On Tue, 6 Jan 2026 16:24:28 -0800
Guenter Roeck <[email protected]> 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. Drop the code calculating the
> actual page count since it is no longer needed, and with it the warning
> backtraces. Also drop the ENTRIES_PER_PAGE definition since it is no longer
> needed either.
>
> Fixes: 4a3efc6baff93 ("ftrace: Update the mcount_loc check of skipped
> entries")
> Cc: Steven Rostedt <[email protected]>
> Signed-off-by: Guenter Roeck <[email protected]>
> ---
> 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().
>
> kernel/trace/ftrace.c | 41 +++++++++--------------------------------
> 1 file changed, 9 insertions(+), 32 deletions(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index ef2d5dca6f70..755a11f13808 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;
>
> @@ -7304,30 +7305,6 @@ 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;
> -
> - /* Count the number of entries unused and compare it to
> skipped. */
> - pg_remaining = (ENTRIES_PER_PAGE << pg->order) - pg->index;
> -
> - if (!WARN(skipped < pg_remaining, "Extra allocated pages for
> ftrace")) {
> -
> - skip = skipped - pg_remaining;
> -
> - for (pg = pg_unuse; pg; pg = pg->next)
> - remaining += 1 << pg->order;
> -
> - 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",
> - remaining, skipped);
> - }
Don't remove this block. It's still needed. A lot of entries are
skipped when adding the records. Weak functions and zero'd pointers
that were part of the count are skipped. This is the code that handles
that. It has nothing to do with rounding errors.
-- Steve
> /* Need to synchronize with ftrace_location_range() */
> synchronize_rcu();
> ftrace_free_pages(pg_unuse);
