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 <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

3 files changed, 47 insertions, 10 deletions

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 7913fdcfc849..1b69359461e9 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -240,6 +240,15 @@ config DM_BUFIO
          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
diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index 7734298bff3c..bccc7e6903e4 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -16,6 +16,7 @@
 #include <linux/shrinker.h>
 #include <linux/module.h>
 #include <linux/rbtree.h>
+#include <linux/stacktrace.h>
 
 #define DM_MSG_PREFIX "bufio"
 
@@ -149,6 +150,11 @@ struct dm_buffer {
         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
 };
 
 /*----------------------------------------------------------------*/
@@ -253,6 +259,17 @@ static LIST_HEAD(dm_bufio_all_clients);
  */
 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.
  *--------------------------------------------------------------*/
@@ -454,6 +471,9 @@ static struct dm_buffer *alloc_buffer(struct dm_bufio_client *c, gfp_t gfp_mask)
 
         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;
 }
 
@@ -1063,6 +1083,10 @@ static void *new_read(struct dm_bufio_client *c, sector_t block,
 
         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);
@@ -1462,6 +1486,7 @@ static void drop_buffers(struct dm_bufio_client *c)
 {
         struct dm_buffer *b;
         int i;
+        bool warned = false;
 
         BUG_ON(dm_bufio_in_request());
 
@@ -1476,9 +1501,21 @@ static void drop_buffers(struct dm_bufio_client *c)
                 __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]));
diff --git a/drivers/md/persistent-data/Kconfig b/drivers/md/persistent-data/Kconfig
index 78c74bb71ba4..a53cbc928af1 100644
--- a/drivers/md/persistent-data/Kconfig
+++ b/drivers/md/persistent-data/Kconfig
@@ -7,12 +7,3 @@ config DM_PERSISTENT_DATA
          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.