xfs: probe the scrub ioctl

Create a probe scrubber with id 0.  This will be used by xfs_scrub to
probe the kernel's abilities to scrub (and repair) the metadata.  We do
this by validating the ioctl inputs from userspace, preparing the
filesystem for a scrub (or a repair) operation, and immediately
returning to userspace.  Userspace can use the returned errno and
structure state to decide (in broad terms) if scrub/repair are
supported by the running kernel.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/Makefile

@@ -143,6 +143,7 @@ ifeq ($(CONFIG_XFS_ONLINE_SCRUB),y)
 
 xfs-y				+= $(addprefix scrub/, \
 				   trace.o \
+				   common.o \
 				   scrub.o \
 				   )
 endif

fs/xfs/libxfs/xfs_fs.h

@@ -483,9 +483,10 @@ struct xfs_scrub_metadata {
  */
 
 /* Scrub subcommands. */
+#define XFS_SCRUB_TYPE_PROBE	0	/* presence test ioctl */
 
 /* Number of scrub subcommands. */
-#define XFS_SCRUB_TYPE_NR	0
+#define XFS_SCRUB_TYPE_NR	1
 
 /* i: Repair this metadata. */
 #define XFS_SCRUB_IFLAG_REPAIR		(1 << 0)

fs/xfs/scrub/common.c

+/*
+ * Copyright (C) 2017 Oracle.  All Rights Reserved.
+ *
+ * Author: Darrick J. Wong <darrick.wong@oracle.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it would be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write the Free Software Foundation,
+ * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+#include "xfs.h"
+#include "xfs_fs.h"
+#include "xfs_shared.h"
+#include "xfs_format.h"
+#include "xfs_trans_resv.h"
+#include "xfs_mount.h"
+#include "xfs_defer.h"
+#include "xfs_btree.h"
+#include "xfs_bit.h"
+#include "xfs_log_format.h"
+#include "xfs_trans.h"
+#include "xfs_sb.h"
+#include "xfs_inode.h"
+#include "xfs_alloc.h"
+#include "xfs_alloc_btree.h"
+#include "xfs_bmap.h"
+#include "xfs_bmap_btree.h"
+#include "xfs_ialloc.h"
+#include "xfs_ialloc_btree.h"
+#include "xfs_refcount.h"
+#include "xfs_refcount_btree.h"
+#include "xfs_rmap.h"
+#include "xfs_rmap_btree.h"
+#include "scrub/xfs_scrub.h"
+#include "scrub/scrub.h"
+#include "scrub/common.h"
+#include "scrub/trace.h"
+
+/* Common code for the metadata scrubbers. */
+
+/* Per-scrubber setup functions */
+
+/* Set us up with a transaction and an empty context. */
+int
+xfs_scrub_setup_fs(
+	struct xfs_scrub_context	*sc,
+	struct xfs_inode		*ip)
+{
+	return xfs_scrub_trans_alloc(sc->sm, sc->mp, &sc->tp);
+}

fs/xfs/scrub/common.h

+/*
+ * Copyright (C) 2017 Oracle.  All Rights Reserved.
+ *
+ * Author: Darrick J. Wong <darrick.wong@oracle.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it would be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write the Free Software Foundation,
+ * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+#ifndef __XFS_SCRUB_COMMON_H__
+#define __XFS_SCRUB_COMMON_H__
+
+/*
+ * We /could/ terminate a scrub/repair operation early.  If we're not
+ * in a good place to continue (fatal signal, etc.) then bail out.
+ * Note that we're careful not to make any judgements about *error.
+ */
+static inline bool
+xfs_scrub_should_terminate(
+	struct xfs_scrub_context	*sc,
+	int				*error)
+{
+	if (fatal_signal_pending(current)) {
+		if (*error == 0)
+			*error = -EAGAIN;
+		return true;
+	}
+	return false;
+}
+
+/*
+ * Grab an empty transaction so that we can re-grab locked buffers if
+ * one of our btrees turns out to be cyclic.
+ */
+static inline int
+xfs_scrub_trans_alloc(
+	struct xfs_scrub_metadata	*sm,
+	struct xfs_mount		*mp,
+	struct xfs_trans		**tpp)
+{
+	return xfs_trans_alloc_empty(mp, tpp);
+}
+
+/* Setup functions */
+int xfs_scrub_setup_fs(struct xfs_scrub_context *sc, struct xfs_inode *ip);
+
+#endif	/* __XFS_SCRUB_COMMON_H__ */

fs/xfs/scrub/scrub.c

@@ -42,6 +42,7 @@
 #include "xfs_rmap_btree.h"
 #include "scrub/xfs_scrub.h"
 #include "scrub/scrub.h"
+#include "scrub/common.h"
 #include "scrub/trace.h"
 
 /*
@@ -108,6 +109,30 @@
  * will be set.
  */
 
+/*
+ * Scrub probe -- userspace uses this to probe if we're willing to scrub
+ * or repair a given mountpoint.  This will be used by xfs_scrub to
+ * probe the kernel's abilities to scrub (and repair) the metadata.  We
+ * do this by validating the ioctl inputs from userspace, preparing the
+ * filesystem for a scrub (or a repair) operation, and immediately
+ * returning to userspace.  Userspace can use the returned errno and
+ * structure state to decide (in broad terms) if scrub/repair are
+ * supported by the running kernel.
+ */
+int
+xfs_scrub_probe(
+	struct xfs_scrub_context	*sc)
+{
+	int				error = 0;
+
+	if (sc->sm->sm_ino || sc->sm->sm_agno)
+		return -EINVAL;
+	if (xfs_scrub_should_terminate(sc, &error))
+		return error;
+
+	return 0;
+}
+
 /* Scrub setup and teardown */
 
 /* Free all the resources and finish the transactions. */
@@ -126,6 +151,10 @@ xfs_scrub_teardown(
 /* Scrubbing dispatch. */
 
 static const struct xfs_scrub_meta_ops meta_scrub_ops[] = {
+	{ /* ioctl presence test */
+		.setup	= xfs_scrub_setup_fs,
+		.scrub	= xfs_scrub_probe,
+	},
 };
 
 /* This isn't a stable feature, warn once per day. */

fs/xfs/scrub/scrub.h

@@ -45,5 +45,6 @@ struct xfs_scrub_context {
 };
 
 /* Metadata scrubbers */
+int xfs_scrub_tester(struct xfs_scrub_context *sc);
 
 #endif	/* __XFS_SCRUB_SCRUB_H__ */

fs/xfs/scrub/trace.c

@@ -32,6 +32,7 @@
 #include "xfs_trans.h"
 #include "scrub/xfs_scrub.h"
 #include "scrub/scrub.h"
+#include "scrub/common.h"
 
 /*
  * We include this last to have the helpers above available for the trace