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: d68e679 ("perf: Cap allocation order at aux_watermark")
Signed-off-by: Adrian Hunter <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ahunter6

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