dm thin: fix discard support to a previously shared block
If a snapshot is created and later deleted the origin dm_thin_device's
snapshotted_time will have been updated to reflect the snapshot's
creation time. The 'shared' flag in the dm_thin_lookup_result struct
returned from dm_thin_find_block() is an approximation based on
snapshotted_time -- this is done to avoid 0(n), or worse, time
complexity. In this case, the shared flag would be true.
But because the 'shared' flag reflects an approximation a block can be
incorrectly assumed to be shared (e.g. false positive for 'shared'
because the snapshot no longer exists). This could result in discards
issued to a thin device not being passed down to the pool's underlying
data device.
To fix this we double check that a thin block is really still in-use
after a mapping is removed using dm_pool_block_is_used(). If the
reference count for a block is now zero the discard is allowed to be
passed down.
Also add a 'definitely_not_shared' member to the dm_thin_new_mapping
structure -- reflects that the 'shared' flag in the response from
dm_thin_find_block() can only be held as definitive if false is
returned.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1043527
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
diff --git a/drivers/md/dm-thin-metadata.c b/drivers/md/dm-thin-metadata.c
index 8a30ad5..7da3476 100644
--- a/drivers/md/dm-thin-metadata.c
+++ b/drivers/md/dm-thin-metadata.c
@@ -1349,6 +1349,12 @@
return td->id;
}
+/*
+ * Check whether @time (of block creation) is older than @td's last snapshot.
+ * If so then the associated block is shared with the last snapshot device.
+ * Any block on a device created *after* the device last got snapshotted is
+ * necessarily not shared.
+ */
static bool __snapshotted_since(struct dm_thin_device *td, uint32_t time)
{
return td->snapshotted_time > time;
@@ -1458,6 +1464,20 @@
return r;
}
+int dm_pool_block_is_used(struct dm_pool_metadata *pmd, dm_block_t b, bool *result)
+{
+ int r;
+ uint32_t ref_count;
+
+ down_read(&pmd->root_lock);
+ r = dm_sm_get_count(pmd->data_sm, b, &ref_count);
+ if (!r)
+ *result = (ref_count != 0);
+ up_read(&pmd->root_lock);
+
+ return r;
+}
+
bool dm_thin_changed_this_transaction(struct dm_thin_device *td)
{
int r;
diff --git a/drivers/md/dm-thin-metadata.h b/drivers/md/dm-thin-metadata.h
index 7bcc0e1..2edf5db 100644
--- a/drivers/md/dm-thin-metadata.h
+++ b/drivers/md/dm-thin-metadata.h
@@ -181,6 +181,8 @@
int dm_pool_get_data_dev_size(struct dm_pool_metadata *pmd, dm_block_t *result);
+int dm_pool_block_is_used(struct dm_pool_metadata *pmd, dm_block_t b, bool *result);
+
/*
* Returns -ENOSPC if the new size is too small and already allocated
* blocks would be lost.
diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index da65feb..51e656a 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -512,6 +512,7 @@
unsigned quiesced:1;
unsigned prepared:1;
unsigned pass_discard:1;
+ unsigned definitely_not_shared:1;
struct thin_c *tc;
dm_block_t virt_block;
@@ -683,7 +684,15 @@
cell_defer_no_holder(tc, m->cell2);
if (m->pass_discard)
- remap_and_issue(tc, m->bio, m->data_block);
+ if (m->definitely_not_shared)
+ remap_and_issue(tc, m->bio, m->data_block);
+ else {
+ bool used = false;
+ if (dm_pool_block_is_used(tc->pool->pmd, m->data_block, &used) || used)
+ bio_endio(m->bio, 0);
+ else
+ remap_and_issue(tc, m->bio, m->data_block);
+ }
else
bio_endio(m->bio, 0);
@@ -1036,7 +1045,8 @@
*/
m = get_next_mapping(pool);
m->tc = tc;
- m->pass_discard = (!lookup_result.shared) && pool->pf.discard_passdown;
+ m->pass_discard = pool->pf.discard_passdown;
+ m->definitely_not_shared = !lookup_result.shared;
m->virt_block = block;
m->data_block = lookup_result.block;
m->cell = cell;