[tip: perf/core] perf/x86: Don't WARN when the ext_regs_buf allocation fails

[tip-bot2 for vineash]

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     f5a78ecccd98a04c47cea1cd63c2a2fdce8a4b48
Gitweb:        https://git.kernel.org/tip/f5a78ecccd98a04c47cea1cd63c2a2fdce8a4b48
Author:        vineash <vineash30640@xxxxxxxxx>
AuthorDate:    Tue, 13 Oct 2026 16:13:20 +08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 16 Oct 2026 09:49:05 +02:00

perf/x86: Don't WARN when the ext_regs_buf allocation fails

reserve_ext_regs_buffers() wraps the per-CPU ext_regs_buf allocation in
WARN_ON_ONCE(). The allocator already prints a warning with a stack dump
when a GFP_KERNEL allocation fails, so the WARN only repeats it, which
Documentation/process/coding-style.rst asks not to do. The failure is
also handled: the buffers are freed again, the event is still created,
and events that ask for SIMD or extended register sampling are rejected
with -ENOMEM in x86_pmu_hw_config() and event_simd_regs_validate().

syzkaller hit this with fault injection on next-20261002. The trigger
is a plain hardware cycles event opened while no other x86 PMU event
exists, with the allocation failed:

  !per_cpu(ext_regs_buf, cpu)
  WARNING: arch/x86/events/core.c:441 at reserve_ext_regs_buffers arch/x86/events/core.c:441 [inline]
  WARNING: arch/x86/events/core.c:441 at x86_reserve_hardware+0x6ff/0x880 arch/x86/events/core.c:458
  Call Trace:
   __x86_pmu_event_init arch/x86/events/core.c:825 [inline]
   x86_pmu_event_init+0x109/0xc50 arch/x86/events/core.c:3003
   perf_try_init_event+0x140/0xbc0 kernel/events/core.c:13261
   perf_init_event kernel/events/core.c:13365 [inline]
   perf_event_alloc+0x2145/0x5680 kernel/events/core.c:13640
   __do_sys_perf_event_open+0x669/0x2d40 kernel/events/core.c:14185
   do_syscall_64+0x12c/0x7b0 arch/x86/entry/syscall_64.c:84
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Drop the WARN_ON_ONCE(). This matches reserve_ds_buffers() and
reserve_lbr_buffers(), which also handle allocation failure without a
warning.

Fixes: b84c96283684 ("perf/x86: Enable XMM register sampling for non-PEBS events")
Assisted-by: Claude:claude-opus-5-5 syzkaller
Signed-off-by: vineash <vineash30640@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/179187920064.3441876.15687522011658301558@xxxxxxxxx
---
 arch/x86/events/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 9b1a903..4df792b 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -438,7 +438,7 @@ static void reserve_ext_regs_buffers(void)
 	for_each_possible_cpu(cpu) {
 		per_cpu(ext_regs_buf, cpu) = kzalloc_node(size, GFP_KERNEL,
 							  cpu_to_node(cpu));
-		if (WARN_ON_ONCE(!per_cpu(ext_regs_buf, cpu)))
+		if (!per_cpu(ext_regs_buf, cpu))
 			goto err;
 	}