Re: [PATCH 02/11] xfs: don't stall cowblocks scan if we can't take locks

Brian Foster
2021-01-26T13:21:41Z

On Mon, Jan 25, 2021 at 11:54:46AM -0800, Darrick J. Wong wrote:
> On Mon, Jan 25, 2021 at 01:14:06PM -0500, Brian Foster wrote:
> > On Sat, Jan 23, 2021 at 10:52:10AM -0800, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <djwong@kernel.org>
> > >
> > > Don't stall the cowblocks scan on a locked inode if we possibly can.
> > > We'd much rather the background scanner keep moving.
> > >
> > > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > > ---
> > > fs/xfs/xfs_icache.c | 21 ++++++++++++++++++---
> > > 1 file changed, 18 insertions(+), 3 deletions(-)
> > >
> > >
> > > diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> > > index c71eb15e3835..89f9e692fde7 100644
> > > --- a/fs/xfs/xfs_icache.c
> > > +++ b/fs/xfs/xfs_icache.c
> > > @@ -1605,17 +1605,31 @@ xfs_inode_free_cowblocks(
> > > void *args)
> > > {
> > > struct xfs_eofblocks *eofb = args;
> > > + bool wait;
> > > int ret = 0;
> > >
> > > + wait = eofb && (eofb->eof_flags & XFS_EOF_FLAGS_SYNC);
> > > +
> > > if (!xfs_prep_free_cowblocks(ip))
> > > return 0;
> > >
> > > if (!xfs_inode_matches_eofb(ip, eofb))
> > > return 0;
> > >
> > > - /* Free the CoW blocks */
> > > - xfs_ilock(ip, XFS_IOLOCK_EXCL);
> > > - xfs_ilock(ip, XFS_MMAPLOCK_EXCL);
> > > + /*
> > > + * If the caller is waiting, return -EAGAIN to keep the background
> > > + * scanner moving and revisit the inode in a subsequent pass.
> > > + */
> > > + if (!xfs_ilock_nowait(ip, XFS_IOLOCK_EXCL)) {
> > > + if (wait)
> > > + return -EAGAIN;
> > > + return 0;
> > > + }
> > > + if (!xfs_ilock_nowait(ip, XFS_MMAPLOCK_EXCL)) {
> > > + if (wait)
> > > + ret = -EAGAIN;
> > > + goto out_iolock;
> > > + }
> >
> > Hmm.. I'd be a little concerned over this allowing a scan to repeat
> > indefinitely with a competing workload because a restart doesn't carry
> > over any state from the previous scan. I suppose the
> > xfs_prep_free_cowblocks() checks make that slightly less likely on a
> > given file, but I more wonder about a scenario with a large set of
> > inodes in a particular AG with a sufficient amount of concurrent
> > activity. All it takes is one trylock failure per scan to have to start
> > the whole thing over again... hm?
>
> I'm not quite sure what to do here -- xfs_inode_free_eofblocks already
> has the ability to return EAGAIN, which (I think) means that it's
> already possible for the low-quota scan to stall indefinitely if the
> scan can't lock the inode.
>

Indeed, that is true.

> I think we already had a stall limiting factor here in that all the
> other threads in the system that hit EDQUOT will drop their IOLOCKs to
> scan the fs, which means that while they loop around the scanner they
> can only be releasing quota and driving us towards having fewer inodes
> with the same dquots and either blockgc tag set.
>

Yeah, that makes sense for the current use case. There's a broader
sequence involved there that provides some throttling and serialization,
along with the fact that the workload is imminently driving into
-ENOSPC.

I think what had me a little concerned upon seeing this is whether the
scanning mechanism is currently suitable for the broader usage
introduced in this series. We've had related issues in the past with
concurrent sync eofblocks scans and iolock (see [1], for example).
Having made it through the rest of the series however, it looks like all
of the new scan invocations are async, so perhaps this is not really an
immediate problem.

I think it would be nice if we could somehow assert that the task that
invokes a sync scan doesn't hold an iolock, but I'm not sure there's a
clean way to do that. We'd probably have to define the interface to
require an inode just for that purpose. It may not be worth that
weirdness, and I suppose if code is tested it should be pretty obvious
that such a scan will never complete..

Brian

[1] c3155097ad89 ("xfs: sync eofblocks scans under iolock are livelock prone")

> --D
>
> > Brian
> >
> > >
> > > /*
> > > * Check again, nobody else should be able to dirty blocks or change
> > > @@ -1625,6 +1639,7 @@ xfs_inode_free_cowblocks(
> > > ret = xfs_reflink_cancel_cow_range(ip, 0, NULLFILEOFF, false);
> > >
> > > xfs_iunlock(ip, XFS_MMAPLOCK_EXCL);
> > > +out_iolock:
> > > xfs_iunlock(ip, XFS_IOLOCK_EXCL);
> > >
> > > return ret;
> > >
> >
>