On Fri, Mar 23 2012 at 8:37am -0400,
Alasdair G Kergon <agk@redhat.com> wrote:

> On Fri, Mar 16, 2012 at 03:22:34PM +0000, Joe Thornber wrote:
> > + 'ignore_discard': disable discard support
> > + 'no_discard_passdown': don't pass discards down to the underlying data device
>
> If someone reloads the pool target changing either of these options, how do connected
> thin targets pick up the change? If they don't pick it up automatically, how do you
> determine whether they did or didn't pick it up - is that internal state not
> exposed to userspace yet?

Here are various fixes for dm_thin-add-pool-target-flags-to-control-discard.patch

o wire up 'discard_passdown' so that it does control whether or not the
discard is passed to the pool's underlying data device
o disallow disabling discards if a pool was already configured with
discards enabled (the default)
- justification is inlined in the code
- required pool_ctr knowing whether pool was created or not; so added
'created' flag to __pool_find()
o if 'discard_passdown' is enabled (the default) verify that the pool's
data device supports discards; if not warn and disable discard_passdown
o the pool device's discard support should _not_ be limited by whether
'discard_passdown' is enabled or not.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
drivers/md/dm-thin.c | 41 ++++++++++++++++++++++++++++++++++-------
1 file changed, 34 insertions(+), 7 deletions(-)

Index: linux/drivers/md/dm-thin.c
================================================== =================
--- linux.orig/drivers/md/dm-thin.c
+++ linux/drivers/md/dm-thin.c
@@ -1209,7 +1209,7 @@ static void process_discard(struct thin_
*/
m = get_next_mapping(pool);
m->tc = tc;
- m->pass_discard = !lookup_result.shared;
+ m->pass_discard = (!lookup_result.shared) & pool->pf.discard_passdown;
m->virt_block = block;
m->data_block = lookup_result.block;
m->cell = cell;
@@ -1790,7 +1790,8 @@ static void __pool_dec(struct pool *pool

static struct pool *__pool_find(struct mapped_device *pool_md,
struct block_device *metadata_dev,
- unsigned long block_size, char **error)
+ unsigned long block_size, char **error,
+ int *created)
{
struct pool *pool = __pool_table_lookup_metadata_dev(metadata_dev);

@@ -1806,8 +1807,10 @@ static struct pool *__pool_find(struct m
return ERR_PTR(-EINVAL);
__pool_inc(pool);

- } else
+ } else {
pool = pool_create(pool_md, metadata_dev, block_size, error);
+ *created = 1;
+ }
}

return pool;
@@ -1887,7 +1890,7 @@ static int parse_pool_features(struct dm
*/
static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
{
- int r;
+ int r, pool_created = 0;
struct pool_c *pt;
struct pool *pool;
struct pool_features pf;
@@ -1961,12 +1964,36 @@ static int pool_ctr(struct dm_target *ti
}

pool = __pool_find(dm_table_get_md(ti->table), metadata_dev->bdev,
- block_size, &ti->error);
+ block_size, &ti->error, &pool_created);
if (IS_ERR(pool)) {
r = PTR_ERR(pool);
goto out_free_pt;
}

+ /*
+ * 'pool_created' reflects whether this is the first table load.
+ * Discard support is not allowed to be disabled after initial load.
+ * Disabling it would require a pool reload to trigger thin device
+ * changes (e.g. ti->discards_supported and QUEUE_FLAG_DISCARD).
+ */
+ if (!pool_created && !pf.discard_enabled && pool->pf.discard_enabled) {
+ ti->error = "Discard support cannot be disabled once enabled";
+ r = -EINVAL;
+ goto out_free_pt;
+ }
+
+ /*
+ * If discard_passdown was enabled verify that the data device
+ * supports discards. Disable discard_passdown if not.
+ */
+ if (pf.discard_passdown) {
+ struct request_queue *q = bdev_get_queue(data_dev->bdev);
+ if (!q || !blk_queue_discard(q)) {
+ DMWARN("Discard unsupported by data device, disabling discard passdown");
+ pf.discard_passdown = 0;
+ }
+ }
+
pt->pool = pool;
pt->ti = ti;
pt->metadata_dev = metadata_dev;
@@ -1974,9 +2001,9 @@ static int pool_ctr(struct dm_target *ti
pt->low_water_blocks = low_water_blocks;
pt->pf = pf;
ti->num_flush_requests = 1;
- if (pf.discard_enabled && pf.discard_passdown) {
- ti->discards_supported = 1;
+ if (pf.discard_enabled) {
ti->num_discard_requests = 1;
+ ti->discards_supported = 1;
}
ti->private = pt;


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