linux-kernel - [tip: perf/core] perf: Fix default aux

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Message-ID: <172021360548.2215.11914104138278947134.tip-bot2@tip-bot2>
Date: Fri, 05 Jul 2024 21:06:45 -0000
From: "tip-bot2 for Adrian Hunter" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Adrian Hunter <adrian.hunter@...el.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: perf/core] perf: Fix default aux_watermark calculation

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

Commit-ID:     43deb76b19663a96ec2189d8f4eb9a9dc2d7623f
Gitweb:        https://git.kernel.org/tip/43deb76b19663a96ec2189d8f4eb9a9dc2d7623f
Author:        Adrian Hunter <adrian.hunter@...el.com>
AuthorDate:    Mon, 24 Jun 2024 23:11:00 +03:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 04 Jul 2024 16:00:23 +02:00

perf: Fix default aux_watermark calculation

The default aux_watermark is half the AUX area buffer size. In general,
on a 64-bit architecture, the AUX area buffer size could be a bigger than
fits in a 32-bit type, but the calculation does not allow for that
possibility.

However the aux_watermark value is recorded in a u32, so should not be
more than U32_MAX either.

Fix by doing the calculation in a correctly sized type, and limiting the
result to U32_MAX.

Fixes: d68e6799a5c8 ("perf: Cap allocation order at aux_watermark")
Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20240624201101.60186-7-adrian.hunter@intel.com
---
 kernel/events/ring_buffer.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 4013408..485cf0a 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -688,7 +688,9 @@ int rb_alloc_aux(struct perf_buffer *rb, struct perf_event *event,
 		 * max_order, to aid PMU drivers in double buffering.
 		 */
 		if (!watermark)
-			watermark = nr_pages << (PAGE_SHIFT - 1);
+			watermark = min_t(unsigned long,
+					  U32_MAX,
+					  (unsigned long)nr_pages << (PAGE_SHIFT - 1));
 
 		/*
 		 * Use aux_watermark as the basis for chunking to