linux-kernel - Re: [PATCH] ref_tracker: add stack_depot_save() failure handling to ref_tracker

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Date:   Tue, 30 May 2023 09:22:50 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ref_tracker: add stack_depot_save() failure handling to ref_tracker_alloc()

On Tue, May 30, 2023 at 4:05 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sat, 27 May 2023 20:04:11 +0900 Tetsuo Handa wrote:
> > stack_depot_save() cannot accept __GFP_NOFAIL flag because
> > __stack_depot_save() drops gfp flags which are not in
> > GFP_KERNEL | GFP_ATOMIC | __GFP_NOWARN. Also, changing
> > __stack_depot_save() to accept __GFP_NOFAIL is not possible
> > because rmqueue() does not want __GFP_NOFAIL flag for
> > order == DEPOT_POOL_ORDER allocation request.
> >
> > Therefore, assume that stack_depot_save(GFP_KERNEL | __GFP_NOFAIL) from
> > ref_tracker_alloc() can silently fail, and emit "unreliable refcount
> > tracker." message.
>
> It's probably a good idea to CC netdev@...r. I'm not sure if anyone
> will pick this up from LKML.
>
> For the patch itself - I'm not sure it's needed, even if we don't
> record the stack we'll have a tracker object and still detect the leak.
> So printing the "unreliable refcount" message is not very precise.
> At least to me; Eric's opinion matters most.

Thanks Jakub (I was on a 3-days week end, computer turned off)

This patch looks wrong to me, or at very least not complete ?

If we really want this, why not remove all the code dealing with
tracker->alloc_stack_handle
being potentially NULL ?

Note: I never assumed stack_depot_save() would enforce/use NOFAIL.

diff --git a/lib/ref_tracker.c b/lib/ref_tracker.c
index dc7b14aa3431e2bf7a97a7e78220f04da144563d..530c51ab31f227a64e1210d108e9780f0bad72f7
100644
--- a/lib/ref_tracker.c
+++ b/lib/ref_tracker.c
@@ -29,8 +29,7 @@ void ref_tracker_dir_exit(struct ref_tracker_dir *dir)
        }
        list_for_each_entry_safe(tracker, n, &dir->list, head) {
                pr_err("leaked reference.\n");
-               if (tracker->alloc_stack_handle)
-                       stack_depot_print(tracker->alloc_stack_handle);
+               stack_depot_print(tracker->alloc_stack_handle);
                leak = true;
                list_del(&tracker->head);
                kfree(tracker);
@@ -53,8 +52,7 @@ void ref_tracker_dir_print(struct ref_tracker_dir *dir,
        list_for_each_entry(tracker, &dir->list, head) {
                if (i < display_limit) {
                        pr_err("leaked reference.\n");
-                       if (tracker->alloc_stack_handle)
-                               stack_depot_print(tracker->alloc_stack_handle);
+                       stack_depot_print(tracker->alloc_stack_handle);
                        i++;
                } else {
                        break;
@@ -124,10 +122,8 @@ int ref_tracker_free(struct ref_tracker_dir *dir,
        spin_lock_irqsave(&dir->lock, flags);
        if (tracker->dead) {
                pr_err("reference already released.\n");
-               if (tracker->alloc_stack_handle) {
-                       pr_err("allocated in:\n");
-                       stack_depot_print(tracker->alloc_stack_handle);
-               }
+               pr_err("allocated in:\n");
+               stack_depot_print(tracker->alloc_stack_handle);
                if (tracker->free_stack_handle) {
                        pr_err("freed in:\n");
                        stack_depot_print(tracker->free_stack_handle);