From c2ad6b51efc5f27d70ce952decd2a15679b83600 Mon Sep 17 00:00:00 2001 From: Alexander Shishkin Date: Tue, 28 Jul 2015 09:00:04 +0300 Subject: [PATCH] perf/ring-buffer: Clarify the use of page::private for high-order AUX allocations A question [1] was raised about the use of page::private in AUX buffer allocations, so let's add a clarification about its intended use. The private field and flag are used by perf's rb_alloc_aux() path to tell the pmu driver the size of each high-order allocation, so that the driver can program those appropriately into its hardware. This only matters for PMUs that don't support hardware scatter tables. Otherwise, every page in the buffer is just a page. This patch adds a comment about the private field to the AUX buffer allocation path. [1] http://marc.info/?l=linux-kernel&m=143803696607968 Reported-by: Mathieu Poirier Signed-off-by: Alexander Shishkin Signed-off-by: Peter Zijlstra (Intel) Cc: Arnaldo Carvalho de Melo Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1438063204-665-1-git-send-email-alexander.shishkin@linux.intel.com Signed-off-by: Ingo Molnar --- kernel/events/ring_buffer.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index c8aa3f75bc4d..182bc30899d5 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -437,7 +437,10 @@ static struct page *rb_alloc_aux_page(int node, int order) if (page && order) { /* - * Communicate the allocation size to the driver + * Communicate the allocation size to the driver: + * if we managed to secure a high-order allocation, + * set its first page's private to this order; + * !PagePrivate(page) means it's just a normal page. */ split_page(page, order); SetPagePrivate(page); -- 2.20.1