xfs: grab active perag ref when reading AG headers

This patch prepares scrub to deal with the possibility of tearing down
entire AGs by changing the order of resource acquisition to match the
rest of the XFS codebase.  In other words, scrub now grabs AG resources
in order of: perag structure, then AGI/AGF/AGFL buffers, then btree
cursors; and releases them in reverse order.

This requires us to distinguish xchk_ag_init callers -- some are
responding to a user request to check AG metadata, in which case we can
return ENOENT to userspace; but other callers have an ondisk reference
to an AG that they're trying to cross-reference.  In this second case,
the lack of an AG means there's ondisk corruption, since ondisk metadata
cannot point into nonexistent space.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

fs/xfs/scrub/agheader.c

@@ -36,7 +36,7 @@ xchk_superblock_xref(
 
 	agbno = XFS_SB_BLOCK(mp);
 
-	error = xchk_ag_init(sc, agno, &sc->sa);
+	error = xchk_ag_init_existing(sc, agno, &sc->sa);
 	if (!xchk_xref_process_error(sc, agno, agbno, &error))
 		return;
 
@@ -63,6 +63,7 @@ xchk_superblock(
 	struct xfs_mount	*mp = sc->mp;
 	struct xfs_buf		*bp;
 	struct xfs_dsb		*sb;
+	struct xfs_perag	*pag;
 	xfs_agnumber_t		agno;
 	uint32_t		v2_ok;
 	__be32			features_mask;
@@ -73,6 +74,15 @@ xchk_superblock(
 	if (agno == 0)
 		return 0;
 
+	/*
+	 * Grab an active reference to the perag structure.  If we can't get
+	 * it, we're racing with something that's tearing down the AG, so
+	 * signal that the AG no longer exists.
+	 */
+	pag = xfs_perag_get(mp, agno);
+	if (!pag)
+		return -ENOENT;
+
 	error = xfs_sb_read_secondary(mp, sc->tp, agno, &bp);
 	/*
 	 * The superblock verifier can return several different error codes
@@ -92,7 +102,7 @@ xchk_superblock(
 		break;
 	}
 	if (!xchk_process_error(sc, agno, XFS_SB_BLOCK(mp), &error))
-		return error;
+		goto out_pag;
 
 	sb = bp->b_addr;
 
@@ -336,7 +346,8 @@ xchk_superblock(
 		xchk_block_set_corrupt(sc, bp);
 
 	xchk_superblock_xref(sc, bp);
-
+out_pag:
+	xfs_perag_put(pag);
 	return error;
 }
 
@@ -527,6 +538,7 @@ xchk_agf(
 	xchk_buffer_recheck(sc, sc->sa.agf_bp);
 
 	agf = sc->sa.agf_bp->b_addr;
+	pag = sc->sa.pag;
 
 	/* Check the AG length */
 	eoag = be32_to_cpu(agf->agf_length);
@@ -582,7 +594,6 @@ xchk_agf(
 		xchk_block_set_corrupt(sc, sc->sa.agf_bp);
 
 	/* Do the incore counters match? */
-	pag = xfs_perag_get(mp, agno);
 	if (pag->pagf_freeblks != be32_to_cpu(agf->agf_freeblks))
 		xchk_block_set_corrupt(sc, sc->sa.agf_bp);
 	if (pag->pagf_flcount != be32_to_cpu(agf->agf_flcount))
@@ -590,7 +601,6 @@ xchk_agf(
 	if (xfs_sb_version_haslazysbcount(&sc->mp->m_sb) &&
 	    pag->pagf_btreeblks != be32_to_cpu(agf->agf_btreeblks))
 		xchk_block_set_corrupt(sc, sc->sa.agf_bp);
-	xfs_perag_put(pag);
 
 	xchk_agf_xref(sc);
 out:
@@ -857,6 +867,7 @@ xchk_agi(
 	xchk_buffer_recheck(sc, sc->sa.agi_bp);
 
 	agi = sc->sa.agi_bp->b_addr;
+	pag = sc->sa.pag;
 
 	/* Check the AG length */
 	eoag = be32_to_cpu(agi->agi_length);
@@ -909,12 +920,10 @@ xchk_agi(
 		xchk_block_set_corrupt(sc, sc->sa.agi_bp);
 
 	/* Do the incore counters match? */
-	pag = xfs_perag_get(mp, agno);
 	if (pag->pagi_count != be32_to_cpu(agi->agi_count))
 		xchk_block_set_corrupt(sc, sc->sa.agi_bp);
 	if (pag->pagi_freecount != be32_to_cpu(agi->agi_freecount))
 		xchk_block_set_corrupt(sc, sc->sa.agi_bp);
-	xfs_perag_put(pag);
 
 	xchk_agi_xref(sc);
 out:

fs/xfs/scrub/agheader_repair.c

@@ -366,7 +366,6 @@ xrep_agf(
 	if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
 		return -EOPNOTSUPP;
 
-	xchk_perag_get(sc->mp, &sc->sa);
 	/*
 	 * Make sure we have the AGF buffer, as scrub might have decided it
 	 * was corrupt after xfs_alloc_read_agf failed with -EFSCORRUPTED.
@@ -641,7 +640,6 @@ xrep_agfl(
 	if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
 		return -EOPNOTSUPP;
 
-	xchk_perag_get(sc->mp, &sc->sa);
 	xbitmap_init(&agfl_extents);
 
 	/*
@@ -896,7 +894,6 @@ xrep_agi(
 	if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
 		return -EOPNOTSUPP;
 
-	xchk_perag_get(sc->mp, &sc->sa);
 	/*
 	 * Make sure we have the AGI buffer, as scrub might have decided it
 	 * was corrupt after xfs_ialloc_read_agi failed with -EFSCORRUPTED.

fs/xfs/scrub/bmap.c

@@ -260,7 +260,7 @@ xchk_bmap_iextent_xref(
 	agbno = XFS_FSB_TO_AGBNO(mp, irec->br_startblock);
 	len = irec->br_blockcount;
 
-	error = xchk_ag_init(info->sc, agno, &info->sc->sa);
+	error = xchk_ag_init_existing(info->sc, agno, &info->sc->sa);
 	if (!xchk_fblock_process_error(info->sc, info->whichfork,
 			irec->br_startoff, &error))
 		return;

fs/xfs/scrub/btree.c

@@ -374,7 +374,7 @@ xchk_btree_check_block_owner(
 
 	init_sa = bs->cur->bc_flags & XFS_BTREE_LONG_PTRS;
 	if (init_sa) {
-		error = xchk_ag_init(bs->sc, agno, &bs->sc->sa);
+		error = xchk_ag_init_existing(bs->sc, agno, &bs->sc->sa);
 		if (!xchk_btree_xref_process_error(bs->sc, bs->cur,
 				level, &error))
 			return error;

fs/xfs/scrub/common.c

@@ -394,11 +394,11 @@ want_ag_read_header_failure(
 }
 
 /*
- * Grab all the headers for an AG.
+ * Grab the perag structure and all the headers for an AG.
  *
- * The headers should be released by xchk_ag_free, but as a fail
- * safe we attach all the buffers we grab to the scrub transaction so
- * they'll all be freed when we cancel it.
+ * The headers should be released by xchk_ag_free, but as a fail safe we attach
+ * all the buffers we grab to the scrub transaction so they'll all be freed
+ * when we cancel it.  Returns ENOENT if we can't grab the perag structure.
  */
 int
 xchk_ag_read_headers(
@@ -409,22 +409,26 @@ xchk_ag_read_headers(
 	struct xfs_mount	*mp = sc->mp;
 	int			error;
 
+	ASSERT(!sa->pag);
+	sa->pag = xfs_perag_get(mp, agno);
+	if (!sa->pag)
+		return -ENOENT;
+
 	sa->agno = agno;
 
 	error = xfs_ialloc_read_agi(mp, sc->tp, agno, &sa->agi_bp);
 	if (error && want_ag_read_header_failure(sc, XFS_SCRUB_TYPE_AGI))
-		goto out;
+		return error;
 
 	error = xfs_alloc_read_agf(mp, sc->tp, agno, 0, &sa->agf_bp);
 	if (error && want_ag_read_header_failure(sc, XFS_SCRUB_TYPE_AGF))
-		goto out;
+		return error;
 
 	error = xfs_alloc_read_agfl(mp, sc->tp, agno, &sa->agfl_bp);
 	if (error && want_ag_read_header_failure(sc, XFS_SCRUB_TYPE_AGFL))
-		goto out;
-	error = 0;
-out:
-	return error;
+		return error;
+
+	return 0;
 }
 
 /* Release all the AG btree cursors. */
@@ -461,7 +465,6 @@ xchk_ag_btcur_init(
 {
 	struct xfs_mount	*mp = sc->mp;
 
-	xchk_perag_get(sc->mp, sa);
 	if (sa->agf_bp &&
 	    xchk_ag_btree_healthy_enough(sc, sa->pag, XFS_BTNUM_BNO)) {
 		/* Set up a bnobt cursor for cross-referencing. */
@@ -532,11 +535,11 @@ xchk_ag_free(
 }
 
 /*
- * For scrub, grab the AGI and the AGF headers, in that order.  Locking
- * order requires us to get the AGI before the AGF.  We use the
- * transaction to avoid deadlocking on crosslinked metadata buffers;
- * either the caller passes one in (bmap scrub) or we have to create a
- * transaction ourselves.
+ * For scrub, grab the perag structure, the AGI, and the AGF headers, in that
+ * order.  Locking order requires us to get the AGI before the AGF.  We use the
+ * transaction to avoid deadlocking on crosslinked metadata buffers; either the
+ * caller passes one in (bmap scrub) or we have to create a transaction
+ * ourselves.  Returns ENOENT if the perag struct cannot be grabbed.
  */
 int
 xchk_ag_init(
@@ -554,19 +557,6 @@ xchk_ag_init(
 	return 0;
 }
 
-/*
- * Grab the per-ag structure if we haven't already gotten it.  Teardown of the
- * xchk_ag will release it for us.
- */
-void
-xchk_perag_get(
-	struct xfs_mount	*mp,
-	struct xchk_ag		*sa)
-{
-	if (!sa->pag)
-		sa->pag = xfs_perag_get(mp, sa->agno);
-}
-
 /* Per-scrubber setup functions */
 
 /*

fs/xfs/scrub/common.h

@@ -107,7 +107,23 @@ int xchk_setup_fscounters(struct xfs_scrub *sc);
 void xchk_ag_free(struct xfs_scrub *sc, struct xchk_ag *sa);
 int xchk_ag_init(struct xfs_scrub *sc, xfs_agnumber_t agno,
 		struct xchk_ag *sa);
-void xchk_perag_get(struct xfs_mount *mp, struct xchk_ag *sa);
+
+/*
+ * Grab all AG resources, treating the inability to grab the perag structure as
+ * a fs corruption.  This is intended for callers checking an ondisk reference
+ * to a given AG, which means that the AG must still exist.
+ */
+static inline int
+xchk_ag_init_existing(
+	struct xfs_scrub	*sc,
+	xfs_agnumber_t		agno,
+	struct xchk_ag		*sa)
+{
+	int			error = xchk_ag_init(sc, agno, sa);
+
+	return error == -ENOENT ? -EFSCORRUPTED : error;
+}
+
 int xchk_ag_read_headers(struct xfs_scrub *sc, xfs_agnumber_t agno,
 		struct xchk_ag *sa);
 void xchk_ag_btcur_free(struct xchk_ag *sa);

fs/xfs/scrub/fscounters.c

@@ -148,7 +148,7 @@ xchk_fscount_btreeblks(
 	xfs_extlen_t		blocks;
 	int			error;
 
-	error = xchk_ag_init(sc, agno, &sc->sa);
+	error = xchk_ag_init_existing(sc, agno, &sc->sa);
 	if (error)
 		return error;
 

fs/xfs/scrub/inode.c

@@ -532,7 +532,7 @@ xchk_inode_xref(
 	agno = XFS_INO_TO_AGNO(sc->mp, ino);
 	agbno = XFS_INO_TO_AGBNO(sc->mp, ino);
 
-	error = xchk_ag_init(sc, agno, &sc->sa);
+	error = xchk_ag_init_existing(sc, agno, &sc->sa);
 	if (!xchk_xref_process_error(sc, agno, agbno, &error))
 		return;