[PATCH] Fix reshape for decreasing data offset

[bugfood-ml@xxxxxxxxxx]

From: Corey Hickey <bugfood-c@xxxxxxxxxx>

...when not changing the number of disks.

This patch needs context to explain. These are the relevant parts of
the original code (condensed and annotated):

if (dir > 0) {
    /* Increase data offset (reshape backwards) */
    if (data_offset < sd->data_offset + min) {
        pr_err("--data-offset too small on %s\n",
               dn);
        goto release;
    }
} else {
    /* Decrease data offset (reshape forwards) */
    if (data_offset < sd->data_offset - min) {
        pr_err("--data-offset too small on %s\n",
               dn);
        goto release;
    }
}

When this code is reached, mdadm has already decided on a reshape
direction. When increasing the data offset, the reshape runs backwards
(dir==1); when decreasing the data offset, the reshape runs forwards
(dir==-1).

The conditional within the backwards reshape is correct: the requested
offset must be larger than the old offset plus a minimum delta; thus the
reshape has room to work.

For the forwards reshape, the requested offset needs to be smaller than
the old offset minus a minimum delta; to do this correctly, the
comparison must be reversed.

Also update the error message.

Note: I have tested this change on a RAID 5 on Linux 4.18.0 and verified
that there were no errors from the kernel and that the device data
remained intact. I do not know if there are considerations for different
RAID levels.

Signed-off-by: Corey Hickey <bugfood-c@xxxxxxxxxx>
---
 Grow.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Grow.c b/Grow.c
index 6d32661..764374f 100644
--- a/Grow.c
+++ b/Grow.c
@@ -2613,8 +2613,8 @@ static int set_new_data_offset(struct mdinfo *sra, struct supertype *st,
 					goto release;
 				}
 				if (data_offset != INVALID_SECTORS &&
-				    data_offset < sd->data_offset - min) {
-					pr_err("--data-offset too small on %s\n",
+				    data_offset > sd->data_offset - min) {
+					pr_err("--data-offset too large on %s\n",
 						dn);
 					goto release;
 				}
-- 
2.20.1