mm,page_owner: display all stacks and their count

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.

Link: https://lkml.kernel.org/r/20240215215907.20121-6-osalvador@suse.de
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Marco Elver <elver@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_owner.c b/mm/page_owner.c
index df6a923af5de6cfe6deab5bb11167b98330becf5..e99fbf822dd69348ba4fdaed7f0bb8444b66ca22 100644 (file)
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -171,7 +171,13 @@ static void add_stack_record_to_list(struct stack_record *stack_record,
 
        spin_lock_irqsave(&stack_list_lock, flags);
        stack->next = stack_list;
-       stack_list = stack;
+       /*
+        * This pairs with smp_load_acquire() from function
+        * 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);
 }
 
@@ -805,8 +811,90 @@ 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);
+       } 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)
+{
+       int i, stack_count;
+       struct stack *stack = v;
+       unsigned long *entries;
+       unsigned long nr_entries;
+       struct stack_record *stack_record = stack->stack_record;
+
+       nr_entries = stack_record->size;
+       entries = stack_record->entries;
+       stack_count = refcount_read(&stack_record->count) - 1;
+
+       if (!nr_entries || nr_entries < 0 || stack_count < 1)
+               return 0;
+
+       for (i = 0; i < nr_entries; i++)
+               seq_printf(m, " %pS\n", (void *)entries[i]);
+       seq_printf(m, "stack_count: %d\n\n", stack_count);
+
+       return 0;
+}
+
+static void stack_stop(struct seq_file *m, void *v)
+{
+}
+
+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;
@@ -814,6 +902,9 @@ 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;
 }