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dm snapshot: track snapshot reads

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Whenever a snapshot read gets mapped through to the origin, track it in
a per-snapshot hash table indexed by chunk number, using memory allocated
from a new per-snapshot mempool.

We need to track these reads to avoid race conditions which will be fixed
by patches that follow.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
tirimbino
Mikulas Patocka 17 years ago committed by Alasdair G Kergon
parent def052d21c
commit cd45daffd1
2 changed files with 106 additions and 10 deletions
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  1. 107
      drivers/md/dm-snap.c
  2. 9
      drivers/md/dm-snap.h

107
drivers/md/dm-snap.c
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@ -40,6 +40,11 @@
*/
#define SNAPSHOT_PAGES (((1UL << 20) >> PAGE_SHIFT) ? : 1)
/*
* The size of the mempool used to track chunks in use.
*/
#define MIN_IOS 256
static struct workqueue_struct *ksnapd;
static void flush_queued_bios(struct work_struct *work);
@ -93,6 +98,42 @@ static struct kmem_cache *exception_cache;
static struct kmem_cache *pending_cache;
static mempool_t *pending_pool;
struct dm_snap_tracked_chunk {
struct hlist_node node;
chunk_t chunk;
};
static struct kmem_cache *tracked_chunk_cache;
static struct dm_snap_tracked_chunk *track_chunk(struct dm_snapshot *s,
chunk_t chunk)
{
struct dm_snap_tracked_chunk *c = mempool_alloc(s->tracked_chunk_pool,
GFP_NOIO);
unsigned long flags;
c->chunk = chunk;
spin_lock_irqsave(&s->tracked_chunk_lock, flags);
hlist_add_head(&c->node,
&s->tracked_chunk_hash[DM_TRACKED_CHUNK_HASH(chunk)]);
spin_unlock_irqrestore(&s->tracked_chunk_lock, flags);
return c;
}
static void stop_tracking_chunk(struct dm_snapshot *s,
struct dm_snap_tracked_chunk *c)
{
unsigned long flags;
spin_lock_irqsave(&s->tracked_chunk_lock, flags);
hlist_del(&c->node);
spin_unlock_irqrestore(&s->tracked_chunk_lock, flags);
mempool_free(c, s->tracked_chunk_pool);
}
/*
* One of these per registered origin, held in the snapshot_origins hash
*/
@ -482,6 +523,7 @@ static int set_chunk_size(struct dm_snapshot *s, const char *chunk_size_arg,
static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
struct dm_snapshot *s;
int i;
int r = -EINVAL;
char persistent;
char *origin_path;
@ -564,11 +606,24 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
goto bad5;
}
s->tracked_chunk_pool = mempool_create_slab_pool(MIN_IOS,
tracked_chunk_cache);
if (!s->tracked_chunk_pool) {
ti->error = "Could not allocate tracked_chunk mempool for "
"tracking reads";
goto bad6;
}
for (i = 0; i < DM_TRACKED_CHUNK_HASH_SIZE; i++)
INIT_HLIST_HEAD(&s->tracked_chunk_hash[i]);
spin_lock_init(&s->tracked_chunk_lock);
/* Metadata must only be loaded into one table at once */
r = s->store.read_metadata(&s->store);
if (r < 0) {
ti->error = "Failed to read snapshot metadata";
goto bad6;
goto bad_load_and_register;
} else if (r > 0) {
s->valid = 0;
DMWARN("Snapshot is marked invalid.");
@ -582,7 +637,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
if (register_snapshot(s)) {
r = -EINVAL;
ti->error = "Cannot register snapshot origin";
goto bad6;
goto bad_load_and_register;
}
ti->private = s;
@ -590,6 +645,9 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
return 0;
bad_load_and_register:
mempool_destroy(s->tracked_chunk_pool);
bad6:
dm_kcopyd_client_destroy(s->kcopyd_client);
@ -624,6 +682,9 @@ static void __free_exceptions(struct dm_snapshot *s)
static void snapshot_dtr(struct dm_target *ti)
{
#ifdef CONFIG_DM_DEBUG
int i;
#endif
struct dm_snapshot *s = ti->private;
flush_workqueue(ksnapd);
@ -632,6 +693,13 @@ static void snapshot_dtr(struct dm_target *ti)
/* After this returns there can be no new kcopyd jobs. */
unregister_snapshot(s);
#ifdef CONFIG_DM_DEBUG
for (i = 0; i < DM_TRACKED_CHUNK_HASH_SIZE; i++)
BUG_ON(!hlist_empty(&s->tracked_chunk_hash[i]));
#endif
mempool_destroy(s->tracked_chunk_pool);
__free_exceptions(s);
dm_put_device(ti, s->origin);
@ -974,14 +1042,10 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
start_copy(pe);
goto out;
}
} else
/*
* FIXME: this read path scares me because we
* always use the origin when we have a pending
* exception. However I can't think of a
* situation where this is wrong - ejt.
*/
} else {
bio->bi_bdev = s->origin->bdev;
map_context->ptr = track_chunk(s, chunk);
}
out_unlock:
up_write(&s->lock);
@ -989,6 +1053,18 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
return r;
}
static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
int error, union map_info *map_context)
{
struct dm_snapshot *s = ti->private;
struct dm_snap_tracked_chunk *c = map_context->ptr;
if (c)
stop_tracking_chunk(s, c);
return 0;
}
static void snapshot_resume(struct dm_target *ti)
{
struct dm_snapshot *s = ti->private;
@ -1266,6 +1342,7 @@ static struct target_type snapshot_target = {
.ctr = snapshot_ctr,
.dtr = snapshot_dtr,
.map = snapshot_map,
.end_io = snapshot_end_io,
.resume = snapshot_resume,
.status = snapshot_status,
};
@ -1306,11 +1383,18 @@ static int __init dm_snapshot_init(void)
goto bad4;
}
tracked_chunk_cache = KMEM_CACHE(dm_snap_tracked_chunk, 0);
if (!tracked_chunk_cache) {
DMERR("Couldn't create cache to track chunks in use.");
r = -ENOMEM;
goto bad5;
}
pending_pool = mempool_create_slab_pool(128, pending_cache);
if (!pending_pool) {
DMERR("Couldn't create pending pool.");
r = -ENOMEM;
goto bad5;
goto bad_pending_pool;
}
ksnapd = create_singlethread_workqueue("ksnapd");
@ -1324,6 +1408,8 @@ static int __init dm_snapshot_init(void)
bad6:
mempool_destroy(pending_pool);
bad_pending_pool:
kmem_cache_destroy(tracked_chunk_cache);
bad5:
kmem_cache_destroy(pending_cache);
bad4:
@ -1355,6 +1441,7 @@ static void __exit dm_snapshot_exit(void)
mempool_destroy(pending_pool);
kmem_cache_destroy(pending_cache);
kmem_cache_destroy(exception_cache);
kmem_cache_destroy(tracked_chunk_cache);
}
/* Module hooks */

9
drivers/md/dm-snap.h
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@ -130,6 +130,10 @@ struct exception_store {
void *context;
};
#define DM_TRACKED_CHUNK_HASH_SIZE 16
#define DM_TRACKED_CHUNK_HASH(x) ((unsigned long)(x) & \
(DM_TRACKED_CHUNK_HASH_SIZE - 1))
struct dm_snapshot {
struct rw_semaphore lock;
struct dm_target *ti;
@ -174,6 +178,11 @@ struct dm_snapshot {
/* Queue of snapshot writes for ksnapd to flush */
struct bio_list queued_bios;
struct work_struct queued_bios_work;
/* Chunks with outstanding reads */
mempool_t *tracked_chunk_pool;
spinlock_t tracked_chunk_lock;
struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];
};
/*

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