linux-kernel - [PATCH] ftrace: Do not over-allocate ftrace memory

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Message-ID: <20260106203533.2896197-1-linux@roeck-us.net>
Date: Tue,  6 Jan 2026 12:35:33 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	Guenter Roeck <linux@...ck-us.net>
Subject: [PATCH] ftrace: Do not over-allocate ftrace memory

The pg_remaining calculation in ftrace_process_locs() assumes that
ENTRIES_PER_PAGE multiplied by 2^order equals the actual capacity of the
allocated page group. However, ENTRIES_PER_PAGE is PAGE_SIZE / ENTRY_SIZE
(integer division). When PAGE_SIZE is not a multiple of ENTRY_SIZE (e.g.
4096 / 24 = 170 with remainder 16), high-order allocations (like 256 pages)
have significantly more capacity than 256 * 170. This leads to pg_remaining
being underestimated, which in turn makes skip (derived from skipped -
pg_remaining) larger than expected, causing the WARN(skip != remaining)
to trigger.

Extra allocated pages for ftrace: 2 with 654 skipped
WARNING: CPU: 0 PID: 0 at kernel/trace/ftrace.c:7295 ftrace_process_locs+0x5bf/0x5e0

A similar problem in ftrace_allocate_records() can result in allocating
too many pages. This can trigger the second warning in
ftrace_process_locs().

Extra allocated pages for ftrace
WARNING: CPU: 0 PID: 0 at kernel/trace/ftrace.c:7276 ftrace_process_locs+0x548/0x580

Use the actual capacity of a page group to determine if too many pages
have been allocated to solve the problem. Also use the actual capacity
of a page group to determine the number of pages needed to avoid over-
allocations in ftrace_allocate_records().

Fixes: 4a3efc6baff93 ("ftrace: Update the mcount_loc check of skipped entries")
Cc: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Guenter Roeck <linux@...ck-us.net>
---
 kernel/trace/ftrace.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index ef2d5dca6f70..211ec7a04f7e 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3844,7 +3844,7 @@ static int ftrace_allocate_records(struct ftrace_page *pg, int count)
 		return -EINVAL;
 
 	/* We want to fill as much as possible, with no empty pages */
-	pages = DIV_ROUND_UP(count, ENTRIES_PER_PAGE);
+	pages = DIV_ROUND_UP(count * ENTRY_SIZE, PAGE_SIZE);
 	order = fls(pages) - 1;
 
  again:
@@ -7308,24 +7308,33 @@ static int ftrace_process_locs(struct module *mod,
 		unsigned long skip;
 
 		/* Count the number of entries unused and compare it to skipped. */
-		pg_remaining = (ENTRIES_PER_PAGE << pg->order) - pg->index;
+		pg_remaining = (PAGE_SIZE << pg->order) / ENTRY_SIZE - pg->index;
 
 		if (!WARN(skipped < pg_remaining, "Extra allocated pages for ftrace")) {
+			unsigned long space = 0;
 
 			skip = skipped - pg_remaining;
 
-			for (pg = pg_unuse; pg; pg = pg->next)
+			for (pg = pg_unuse; pg; pg = pg->next) {
 				remaining += 1 << pg->order;
+				/*
+				 * The capacity of a page group is
+				 *     (PAGE_SIZE << order) / ENTRY_SIZE
+				 * Accumulate the total capacity of unused pages.
+				 */
+				space += (PAGE_SIZE << pg->order) / ENTRY_SIZE;
+			}
 
 			pages -= remaining;
 
-			skip = DIV_ROUND_UP(skip, ENTRIES_PER_PAGE);
-
 			/*
-			 * Check to see if the number of pages remaining would
-			 * just fit the number of entries skipped.
+			 * Check to see if extra pages have been allocated.
+			 * Only warn if the number of unused entries is larger
+			 * than the number of entries per page to avoid false
+			 * positives due to rounding.
 			 */
-			WARN(skip != remaining, "Extra allocated pages for ftrace: %lu with %lu skipped",
+			WARN(space - skip > ENTRIES_PER_PAGE,
+			     "Extra allocated pages for ftrace: %lu with %lu skipped",
 			     remaining, skipped);
 		}
 		/* Need to synchronize with ftrace_location_range() */
-- 
2.45.2