[PATCH] blktrace: use debug print to report dropped events

[Chaitanya Kulkarni <ckulkarnilinux@gmail.com>]

The WARN_ON_ONCE introduced in
commit f9ee38bbf70f ("blktrace: add block trace commands for zone operations") 
triggers kernel warnings when zone operations are traced with blktrace
version 1. This can spam the kernel log during normal operation with
zoned block devices when userspace is using the legacy blktrace
protocol.

Currently blktrace implementation drops newly added REQ_OP_ZONE_XXX
when blktrace userspce version is set to 1.

Remove the WARN_ON_ONCE and quietly filter these events. Add a
rate-limited debug message to help diagnose potential issues without
flooding the kernel log. The debug message can be enabled via dynamic
debug when needed for troubleshooting.

This approach is more appropriate as encountering zone operations with
blktrace v1 is an expected condition that should be handled gracefully
rather than warned about, since users may be running older blktrace
userspace tools that only support version 1 of the protocol.

With this patch :-
linux-block (for-next) # git log -1 
commit c8966006a0971d2b4bf94c0426eb7e4407c6853f (HEAD -> for-next)
Author: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Date:   Mon Oct 27 19:26:53 2025 -0700

    blktrace: use debug print to report dropped events
linux-block (for-next) # cdblktests
blktests (master) # ./check blktrace
blktrace/001 (blktrace zone management command tracing)      [passed]
    runtime  3.805s  ...  3.889s
blktests (master) # dmesg  -c
blktests (master) #  echo "file kernel/trace/blktrace.c +p" > /sys/kernel/debug/dynamic_debug/control
blktests (master) # ./check blktrace
blktrace/001 (blktrace zone management command tracing)      [passed]
    runtime  3.889s  ...  3.881s
blktests (master) # dmesg  -c
[   77.826237] blktrace: blktrace v1 cannot trace zone operation 0x1000190001
[   77.826260] blktrace: blktrace v1 cannot trace zone operation 0x1000190004
[   77.826282] blktrace: blktrace v1 cannot trace zone operation 0x1001490007
[   77.826288] blktrace: blktrace v1 cannot trace zone operation 0x1001890008
[   77.826343] blktrace: blktrace v1 cannot trace zone operation 0x1000190001
[   77.826347] blktrace: blktrace v1 cannot trace zone operation 0x1000190004
[   77.826350] blktrace: blktrace v1 cannot trace zone operation 0x1001490007
[   77.826354] blktrace: blktrace v1 cannot trace zone operation 0x1001890008
[   77.826373] blktrace: blktrace v1 cannot trace zone operation 0x1000190001
[   77.826377] blktrace: blktrace v1 cannot trace zone operation 0x1000190004
blktests (master) #  echo "file kernel/trace/blktrace.c -p" > /sys/kernel/debug/dynamic_debug/control
blktests (master) # ./check blktrace
blktrace/001 (blktrace zone management command tracing)      [passed]
    runtime  3.881s  ...  3.824s
blktests (master) # dmesg  -c 
blktests (master) # 

Reported-by: syzbot+153e64c0aa875d7e4c37@syzkaller.appspotmail.com
Fixes: f9ee38bbf70f ("blktrace: add block trace commands for zone operations")
Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
---
 kernel/trace/blktrace.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 6ad3807a5b73..776ae4190f36 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -364,9 +364,12 @@ static void __blk_add_trace(struct blk_trace *bt, sector_t sector, int bytes,
 		break;
 	}
 
-	if (WARN_ON_ONCE(bt->version == 1 &&
-		     (what >> BLK_TC_SHIFT) > BLK_TC_END_V1))
+	/* Drop trace events for zone operations with blktrace v1 */
+	if (bt->version == 1 && (what >> BLK_TC_SHIFT) > BLK_TC_END_V1) {
+		pr_debug_ratelimited("blktrace v1 cannot trace zone operation 0x%llx\n",
+				(unsigned long long)what);
 		return;
+	}
 
 	if (cgid)
 		what |= __BLK_TA_CGROUP;
-- 
2.40.0