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[188.141.5.72]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-486f8c40ccbsm6848125e9.9.2026.03.18.10.19.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 18 Mar 2026 10:19:10 -0700 (PDT) From: David Carlier To: Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers Cc: linux-kernel@vger.kernel.org, David Carlier Subject: [PATCH] tracing: Fix nr_subbufs initialization in simple_ring_buffer_init_mm() Date: Wed, 18 Mar 2026 17:19:07 +0000 Message-ID: <20260318171907.19640-1-devnexen@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit In simple_ring_buffer_init_mm(), meta->nr_subbufs is assigned from cpu_buffer->nr_pages at line 398, but cpu_buffer was just zeroed by memset() at line 390. The actual page count is only computed later in the loop (lines 410-429), so nr_subbufs is always set to 0. This field is part of struct trace_buffer_meta (UAPI), exposed to consumers who rely on it for buffer geometry calculations (e.g., data_len = subbuf_size * nr_subbufs). A value of 0 breaks ring buffer consumption entirely. Fix by using desc->nr_page_va directly, which holds the correct total page count (reader + ring pages) and is available at this point. This matches the UAPI documentation: "Number of subbufs in the ring-buffer, including the reader." Fixes: 34e5b958bdad ("tracing: Introduce simple_ring_buffer") Signed-off-by: David Carlier --- kernel/trace/simple_ring_buffer.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c index 02af2297ae5a..e991a0d8def2 100644 --- a/kernel/trace/simple_ring_buffer.c +++ b/kernel/trace/simple_ring_buffer.c @@ -395,7 +395,7 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer, memset(cpu_buffer->meta, 0, sizeof(*cpu_buffer->meta)); cpu_buffer->meta->meta_page_size = PAGE_SIZE; - cpu_buffer->meta->nr_subbufs = cpu_buffer->nr_pages; + cpu_buffer->meta->nr_subbufs = desc->nr_page_va; /* The reader page is not part of the ring initially */ page = load_page(desc->page_va[0]); -- 2.53.0