[PATCH] btrfs: add EM_WARN_ON_ONCE() to dump the extent map on a warning

When an extent_map trips a consistency check in the free path, the bare
WARN_ON() only gives a stack trace.  For the checks in
btrfs_free_extent_map():

	WARN_ON(btrfs_extent_map_in_tree(em));
	WARN_ON(!list_empty(&em->list));

that means the report tells us an extent map was freed while still in the
tree, or while still linked onto a list (e.g. an inode's modified_extents
list), but nothing about the map itself: its flags, refcount, or its
geometry.

Borrow the pattern the mm code uses with VM_WARN_ON_ONCE_FOLIO() and
friends, which dump the offending object via dump_page() before the
warning.  Add btrfs_dump_extent_map(), a helper that prints an extent
map's geometry, flags, refcount and list/tree membership, and an
EM_WARN_ON_ONCE(cond, em) macro that dumps the map before triggering the
warning and stack trace.  As with the VM_*_ONCE helpers it fires only
once, which keeps these checks from spamming the log on a filesystem
that is repeatedly hitting them.

Unlike the existing dump_extent_map() helper (which is gated behind
CONFIG_BTRFS_DEBUG and calls ASSERT(0)) and unlike the VM_* helpers
(gated behind CONFIG_DEBUG_VM), EM_WARN_ON_ONCE() is always compiled in,
since the conditions it guards can be hit on production kernels and the
extra context is what makes those reports actionable.  No btrfs_fs_info
is available at the call site, so the helper uses the plain printk helpers
rather than the device-aware btrfs_*() ones.

Convert both checks in btrfs_free_extent_map() to use it.

Signed-off-by: Jeff Layton <[email protected]>
---
We've seen a number of crashes in our fleet that have shown WARN_ON()
pops from the list_empty() check here. I'm hoping this might give us a
bit more info.
---
 fs/btrfs/extent_map.c | 20 ++++++++++++++++++--
 fs/btrfs/extent_map.h | 22 ++++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 6b79bff241f2..670143f7d473 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -63,8 +63,8 @@ void btrfs_free_extent_map(struct extent_map *em)
 	if (!em)
 		return;
 	if (refcount_dec_and_test(&em->refs)) {
-		WARN_ON(btrfs_extent_map_in_tree(em));
-		WARN_ON(!list_empty(&em->list));
+		EM_WARN_ON_ONCE(btrfs_extent_map_in_tree(em), em);
+		EM_WARN_ON_ONCE(!list_empty(&em->list), em);
 		kmem_cache_free(extent_map_cache, em);
 	}
 }
@@ -316,6 +316,22 @@ static void dump_extent_map(struct btrfs_fs_info *fs_info, const char *prefix,
 	ASSERT(0);
 }
 
+/*
+ * Dump the contents of an extent map to the kernel log.  Used by
+ * EM_WARN_ON_ONCE() to print context about the offending map before its
+ * warning and stack trace.
+ */
+void btrfs_dump_extent_map(const struct extent_map *em, const char *prefix)
+{
+	pr_crit(
+"BTRFS: %s: em=%p start=%llu len=%llu disk_bytenr=%llu disk_num_bytes=%llu offset=%llu ram_bytes=%llu generation=%llu flags=0x%x refs=%u in_tree=%d list_empty=%d list=(next=%p prev=%p)\n",
+		prefix, em, em->start, em->len, em->disk_bytenr,
+		em->disk_num_bytes, em->offset, em->ram_bytes, em->generation,
+		em->flags, refcount_read(&em->refs),
+		btrfs_extent_map_in_tree(em), list_empty(&em->list),
+		em->list.next, em->list.prev);
+}
+
 /* Internal sanity checks for btrfs debug builds. */
 static void validate_extent_map(struct btrfs_fs_info *fs_info, struct extent_map *em)
 {
diff --git a/fs/btrfs/extent_map.h b/fs/btrfs/extent_map.h
index 6f685f3c9327..1d2efd4a5665 100644
--- a/fs/btrfs/extent_map.h
+++ b/fs/btrfs/extent_map.h
@@ -8,6 +8,8 @@
 #include <linux/rbtree.h>
 #include <linux/list.h>
 #include <linux/refcount.h>
+#include <linux/bug.h>
+#include <linux/stringify.h>
 #include "fs.h"
 
 struct btrfs_inode;
@@ -175,6 +177,26 @@ int btrfs_split_extent_map(struct btrfs_inode *inode, u64 start, u64 len, u64 pr
 
 struct extent_map *btrfs_alloc_extent_map(void);
 void btrfs_free_extent_map(struct extent_map *em);
+void btrfs_dump_extent_map(const struct extent_map *em, const char *prefix);
+
+/*
+ * Warn once about a condition involving an extent map, dumping the offending
+ * map's contents (geometry, flags, refcount and list/tree membership) before
+ * the warning and stack trace, much like VM_WARN_ON_ONCE_FOLIO() does for
+ * folios.
+ */
+#define EM_WARN_ON_ONCE(cond, em) ({					\
+	static bool __section(".data..once") __warned;			\
+	int __ret_warn_once = !!(cond);					\
+									\
+	if (unlikely(__ret_warn_once && !__warned)) {			\
+		btrfs_dump_extent_map((em),				\
+			"EM_WARN_ON_ONCE(" __stringify(cond) ")");	\
+		__warned = true;					\
+		WARN_ON(1);						\
+	}								\
+	unlikely(__ret_warn_once);					\
+})
 int __init btrfs_extent_map_init(void);
 void __cold btrfs_extent_map_exit(void);
 int btrfs_unpin_extent_cache(struct btrfs_inode *inode, u64 start, u64 len, u64 gen);

---
base-commit: eae9071be4d8d386e6530120bdaee42f6da1f70f
change-id: 20260626-em-warn-on-dump-e82922269f25

Best regards,
-- 
Jeff Layton <[email protected]>