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Commit 19fa1a67 authored by Joe Thornber's avatar Joe Thornber Committed by Mike Snitzer
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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: default avatarJoe Thornber <ejt@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
parent 16961b04

drivers/md/dm-thin-metadata.c

+20 −0
Original line number Diff line number Diff line
@@ -1349,6 +1349,12 @@ dm_thin_id dm_thin_dev_id(struct dm_thin_device *td) 
	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 @@ int dm_thin_remove_block(struct dm_thin_device *td, dm_block_t block) 
	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;

drivers/md/dm-thin-metadata.h

+2 −0
Original line number Diff line number Diff line
@@ -181,6 +181,8 @@ int dm_pool_get_data_block_size(struct dm_pool_metadata *pmd, sector_t *result); 


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.

drivers/md/dm-thin.c

+12 −2
Original line number Diff line number Diff line
@@ -512,6 +512,7 @@ struct dm_thin_new_mapping { 
	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 @@ static void process_prepared_discard_passdown(struct dm_thin_new_mapping *m) 
	cell_defer_no_holder(tc, m->cell2);



	if (m->pass_discard)

		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 @@ static void process_discard(struct thin_c *tc, struct bio *bio) 
			 */

			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;