[PATCH v4] ftrace: Fix possible warning on checking all pages used in ftrace_process_locs()

[Zheng Yejian]

As comments in ftrace_process_locs(), there may be NULL pointers in
mcount_loc section:
 > Some architecture linkers will pad between
 > the different mcount_loc sections of different
 > object files to satisfy alignments.
 > Skip any NULL pointers.

After commit 20e5227e9f55 ("ftrace: allow NULL pointers in mcount_loc"),
NULL pointers will be accounted when allocating ftrace pages but skipped
before adding into ftrace pages, this may result in some pages not being
used. Then after commit 706c81f87f84 ("ftrace: Remove extra helper
functions"), warning may occur at:
  WARN_ON(pg->next);

To fix it, only warn for case that no pointers skipped but pages not used
up, then free those unused pages after releasing ftrace_lock.

Fixes: 706c81f87f84 ("ftrace: Remove extra helper functions")
Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
---
Changes v3 [3] => v4:
  - Keep the upside-down-xmas-tree format as Steve suggested.
    Link: https://lore.kernel.org/all/20230711095802.71406422@xxxxxxxxxxxxxxxxxx/

Changes v2 [2] => v3:
  - Check NULL for 'pg->next' before assigning it to variable 'pg_unuse'.

Changes v1 [1] => v2:
  - As Steve suggested, only warn for case that no pointers skipped
    but pages not used up then free those unused pages. But I move
    the free process after releasing ftrace_lock.
    Link: https://lore.kernel.org/all/20230710104625.421c851a@xxxxxxxxxxxxxxxxxx/
  - Update commit messages about the new solution.

[1] https://lore.kernel.org/all/20230710212958.274126-1-zhengyejian1@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20230711201630.1837109-1-zhengyejian1@xxxxxxxxxx/
[3] https://lore.kernel.org/all/20230711112752.2595316-1-zhengyejian1@xxxxxxxxxx/

 kernel/trace/ftrace.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 3740aca79fe7..6fc238f6ef3e 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -6471,9 +6471,11 @@ static int ftrace_process_locs(struct module *mod,
 			       unsigned long *start,
 			       unsigned long *end)
 {
+	struct ftrace_page *pg_unuse = NULL;
 	struct ftrace_page *start_pg;
 	struct ftrace_page *pg;
 	struct dyn_ftrace *rec;
+	unsigned long skipped = 0;
 	unsigned long count;
 	unsigned long *p;
 	unsigned long addr;
@@ -6536,8 +6538,10 @@ static int ftrace_process_locs(struct module *mod,
 		 * object files to satisfy alignments.
 		 * Skip any NULL pointers.
 		 */
-		if (!addr)
+		if (!addr) {
+			skipped++;
 			continue;
+		}
 
 		end_offset = (pg->index+1) * sizeof(pg->records[0]);
 		if (end_offset > PAGE_SIZE << pg->order) {
@@ -6551,8 +6555,10 @@ static int ftrace_process_locs(struct module *mod,
 		rec->ip = addr;
 	}
 
-	/* We should have used all pages */
-	WARN_ON(pg->next);
+	if (pg->next) {
+		pg_unuse = pg->next;
+		pg->next = NULL;
+	}
 
 	/* Assign the last page to ftrace_pages */
 	ftrace_pages = pg;
@@ -6574,6 +6580,20 @@ static int ftrace_process_locs(struct module *mod,
  out:
 	mutex_unlock(&ftrace_lock);
 
+	/* We should have used all pages unless we skipped some */
+	if (pg_unuse) {
+		WARN_ON(!skipped);
+		while (pg_unuse) {
+			pg = pg_unuse;
+			pg_unuse = pg->next;
+			if (pg->records) {
+				free_pages((unsigned long)pg->records, pg->order);
+				ftrace_number_of_pages -= 1 << pg->order;
+			}
+			kfree(pg);
+			ftrace_number_of_groups--;
+		}
+	}
 	return ret;
 }
 
-- 
2.25.1