCOFFEE I guess. :P That seems reasonable to me, but
tbh I'm not sure it's worth saving 1 LOC either. *shrug*

Brian

> --D
>
> > Brian
> >
> > > > This means that If the quotaoff start transaction aborts, it frees
> > > > the start intent in the transaction commit path. If the filesystem
> > > > shuts down before the end transaction allocates, the quotaoff
> > > > sequence removes and frees the start intent. If the end transaction
> > > > aborts, it removes the start intent and frees both. This ensures
> > > > that a shutdown does not result in a hung unmount and that memory is
> > > > not leaked regardless of when a quotaoff error occurs.
> > > >
> > > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > >
> > > FWIW, the code looks reasonable.
> > >
> > > --D
> > >
> > > > ---
> > > > fs/xfs/xfs_dquot_item.c | 15 +++++++++++++++
> > > > fs/xfs/xfs_qm_syscalls.c | 13 +++++++------
> > > > 2 files changed, 22 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/fs/xfs/xfs_dquot_item.c b/fs/xfs/xfs_dquot_item.c
> > > > index 2b816e9b4465..cf65e2e43c6e 100644
> > > > --- a/fs/xfs/xfs_dquot_item.c
> > > > +++ b/fs/xfs/xfs_dquot_item.c
> > > > @@ -315,17 +315,32 @@ xfs_qm_qoffend_logitem_committed(
> > > > return (xfs_lsn_t)-1;
> > > > }
> > > >
> > > > +STATIC void
> > > > +xfs_qm_qoff_logitem_release(
> > > > + struct xfs_log_item *lip)
> > > > +{
> > > > + struct xfs_qoff_logitem *qoff = QOFF_ITEM(lip);
> > > > +
> > > > + if (test_bit(XFS_LI_ABORTED, &lip->li_flags)) {
> > > > + if (qoff->qql_start_lip)
> > > > + xfs_qm_qoff_logitem_relse(qoff->qql_start_lip);
> > > > + xfs_qm_qoff_logitem_relse(qoff);
> > > > + }
> > > > +}
> > > > +
> > > > static const struct xfs_item_ops xfs_qm_qoffend_logitem_ops = {
> > > > .iop_size = xfs_qm_qoff_logitem_size,
> > > > .iop_format = xfs_qm_qoff_logitem_format,
> > > > .iop_committed = xfs_qm_qoffend_logitem_committed,
> > > > .iop_push = xfs_qm_qoff_logitem_push,
> > > > + .iop_release = xfs_qm_qoff_logitem_release,
> > > > };
> > > >
> > > > static const struct xfs_item_ops xfs_qm_qoff_logitem_ops = {
> > > > .iop_size = xfs_qm_qoff_logitem_size,
> > > > .iop_format = xfs_qm_qoff_logitem_format,
> > > > .iop_push = xfs_qm_qoff_logitem_push,
> > > > + .iop_release = xfs_qm_qoff_logitem_release,
> > > > };
> > > >
> > > > /*
> > > > diff --git a/fs/xfs/xfs_qm_syscalls.c b/fs/xfs/xfs_qm_syscalls.c
> > > > index 1ea82764bf89..5d5ac65aa1cc 100644
> > > > --- a/fs/xfs/xfs_qm_syscalls.c
> > > > +++ b/fs/xfs/xfs_qm_syscalls.c
> > > > @@ -29,8 +29,6 @@ xfs_qm_log_quotaoff(
> > > > int error;
> > > > struct xfs_qoff_logitem *qoffi;
> > > >
> > > > - *qoffstartp = NULL;
> > > > -
> > > > error = xfs_trans_alloc(mp, &M_RES(mp)->tr_qm_quotaoff, 0, 0, 0, &tp);
> > > > if (error)
> > > > goto out;
> > > > @@ -62,7 +60,7 @@ xfs_qm_log_quotaoff(
> > > > STATIC int
> > > > xfs_qm_log_quotaoff_end(
> > > > struct xfs_mount *mp,
> > > > - struct xfs_qoff_logitem *startqoff,
> > > > + struct xfs_qoff_logitem **startqoff,
> > > > uint flags)
> > > > {
> > > > struct xfs_trans *tp;
> > > > @@ -73,9 +71,10 @@ xfs_qm_log_quotaoff_end(
> > > > if (error)
> > > > return error;
> > > >
> > > > - qoffi = xfs_trans_get_qoff_item(tp, startqoff,
> > > > + qoffi = xfs_trans_get_qoff_item(tp, *startqoff,
> > > > flags & XFS_ALL_QUOTA_ACCT);
> > > > xfs_trans_log_quotaoff_item(tp, qoffi);
> > > > + *startqoff = NULL;
> > > >
> > > > /*
> > > > * We have to make sure that the transaction is secure on disk before we
> > > > @@ -103,7 +102,7 @@ xfs_qm_scall_quotaoff(
> > > > uint dqtype;
> > > > int error;
> > > > uint inactivate_flags;
> > > > - struct xfs_qoff_logitem *qoffstart;
> > > > + struct xfs_qoff_logitem *qoffstart = NULL;
> > > >
> > > > /*
> > > > * No file system can have quotas enabled on disk but not in core.
> > > > @@ -228,7 +227,7 @@ xfs_qm_scall_quotaoff(
> > > > * 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);
> > > > + 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);
> > > > @@ -261,6 +260,8 @@ xfs_qm_scall_quotaoff(
> > > > }
> > > >
> > > > out_unlock:
> > > > + if (error && qoffstart)
> > > > + xfs_qm_qoff_logitem_relse(qoffstart);
> > > > mutex_unlock(&q->qi_quotaofflock);
> > > > return error;
> > > > }
> > > > --
> > > > 2.21.1
> > > >
> > >
> >
>