dm bufio: store stacktrace in buffers to help find buffer leaks

The option DM_DEBUG_BLOCK_STACK_TRACING is moved from persistent-data
directory to device mapper directory because it will now be used by
persistent-data and bufio. When the option is enabled, each bufio buffer
stores the stacktrace of the last dm_bufio_get(), dm_bufio_read() or
dm_bufio_new() call that increased the hold count to 1. The buffer's
stacktrace is printed if the buffer was not released before the bufio
client is destroyed.

When DM_DEBUG_BLOCK_STACK_TRACING is enabled, any bufio buffer leaks are
considered warnings - i.e. the kernel continues afterwards. If not
enabled, buffer leaks are considered BUGs and the kernel with crash.
Reasoning on this disposition is: if we only ever warned on buffer leaks
users would generally ignore them and the problematic code would never
get fixed.

Successfully used to find source of bufio leaks fixed with commit
fce079f63c3 ("dm btree: fix bufio buffer leaks in dm_btree_del() error
path").

Signed-off-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/Kconfig

···
 	 as a cache, holding recently-read blocks in memory and performing
 	 delayed writes.
 
+config DM_DEBUG_BLOCK_STACK_TRACING
+       bool "Keep stack trace of persistent data block lock holders"
+       depends on STACKTRACE_SUPPORT && DM_BUFIO
+       select STACKTRACE
+       ---help---
+	 Enable this for messages that may help debug problems with the
+	 block manager locking used by thin provisioning and caching.
+
+	 If unsure, say N.
 config DM_BIO_PRISON
        tristate
        depends on BLK_DEV_DM

drivers/md/dm-bufio.c

···
 #include <linux/shrinker.h>
 #include <linux/module.h>
 #include <linux/rbtree.h>
+#include <linux/stacktrace.h>
 
 #define DM_MSG_PREFIX "bufio"
 
···
 	struct list_head write_list;
 	struct bio bio;
 	struct bio_vec bio_vec[DM_BUFIO_INLINE_VECS];
+#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
+#define MAX_STACK 10
+	struct stack_trace stack_trace;
+	unsigned long stack_entries[MAX_STACK];
+#endif
 };
 
 /*----------------------------------------------------------------*/
···
  * dm_bufio_cache_size_per_client and dm_bufio_client_count
  */
 static DEFINE_MUTEX(dm_bufio_clients_lock);
+
+#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
+static void buffer_record_stack(struct dm_buffer *b)
+{
+	b->stack_trace.nr_entries = 0;
+	b->stack_trace.max_entries = MAX_STACK;
+	b->stack_trace.entries = b->stack_entries;
+	b->stack_trace.skip = 2;
+	save_stack_trace(&b->stack_trace);
+}
+#endif
 
 /*----------------------------------------------------------------
  * A red/black tree acts as an index for all the buffers.
···
 
 	adjust_total_allocated(b->data_mode, (long)c->block_size);
 
+#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
+	memset(&b->stack_trace, 0, sizeof(b->stack_trace));
+#endif
 	return b;
 }
 
···
 
 	dm_bufio_lock(c);
 	b = __bufio_new(c, block, nf, &need_submit, &write_list);
+#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
+	if (b && b->hold_count == 1)
+		buffer_record_stack(b);
+#endif
 	dm_bufio_unlock(c);
 
 	__flush_write_list(&write_list);
···
 {
 	struct dm_buffer *b;
 	int i;
+	bool warned = false;
 
 	BUG_ON(dm_bufio_in_request());
 
···
 		__free_buffer_wake(b);
 
 	for (i = 0; i < LIST_SIZE; i++)
-		list_for_each_entry(b, &c->lru[i], lru_list)
+		list_for_each_entry(b, &c->lru[i], lru_list) {
+			WARN_ON(!warned);
+			warned = true;
 			DMERR("leaked buffer %llx, hold count %u, list %d",
 			      (unsigned long long)b->block, b->hold_count, i);
+#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
+			print_stack_trace(&b->stack_trace, 1);
+			b->hold_count = 0; /* mark unclaimed to avoid BUG_ON below */
+#endif
+		}
+
+#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
+	while ((b = __get_unclaimed_buffer(c)))
+		__free_buffer_wake(b);
+#endif
 
 	for (i = 0; i < LIST_SIZE; i++)
 		BUG_ON(!list_empty(&c->lru[i]));

drivers/md/persistent-data/Kconfig

···
 	 Library providing immutable on-disk data structure support for
 	 device-mapper targets such as the thin provisioning target.
 
-config DM_DEBUG_BLOCK_STACK_TRACING
-       bool "Keep stack trace of persistent data block lock holders"
-       depends on STACKTRACE_SUPPORT && DM_PERSISTENT_DATA
-       select STACKTRACE
-       ---help---
-	 Enable this for messages that may help debug problems with the
-	 block manager locking used by thin provisioning and caching.
-
-	 If unsure, say N.