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From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Dave Chinner <david@fromorbit.com>
Cc: xfs <xfs@oss.sgi.com>
Subject: [PATCH] xfs: remove OWN_AG rmap when allocating a block from the AGFL
Date: Fri, 5 Aug 2016 13:22:02 -0700
Message-ID: <20160805202202.GJ25565@birch.djwong.org> (raw)
In-Reply-To: <20160805202104.GB8590@birch.djwong.org>

[resend, cc list this time]

I ftrace'd the rmap code while trying to repro the g/299 assert you
saw.  Eventually I got it to reproduce and it looks like a block would
go onto the AGFL and we'd add an OWN_AG rmap for the block.  ~20s
later the block mysteriously ends up being mapped in a second time as
some inode's BMBT block without the OWN_AG rmap being removed.  So I
looked around for anything that grabbed a block off the AGFL and
noticed that the regular allocation function can do that if we get
really tight on space.  I also noticed that it doesn't remove the
OWN_AG rmap, nor does it credit the per-AG reservation for the block
it used.  So here's a test patch that updates the rmap.  (The per-AG
stuff is in the reflink patchset so we don't need it here.)

---
When we're really tight on space, xfs_alloc_ag_vextent_small() can
allocate a block from the AGFL and give it to the caller.  Since the
caller is never the AGFL-fixing method, we must remove the OWN_AG
reverse mapping because it will clash with whatever rmap the caller
wants to set up.  This bug was discovered by running generic/299
repeatedly.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/xfs/libxfs/xfs_alloc.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index af1a72e..3dd8f1d 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -1582,6 +1582,7 @@ xfs_alloc_ag_vextent_small(
 	xfs_extlen_t	*flenp,	/* result length */
 	int		*stat)	/* status: 0-freelist, 1-normal/none */
 {
+	struct xfs_owner_info	oinfo;
 	int		error;
 	xfs_agblock_t	fbno;
 	xfs_extlen_t	flen;
@@ -1624,6 +1625,18 @@ xfs_alloc_ag_vextent_small(
 				error0);
 			args->wasfromfl = 1;
 			trace_xfs_alloc_small_freelist(args);
+
+			/*
+			 * If we're feeding an AGFL block to something that
+			 * doesn't live in the free space, we need to clear
+			 * out the OWN_AG rmap.
+			 */
+			xfs_rmap_ag_owner(&oinfo, XFS_RMAP_OWN_AG);
+			error = xfs_rmap_free(args->tp, args->agbp, args->agno,
+					fbno, 1, &oinfo);
+			if (error)
+				goto error0;
+
 			*stat = 0;
 			return 0;
 		}

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