On Mon, 12 Feb 2024 at 23:29, Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> This patch adds a new directory called 'page_owner_stacks' under
> /sys/kernel/debug/, with a file called 'show_stacks' in it.
> Reading from that file will show all stacks that were added by page_owner
> followed by their counting, giving us a clear overview of stack <-> count
> relationship.
>
> E.g:
>
> prep_new_page+0xa9/0x120
> get_page_from_freelist+0x801/0x2210
> __alloc_pages+0x18b/0x350
> alloc_pages_mpol+0x91/0x1f0
> folio_alloc+0x14/0x50
> filemap_alloc_folio+0xb2/0x100
> __filemap_get_folio+0x14a/0x490
> ext4_write_begin+0xbd/0x4b0 [ext4]
> generic_perform_write+0xc1/0x1e0
> ext4_buffered_write_iter+0x68/0xe0 [ext4]
> ext4_file_write_iter+0x70/0x740 [ext4]
> vfs_write+0x33d/0x420
> ksys_write+0xa5/0xe0
> do_syscall_64+0x80/0x160
> entry_SYSCALL_64_after_hwframe+0x6e/0x76
> stack_count: 4578
>
> The seq stack_{start,next} functions will iterate through the list
> stack_list in order to print all stacks.
>
> Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
Minor comments below.
> ---
> mm/page_owner.c | 99 ++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 98 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 7d1b3f75cef3..3e4b7cd7c8f8 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -84,7 +84,12 @@ static void add_stack_record_to_list(struct stack_record *stack_record)
> stack_list = stack;
> } else {
> stack->next = stack_list;
> - stack_list = stack;
> + /* This pairs with smp_load_acquire() from function
Comment should be
/*
*
...
*/
(Unless in networking or other special subsystems with their own comment style.)
> + * stack_start(). This guarantees that stack_start()
> + * will see an updated stack_list before starting to
> + * traverse the list.
> + */
> + smp_store_release(&stack_list, stack);
> }
> spin_unlock_irqrestore(&stack_list_lock, flags);
> }
> @@ -792,8 +797,97 @@ static const struct file_operations proc_page_owner_operations = {
> .llseek = lseek_page_owner,
> };
>
> +static void *stack_start(struct seq_file *m, loff_t *ppos)
> +{
> + struct stack *stack;
> +
> + if (*ppos == -1UL)
> + return NULL;
> +
> + if (!*ppos) {
> + /*
> + * This pairs with smp_store_release() from function
> + * add_stack_record_to_list(), so we get a consistent
> + * value of stack_list.
> + */
> + stack = smp_load_acquire(&stack_list);
I'm not sure if it'd make your code simpler or not: there is
<linux/llist.h> for singly-linked linked lists, although the code to
manage the list is simple enough I'm indifferent here. Only consider
it if it helps you make the code simpler.
> + } else {
> + stack = m->private;
> + stack = stack->next;
> + }
> +
> + m->private = stack;
> +
> + return stack;
> +}
> +
> +static void *stack_next(struct seq_file *m, void *v, loff_t *ppos)
> +{
> + struct stack *stack = v;
> +
> + stack = stack->next;
> + *ppos = stack ? *ppos + 1 : -1UL;
> + m->private = stack;
> +
> + return stack;
> +}
> +
> +static int stack_print(struct seq_file *m, void *v)
> +{
> + char *buf;
> + int ret = 0;
> + struct stack *stack = v;
> + struct stack_record *stack_record = stack->stack_record;
> +
> + if (!stack_record->size || stack_record->size < 0 ||
> + refcount_read(&stack_record->count) < 2)
> + return 0;
> +
> + buf = kzalloc(PAGE_SIZE, GFP_KERNEL);
> +
> + ret += stack_trace_snprint(buf, PAGE_SIZE, stack_record->entries,
> + stack_record->size, 0);
> + if (!ret)
> + goto out;
> +
> + scnprintf(buf + ret, PAGE_SIZE - ret, "stack_count: %d\n\n",
> + refcount_read(&stack_record->count));
> +
> + seq_printf(m, buf);
> + seq_puts(m, "\n\n");
> +out:
> + kfree(buf);
> +
> + return 0;
> +}
> +
> +static void stack_stop(struct seq_file *m, void *v)
> +{
> +}
Is this function even needed if it's empty? I recall there were some
boilerplate "nop" functions that could be used.
> +static const struct seq_operations page_owner_stack_op = {
> + .start = stack_start,
> + .next = stack_next,
> + .stop = stack_stop,
> + .show = stack_print
> +};
> +
> +static int page_owner_stack_open(struct inode *inode, struct file *file)
> +{
> + return seq_open_private(file, &page_owner_stack_op, 0);
> +}
> +
> +static const struct file_operations page_owner_stack_operations = {
> + .open = page_owner_stack_open,
> + .read = seq_read,
> + .llseek = seq_lseek,
> + .release = seq_release,
> +};
> +
> static int __init pageowner_init(void)
> {
> + struct dentry *dir;
> +
> if (!static_branch_unlikely(&page_owner_inited)) {
> pr_info("page_owner is disabled\n");
> return 0;
> @@ -801,6 +895,9 @@ static int __init pageowner_init(void)
>
> debugfs_create_file("page_owner", 0400, NULL, NULL,
> &proc_page_owner_operations);
> + dir = debugfs_create_dir("page_owner_stacks", NULL);
> + debugfs_create_file("show_stacks", 0400, dir, NULL,
> + &page_owner_stack_operations);
>
> return 0;
> }
> --
> 2.43.0
>
