[dm-devel] [PATCH 1/4] dm-bufio: make it possible to query multiple blocks with just one lock

[Mikulas Patocka]

In order to reduce locking overhead, we introduce a new API:
dm_bufio_lock_read, dm_bufio_unlock_read and dm_bufio_get_unlocked.

The user calls dm_bufio_lock_read, then he might call
dm_bufio_get_unlocked several times and finally calls
dm_bufio_unlock_read. This is intended to simplify btree walks in dm-thin.

The follow-up patch will change c->lock from mutex to rw-semaphore, so
that there could be multiple processes walking the btree concurently.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

---
 drivers/md/dm-bufio.c    |   45 ++++++++++++++++++++++++++++++++++++++-------
 include/linux/dm-bufio.h |   15 +++++++++++++++
 2 files changed, 53 insertions(+), 7 deletions(-)

Index: linux-2.6/drivers/md/dm-bufio.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-bufio.c	2022-10-10 11:08:10.000000000 +0200
+++ linux-2.6/drivers/md/dm-bufio.c	2022-10-10 11:13:10.000000000 +0200
@@ -145,7 +145,7 @@ struct dm_buffer {
 	unsigned char list_mode;		/* LIST_* */
 	blk_status_t read_error;
 	blk_status_t write_error;
-	unsigned accessed;
+	volatile unsigned accessed;
 	unsigned hold_count;
 	unsigned long state;
 	unsigned long last_accessed;
@@ -193,6 +193,18 @@ static void dm_bufio_unlock(struct dm_bu
 		mutex_unlock(&c->lock);
 }
 
+void dm_bufio_lock_read(struct dm_bufio_client *c)
+{
+	mutex_lock(&c->lock);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_lock_read);
+
+void dm_bufio_unlock_read(struct dm_bufio_client *c)
+{
+	mutex_unlock(&c->lock);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_unlock_read);
+
 /*----------------------------------------------------------------*/
 
 /*
@@ -870,7 +882,8 @@ enum new_flag {
 	NF_FRESH = 0,
 	NF_READ = 1,
 	NF_GET = 2,
-	NF_PREFETCH = 3
+	NF_GET_UNLOCKED = 3,
+	NF_PREFETCH = 4
 };
 
 /*
@@ -1013,7 +1026,7 @@ static struct dm_buffer *__bufio_new(str
 	if (b)
 		goto found_buffer;
 
-	if (nf == NF_GET)
+	if (nf == NF_GET || nf == NF_GET_UNLOCKED)
 		return NULL;
 
 	new_b = __alloc_buffer_wait(c, nf);
@@ -1058,12 +1071,17 @@ found_buffer:
 	 * If the user called both dm_bufio_prefetch and dm_bufio_get on
 	 * the same buffer, it would deadlock if we waited.
 	 */
-	if (nf == NF_GET && unlikely(test_bit(B_READING, &b->state)))
+	if ((nf == NF_GET || nf == NF_GET_UNLOCKED) && unlikely(test_bit(B_READING, &b->state)))
 		return NULL;
 
-	b->hold_count++;
-	__relink_lru(b, test_bit(B_DIRTY, &b->state) ||
-		     test_bit(B_WRITING, &b->state));
+	if (nf == NF_GET_UNLOCKED) {
+		if (!b->accessed)
+			b->accessed = 1;
+	} else {
+		b->hold_count++;
+		__relink_lru(b, test_bit(B_DIRTY, &b->state) ||
+			     test_bit(B_WRITING, &b->state));
+	}
 	return b;
 }
 
@@ -1154,6 +1172,19 @@ void *dm_bufio_new(struct dm_bufio_clien
 }
 EXPORT_SYMBOL_GPL(dm_bufio_new);
 
+void *dm_bufio_get_unlocked(struct dm_bufio_client *c, sector_t block,
+			    struct dm_buffer **bp)
+{
+	int need_submit;
+	struct dm_buffer *b = __bufio_new(c, block, NF_GET_UNLOCKED, &need_submit, NULL);
+	if (b) {
+		*bp = b;
+		return b->data;
+	}
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(dm_bufio_get_unlocked);
+
 void dm_bufio_prefetch(struct dm_bufio_client *c,
 		       sector_t block, unsigned n_blocks)
 {
Index: linux-2.6/include/linux/dm-bufio.h
===================================================================
--- linux-2.6.orig/include/linux/dm-bufio.h	2022-10-10 11:13:10.000000000 +0200
+++ linux-2.6/include/linux/dm-bufio.h	2022-10-10 11:13:25.000000000 +0200
@@ -38,6 +38,13 @@ dm_bufio_client_create(struct block_devi
 void dm_bufio_client_destroy(struct dm_bufio_client *c);
 
 /*
+ * Lock and unlock the bufio client - this is needed if we want to call
+ * dm_bufio_get_unlocked.
+ */
+void dm_bufio_lock_read(struct dm_bufio_client *c);
+void dm_bufio_unlock_read(struct dm_bufio_client *c);
+
+/*
  * Set the sector range.
  * When this function is called, there must be no I/O in progress on the bufio
  * client.
@@ -76,6 +83,14 @@ void *dm_bufio_new(struct dm_bufio_clien
 		   struct dm_buffer **bp);
 
 /*
+ * Like dm_bufio_get, but assume that the client is already locked with
+ * dm_bufio_lock_read/dm_bufio_unlock_read. dm_bufio_release should not be
+ * called; the caller should call dm_bufio_unlock_read to release the buffer.
+ */
+void *dm_bufio_get_unlocked(struct dm_bufio_client *c, sector_t block,
+			    struct dm_buffer **bp);
+
+/*
  * Prefetch the specified blocks to the cache.
  * The function starts to read the blocks and returns without waiting for
  * I/O to finish.