xfs: allow the user to cancel repairs before we start writing

All online repair functions have the same structure: walk filesystem
metadata structures gathering enough data to rebuild the structure,
stage a new copy, and then commit the new copy.

The gathering steps do not write anything to disk, so they are peppered
with xchk_should_terminate calls to avoid softlockup warnings and to
provide an opportunity to abort the repair (by killing xfs_scrub).
However, it's not clear in the code base when is the last chance to
abort cleanly without having to undo a bunch of structure.

Therefore, add one more call to xchk_should_terminate (along with a
comment) providing the sysadmin with the ability to abort before it's
too late and to make it clear in the source code when it's no longer
convenient or safe to abort a repair.   As there are only four repair
functions right now, this patch exists more to establish a precedent for
subsequent additions than to deliver practical functionality.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

Author: Darrick J. Wong

fs/xfs/scrub/agheader_repair.c

@@ -49,6 +49,10 @@ xrep_superblock(
 	if (error)
 		return error;
 
+	/* Last chance to abort before we start committing fixes. */
+	if (xchk_should_terminate(sc, &error))
+		return error;
+
 	/* Copy AG 0's superblock to this one. */
 	xfs_buf_zero(bp, 0, BBTOB(bp->b_length));
 	xfs_sb_to_disk(bp->b_addr, &mp->m_sb);
@@ -424,6 +428,10 @@ xrep_agf(
 	if (error)
 		return error;
 
+	/* Last chance to abort before we start committing fixes. */
+	if (xchk_should_terminate(sc, &error))
+		return error;
+
 	/* Start rewriting the header and implant the btrees we found. */
 	xrep_agf_init_header(sc, agf_bp, &old_agf);
 	xrep_agf_set_roots(sc, agf, fab);
@@ -748,6 +756,10 @@ xrep_agfl(
 	if (error)
 		goto err;
 
+	/* Last chance to abort before we start committing fixes. */
+	if (xchk_should_terminate(sc, &error))
+		goto err;
+
 	/*
 	 * Update AGF and AGFL.  We reset the global free block counter when
 	 * we adjust the AGF flcount (which can fail) so avoid updating any
@@ -995,6 +1007,10 @@ xrep_agi(
 	if (error)
 		return error;
 
+	/* Last chance to abort before we start committing fixes. */
+	if (xchk_should_terminate(sc, &error))
+		return error;
+
 	/* Start rewriting the header and implant the btrees we found. */
 	xrep_agi_init_header(sc, agi_bp, &old_agi);
 	xrep_agi_set_roots(sc, agi, fab);