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Subject[PATCH 3.16.y-ckt 006/144] dm snapshot: suspend merging snapshot when doing exception handover
Date
3.16.7-ckt10 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mikulas Patocka <mpatocka@redhat.com>

commit 09ee96b21456883e108c3b00597bb37ec512151b upstream.

The "dm snapshot: suspend origin when doing exception handover" commit
fixed a exception store handover bug associated with pending exceptions
to the "snapshot-origin" target.

However, a similar problem exists in snapshot merging. When snapshot
merging is in progress, we use the target "snapshot-merge" instead of
"snapshot-origin". Consequently, during exception store handover, we
must find the snapshot-merge target and suspend its associated
mapped_device.

To avoid lockdep warnings, the target must be suspended and resumed
without holding _origins_lock.

Introduce a dm_hold() function that grabs a reference on a
mapped_device, but unlike dm_get(), it doesn't crash if the device has
the DMF_FREEING flag set, it returns an error in this case.

In snapshot_resume() we grab the reference to the origin device using
dm_hold() while holding _origins_lock (_origins_lock guarantees that the
device won't disappear). Then we release _origins_lock, suspend the
device and grab _origins_lock again.

NOTE to stable@ people:
When backporting to kernels 3.18 and older, use dm_internal_suspend and
dm_internal_resume instead of dm_internal_suspend_fast and
dm_internal_resume_fast.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
[ luis: backported to 3.16: as suggested by the author:
- replaced dm_internal_suspend_fast by dm_internal_suspend
- replaced dm_internal_resume_fast by dm_internal_resume ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
drivers/md/dm-snap.c | 35 +++++++++++++++++++++++++++++------
drivers/md/dm.c | 13 +++++++++++++
include/linux/device-mapper.h | 1 +
3 files changed, 43 insertions(+), 6 deletions(-)

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 1adf4bb21797..3363fa199a19 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1896,20 +1896,39 @@ static int snapshot_preresume(struct dm_target *ti)
static void snapshot_resume(struct dm_target *ti)
{
struct dm_snapshot *s = ti->private;
- struct dm_snapshot *snap_src = NULL, *snap_dest = NULL;
+ struct dm_snapshot *snap_src = NULL, *snap_dest = NULL, *snap_merging = NULL;
struct dm_origin *o;
struct mapped_device *origin_md = NULL;
+ bool must_restart_merging = false;

down_read(&_origins_lock);

o = __lookup_dm_origin(s->origin->bdev);
if (o)
origin_md = dm_table_get_md(o->ti->table);
+ if (!origin_md) {
+ (void) __find_snapshots_sharing_cow(s, NULL, NULL, &snap_merging);
+ if (snap_merging)
+ origin_md = dm_table_get_md(snap_merging->ti->table);
+ }
if (origin_md == dm_table_get_md(ti->table))
origin_md = NULL;
+ if (origin_md) {
+ if (dm_hold(origin_md))
+ origin_md = NULL;
+ }

- if (origin_md)
+ up_read(&_origins_lock);
+
+ if (origin_md) {
dm_internal_suspend(origin_md);
+ if (snap_merging && test_bit(RUNNING_MERGE, &snap_merging->state_bits)) {
+ must_restart_merging = true;
+ stop_merge(snap_merging);
+ }
+ }
+
+ down_read(&_origins_lock);

(void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
if (snap_src && snap_dest) {
@@ -1920,11 +1939,15 @@ static void snapshot_resume(struct dm_target *ti)
up_write(&snap_src->lock);
}

- if (origin_md)
- dm_internal_resume(origin_md);
-
up_read(&_origins_lock);

+ if (origin_md) {
+ if (must_restart_merging)
+ start_merge(snap_merging);
+ dm_internal_resume(origin_md);
+ dm_put(origin_md);
+ }
+
/* Now we have correct chunk size, reregister */
reregister_snapshot(s);

@@ -2368,7 +2391,7 @@ static struct target_type snapshot_target = {

static struct target_type merge_target = {
.name = dm_snapshot_merge_target_name,
- .version = {1, 2, 0},
+ .version = {1, 3, 0},
.module = THIS_MODULE,
.ctr = snapshot_ctr,
.dtr = snapshot_dtr,
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 87791fc992ac..b3fdf9cfbeb9 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2386,6 +2386,19 @@ void dm_get(struct mapped_device *md)
BUG_ON(test_bit(DMF_FREEING, &md->flags));
}

+int dm_hold(struct mapped_device *md)
+{
+ spin_lock(&_minor_lock);
+ if (test_bit(DMF_FREEING, &md->flags)) {
+ spin_unlock(&_minor_lock);
+ return -EBUSY;
+ }
+ dm_get(md);
+ spin_unlock(&_minor_lock);
+ return 0;
+}
+EXPORT_SYMBOL_GPL(dm_hold);
+
const char *dm_device_name(struct mapped_device *md)
{
return md->name;
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index e1707de043ae..c51706b7a36e 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -366,6 +366,7 @@ int dm_create(int minor, struct mapped_device **md);
*/
struct mapped_device *dm_get_md(dev_t dev);
void dm_get(struct mapped_device *md);
+int dm_hold(struct mapped_device *md);
void dm_put(struct mapped_device *md);

/*

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