[PATCH] md: fix two problems with setting the "re-add" device state.

From: NeilBrown
Date: Thu Apr 26 2018 - 00:46:50 EST

If "re-add" is written to the "state" file for a device
which is faulty, this has an effect similar to removing
and re-adding the device. It should take up the
same slot in the array that it previously had, and
an accelerated (e.g. bitmap-based) rebuild should happen.

The slot that "it previously had" is determined by
However this is not set when a device fails (only when a device
is added), and it is cleared when resync completes.
This means that "re-add" will normally work once, but may not work a
second time.

This patch includes two fixes.
1/ when a device fails, record the ->raid_disk value in
->saved_raid_disk before clearing ->raid_disk
2/ when "re-add" is written to a device for which
->saved_raid_disk is not set, fail.

I think this is suitable for stable as it can
cause re-adding a device to be forced to do a full
resync which takes a lot longer and so puts data at
more risk.

Cc: <stable@xxxxxxxxxxxxxxx> (v4.1)
Fixes: 97f6cd39da22 ("md-cluster: re-add capabilities")
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
drivers/md/md.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 3bea45e8ccff..ecd4235c6e30 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2853,7 +2853,8 @@ state_store(struct md_rdev *rdev, const char *buf, size_t len)
err = 0;
} else if (cmd_match(buf, "re-add")) {
- if (test_bit(Faulty, &rdev->flags) && (rdev->raid_disk == -1)) {
+ if (test_bit(Faulty, &rdev->flags) && (rdev->raid_disk == -1) &&
+ rdev->saved_raid_disk >= 0) {
/* clear_bit is performed _after_ all the devices
* have their local Faulty bit cleared. If any writes
* happen in the meantime in the local node, they
@@ -8641,6 +8642,7 @@ static int remove_and_add_spares(struct mddev *mddev,
if (mddev->pers->hot_remove_disk(
mddev, rdev) == 0) {
sysfs_unlink_rdev(mddev, rdev);
+ rdev->saved_raid_disk = rdev->raid_disk;
rdev->raid_disk = -1;

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