Re: [PATCH v3] ftrace: Do not over-allocate ftrace memory

[Guenter Roeck]

Hi Steven,

On 1/6/26 18:22, Guenter Roeck wrote:

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 the number of pages
to allocate. Have ftrace_allocate_pages() return the number of allocated
pages to avoid having to calculate it. Use the actual page group capacity
when validating the number of unused pages due to skipped entries.
Drop the definition of ENTRIES_PER_PAGE since it is no longer used.

Fixes: 4a3efc6baff93 ("ftrace: Update the mcount_loc check of skipped entries")
Cc: Steven Rostedt <rost...@goodmis.org>
Signed-off-by: Guenter Roeck <li...@roeck-us.net>
---
v3: Restore the code calculating the number of unused pages due to skipped
     entries. Use the actual number of entries per page group when
     calculating the number of entries in unused page groups.
v2: Have ftrace_allocate_pages() return the number of allocated pages,
     and drop the page count calculation code as well as the associated
     warnings from ftrace_process_locs().

Any additional feedback / comments / suggestions ?

Thanks,
Guenter

  kernel/trace/ftrace.c | 29 +++++++++++++++--------------
  1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index ef2d5dca6f70..2c9692b45215 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1148,7 +1148,6 @@ struct ftrace_page {
  };

#define ENTRY_SIZE sizeof(struct dyn_ftrace)

-#define ENTRIES_PER_PAGE (PAGE_SIZE / ENTRY_SIZE)

static struct ftrace_page *ftrace_pages_start;

  static struct ftrace_page     *ftrace_pages;
@@ -3834,7 +3833,8 @@ static int ftrace_update_code(struct module *mod, struct 
ftrace_page *new_pgs)
        return 0;
  }

-static int ftrace_allocate_records(struct ftrace_page *pg, int count)

+static int ftrace_allocate_records(struct ftrace_page *pg, int count,
+                                  unsigned long *num_pages)
  {
        int order;
        int pages;
@@ -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:

@@ -3859,6 +3859,7 @@ static int ftrace_allocate_records(struct ftrace_page 
*pg, int count)
        }

ftrace_number_of_pages += 1 << order;

+       *num_pages += 1 << order;
        ftrace_number_of_groups++;

cnt = (PAGE_SIZE << order) / ENTRY_SIZE;

@@ -3887,12 +3888,14 @@ static void ftrace_free_pages(struct ftrace_page *pages)
  }

static struct ftrace_page *

-ftrace_allocate_pages(unsigned long num_to_init)
+ftrace_allocate_pages(unsigned long num_to_init, unsigned long *pages)
  {
        struct ftrace_page *start_pg;
        struct ftrace_page *pg;
        int cnt;

+ *pages = 0;

+
        if (!num_to_init)
                return NULL;

@@ -3906,7 +3909,7 @@ ftrace_allocate_pages(unsigned long num_to_init)

         * waste as little space as possible.
         */
        for (;;) {
-               cnt = ftrace_allocate_records(pg, num_to_init);
+               cnt = ftrace_allocate_records(pg, num_to_init, pages);
                if (cnt < 0)
                        goto free_pages;

@@ -7192,8 +7195,6 @@ static int ftrace_process_locs(struct module *mod,

        if (!count)
                return 0;

- pages = DIV_ROUND_UP(count, ENTRIES_PER_PAGE);

-
        /*
         * Sorting mcount in vmlinux at build time depend on
         * CONFIG_BUILDTIME_MCOUNT_SORT, while mcount loc in
@@ -7206,7 +7207,7 @@ static int ftrace_process_locs(struct module *mod,
                test_is_sorted(start, count);
        }

- start_pg = ftrace_allocate_pages(count);

+       start_pg = ftrace_allocate_pages(count, &pages);
        if (!start_pg)
                return -ENOMEM;

@@ -7305,27 +7306,27 @@ static int ftrace_process_locs(struct module *mod,

        /* We should have used all pages unless we skipped some */
        if (pg_unuse) {
                unsigned long pg_remaining, remaining = 0;
-               unsigned long skip;
+               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")) { skip = skipped - pg_remaining; - for (pg = pg_unuse; pg; pg = pg->next)

+                       for (pg = pg_unuse; pg && skip > 0; pg = pg->next) {
                                remaining += 1 << pg->order;
+                               skip -= (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.
                         */
-                       WARN(skip != remaining, "Extra allocated pages for ftrace: 
%lu with %lu skipped",
+                       WARN(pg || skip > 0, "Extra allocated pages for ftrace: %lu 
with %lu skipped",
                             remaining, skipped);
                }
                /* Need to synchronize with ftrace_location_range() */