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dm cache: add support for discard passdown to the origin device
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DM cache now defaults to passing discards down to the origin device.
User may disable this using the "no_discard_passdown" feature when
creating the cache device.

If the cache's underlying origin device doesn't support discards then
passdown is disabled (with warning).  Similarly, if the underlying
origin device's max_discard_sectors is less than a cache block discard
passdown will be disabled (this is required because sizing of the cache
internal discard bitset depends on it).

Signed-off-by: Mike Snitzer <[email protected]>
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snitm committed Mar 5, 2019
1 parent f87e033 commit de7180f
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Showing 2 changed files with 103 additions and 26 deletions.
3 changes: 3 additions & 0 deletions Documentation/device-mapper/cache.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -206,6 +206,9 @@ Optional feature arguments are:
in a separate btree, which improves speed of shutting
down the cache.

no_discard_passdown : disable passing down discards from the cache
to the origin's data device.

A policy called 'default' is always registered. This is an alias for
the policy we currently think is giving best all round performance.

Expand Down
126 changes: 100 additions & 26 deletions drivers/md/dm-cache-target.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -353,6 +353,7 @@ struct cache_features {
enum cache_metadata_mode mode;
enum cache_io_mode io_mode;
unsigned metadata_version;
bool discard_passdown:1;
};

struct cache_stats {
Expand Down Expand Up @@ -1899,7 +1900,11 @@ static bool process_discard_bio(struct cache *cache, struct bio *bio)
b = to_dblock(from_dblock(b) + 1);
}

bio_endio(bio);
if (cache->features.discard_passdown) {
remap_to_origin(cache, bio);
generic_make_request(bio);
} else
bio_endio(bio);

return false;
}
Expand Down Expand Up @@ -2233,13 +2238,14 @@ static void init_features(struct cache_features *cf)
cf->mode = CM_WRITE;
cf->io_mode = CM_IO_WRITEBACK;
cf->metadata_version = 1;
cf->discard_passdown = true;
}

static int parse_features(struct cache_args *ca, struct dm_arg_set *as,
char **error)
{
static const struct dm_arg _args[] = {
{0, 2, "Invalid number of cache feature arguments"},
{0, 3, "Invalid number of cache feature arguments"},
};

int r, mode_ctr = 0;
Expand Down Expand Up @@ -2274,6 +2280,9 @@ static int parse_features(struct cache_args *ca, struct dm_arg_set *as,
else if (!strcasecmp(arg, "metadata2"))
cf->metadata_version = 2;

else if (!strcasecmp(arg, "no_discard_passdown"))
cf->discard_passdown = false;

else {
*error = "Unrecognised cache feature requested";
return -EINVAL;
Expand Down Expand Up @@ -3119,6 +3128,39 @@ static void cache_resume(struct dm_target *ti)
do_waker(&cache->waker.work);
}

static void emit_flags(struct cache *cache, char *result,
unsigned maxlen, ssize_t *sz_ptr)
{
ssize_t sz = *sz_ptr;
struct cache_features *cf = &cache->features;
unsigned count = (cf->metadata_version == 2) + !cf->discard_passdown + 1;

DMEMIT("%u ", count);

if (cf->metadata_version == 2)
DMEMIT("metadata2 ");

if (writethrough_mode(cache))
DMEMIT("writethrough ");

else if (passthrough_mode(cache))
DMEMIT("passthrough ");

else if (writeback_mode(cache))
DMEMIT("writeback ");

else {
DMEMIT("unknown ");
DMERR("%s: internal error: unknown io mode: %d",
cache_device_name(cache), (int) cf->io_mode);
}

if (!cf->discard_passdown)
DMEMIT("no_discard_passdown ");

*sz_ptr = sz;
}

/*
* Status format:
*
Expand Down Expand Up @@ -3185,25 +3227,7 @@ static void cache_status(struct dm_target *ti, status_type_t type,
(unsigned) atomic_read(&cache->stats.promotion),
(unsigned long) atomic_read(&cache->nr_dirty));

if (cache->features.metadata_version == 2)
DMEMIT("2 metadata2 ");
else
DMEMIT("1 ");

if (writethrough_mode(cache))
DMEMIT("writethrough ");

else if (passthrough_mode(cache))
DMEMIT("passthrough ");

else if (writeback_mode(cache))
DMEMIT("writeback ");

else {
DMERR("%s: internal error: unknown io mode: %d",
cache_device_name(cache), (int) cache->features.io_mode);
goto err;
}
emit_flags(cache, result, maxlen, &sz);

DMEMIT("2 migration_threshold %llu ", (unsigned long long) cache->migration_threshold);

Expand Down Expand Up @@ -3432,14 +3456,62 @@ static int cache_iterate_devices(struct dm_target *ti,
return r;
}

static bool origin_dev_supports_discard(struct block_device *origin_bdev)
{
struct request_queue *q = bdev_get_queue(origin_bdev);

return q && blk_queue_discard(q);
}

/*
* If discard_passdown was enabled verify that the origin device
* supports discards. Disable discard_passdown if not.
*/
static void disable_passdown_if_not_supported(struct cache *cache)
{
struct block_device *origin_bdev = cache->origin_dev->bdev;
struct queue_limits *origin_limits = &bdev_get_queue(origin_bdev)->limits;
const char *reason = NULL;
char buf[BDEVNAME_SIZE];

if (!cache->features.discard_passdown)
return;

if (!origin_dev_supports_discard(origin_bdev))
reason = "discard unsupported";

else if (origin_limits->max_discard_sectors < cache->sectors_per_block)
reason = "max discard sectors smaller than a block";

if (reason) {
DMWARN("Origin device (%s) %s: Disabling discard passdown.",
bdevname(origin_bdev, buf), reason);
cache->features.discard_passdown = false;
}
}

static void set_discard_limits(struct cache *cache, struct queue_limits *limits)
{
struct block_device *origin_bdev = cache->origin_dev->bdev;
struct queue_limits *origin_limits = &bdev_get_queue(origin_bdev)->limits;

if (!cache->features.discard_passdown) {
/* No passdown is done so setting own virtual limits */
limits->max_discard_sectors = min_t(sector_t, cache->discard_block_size * 1024,
cache->origin_sectors);
limits->discard_granularity = cache->discard_block_size << SECTOR_SHIFT;
return;
}

/*
* FIXME: these limits may be incompatible with the cache device
* cache_iterate_devices() is stacking both origin and fast device limits
* but discards aren't passed to fast device, so inherit origin's limits.
*/
limits->max_discard_sectors = min_t(sector_t, cache->discard_block_size * 1024,
cache->origin_sectors);
limits->discard_granularity = cache->discard_block_size << SECTOR_SHIFT;
limits->max_discard_sectors = origin_limits->max_discard_sectors;
limits->max_hw_discard_sectors = origin_limits->max_hw_discard_sectors;
limits->discard_granularity = origin_limits->discard_granularity;
limits->discard_alignment = origin_limits->discard_alignment;
limits->discard_misaligned = origin_limits->discard_misaligned;
}

static void cache_io_hints(struct dm_target *ti, struct queue_limits *limits)
Expand All @@ -3456,14 +3528,16 @@ static void cache_io_hints(struct dm_target *ti, struct queue_limits *limits)
blk_limits_io_min(limits, cache->sectors_per_block << SECTOR_SHIFT);
blk_limits_io_opt(limits, cache->sectors_per_block << SECTOR_SHIFT);
}

disable_passdown_if_not_supported(cache);
set_discard_limits(cache, limits);
}

/*----------------------------------------------------------------*/

static struct target_type cache_target = {
.name = "cache",
.version = {2, 0, 0},
.version = {2, 1, 0},
.module = THIS_MODULE,
.ctr = cache_ctr,
.dtr = cache_dtr,
Expand Down

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