[PATCH RFC] btrfs-progs: scrub: Correct tracking of last_physical across scrub cancel/resume

["Graham R. Cobb" <g.btrfs@cobb.uk.net>]

When a scrub completes or is cancelled, statistics are updated for reporting
in a later btrfs scrub status command. Most statistics (such as bytes scrubbed)
are additive so scrub adds the statistics from the current run to the
saved statistics.

However, the last_physical statistic is not additive. The value from the
current run should replace the saved value. The current code incorrectly
adds the last_physical from the current run to the saved value.

This bug not only affects user status reporting but also has the effect that
subsequent resumes start from the wrong place and large amounts of the
filesystem are not scrubbed.

This patch changes the saved last_physical to track the last reported value
from the kernel.

Signed-off-by: Graham R. Cobb <g.btrfs@cobb.uk.net>
---
 cmds-scrub.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/cmds-scrub.c b/cmds-scrub.c
index f21d2d89..2800e796 100644
--- a/cmds-scrub.c
+++ b/cmds-scrub.c
@@ -171,6 +171,10 @@ static void print_scrub_summary(struct btrfs_scrub_progress *p)
 	fs_stat->p.name += p->name;		\
 } while (0)
 
+#define _SCRUB_FS_STAT_COPY(p, name, fs_stat) do {	\
+	fs_stat->p.name = p->name;		\
+} while (0)
+
 #define _SCRUB_FS_STAT_MIN(ss, name, fs_stat)	\
 do {						\
 	if (fs_stat->s.name > ss->name) {	\
@@ -209,7 +213,7 @@ static void add_to_fs_stat(struct btrfs_scrub_progress *p,
 	_SCRUB_FS_STAT(p, malloc_errors, fs_stat);
 	_SCRUB_FS_STAT(p, uncorrectable_errors, fs_stat);
 	_SCRUB_FS_STAT(p, corrected_errors, fs_stat);
-	_SCRUB_FS_STAT(p, last_physical, fs_stat);
+	_SCRUB_FS_STAT_COPY(p, last_physical, fs_stat);
 	_SCRUB_FS_STAT_ZMIN(ss, t_start, fs_stat);
 	_SCRUB_FS_STAT_ZMIN(ss, t_resumed, fs_stat);
 	_SCRUB_FS_STAT_ZMAX(ss, duration, fs_stat);
@@ -683,6 +687,8 @@ static int scrub_writev(int fd, char *buf, int max, const char *fmt, ...)
 
 #define _SCRUB_SUM(dest, data, name) dest->scrub_args.progress.name =	\
 			data->resumed->p.name + data->scrub_args.progress.name
+#define _SCRUB_COPY(dest, data, name) dest->scrub_args.progress.name =	\
+			data->scrub_args.progress.name
 
 static struct scrub_progress *scrub_resumed_stats(struct scrub_progress *data,
 						  struct scrub_progress *dest)
@@ -703,7 +709,7 @@ static struct scrub_progress *scrub_resumed_stats(struct scrub_progress *data,
 	_SCRUB_SUM(dest, data, malloc_errors);
 	_SCRUB_SUM(dest, data, uncorrectable_errors);
 	_SCRUB_SUM(dest, data, corrected_errors);
-	_SCRUB_SUM(dest, data, last_physical);
+	_SCRUB_COPY(dest, data, last_physical);
 	dest->stats.canceled = data->stats.canceled;
 	dest->stats.finished = data->stats.finished;
 	dest->stats.t_resumed = data->stats.t_start;
-- 
2.20.1