Re: [PATCH] mm: kfence: print the age time for alloacted objectes to trace memleak

[chenqiwu <qiwuchen55@gmail.com>]

On Sun, Aug 04, 2024 at 10:37:43AM +0200, Marco Elver wrote:
> 
> Well, what I'm saying, having this info also for FREED objects on the
> free stack can be useful in some debugging scenarios when you get a
> use-after-free, and you want to know the elapsed time since the free
> happened. I have done this calculation manually before, which is why I
> suggested it. Maybe it's not useful for you for finding leaks, but
> that's just one usecase.
>
Agreed with your concern scenarios.
How about the following change with additonal object state info?

+       u64 interval_nsec = local_clock() - meta->alloc_track.ts_nsec;
+       unsigned long rem_interval_nsec = do_div(interval_nsec, NSEC_PER_SEC);

        /* Timestamp matches printk timestamp format. */
-       seq_con_printf(seq, "%s by task %d on cpu %d at %lu.%06lus:\n",
+       seq_con_printf(seq, "%s by task %d on cpu %d at %lu.%06lus (%lu.%06lus ago) for %s object:\n",
                       show_alloc ? "allocated" : "freed", track->pid,
-                      track->cpu, (unsigned long)ts_sec, rem_nsec / 1000);
+                      track->cpu, (unsigned long)ts_sec, rem_nsec / 1000,
+                      (unsigned long)interval_nsec, rem_interval_nsec / 1000,
+                      meta->state == KFENCE_OBJECT_ALLOCATED? "allocated" : "freed");

In this way, we can find leaks by grep "allocated object" and inspect the elapsed time of
use-after-free by grep "freed object".

Thanks
Qiwu