On Sat, 23 Nov 2024 22:38:59 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote: > On 11/23/24 22:35, Shakeel Butt wrote: > > On Sat, Nov 23, 2024 at 05:01:57PM +0000, Matthew Wilcox wrote: > >> On Fri, Nov 22, 2024 at 10:09:39PM -0800, Shakeel Butt wrote: > >> > TP_printk( > >> > - "mm=%p memcg_path=%s write=%s", > >> > - __entry->mm, > >> > - __get_str(memcg_path), > >> > + "mm=%p memcg_id=%llu write=%s", > >> > + __entry->mm, __entry->memcg_id, > >> > __entry->write ? "true" : "false" > >> > >> Is it actually useful to print out the (hashed) pointer of the mm? > >> Wouldn't the PID be more useful so you could actually associate it with > >> a task? > >> > > > > For our usecase i.e. bpftrace, we don't really care about these prints > > as we can directly access the arguments like mm in bpftrace. I wonder if > > others are using this hased pointer in some other way. I don't mind > > chaning it but I think that would be a separate patch. > > I wonder if it's actually hashed when trace events are obtained in binary > form, i.e. via trace-cmd. Might be hashed only when doing e.g. cat > trace_pipe as that's when the kernel's printk with its hashing is used? > > I guess that would be another argument for not using it in the tracepoint, > as it would be a sidechannel... This is no more a sidechannel than /proc/kallsyms. It is only accessible via the privileged users. It's very common and useful to show pointers in trace events. You can use eprobes to get information off of pointers too: echo 'e:mmap_lock_count mmap_lock/mmap_lock_start_locking count=+0($mm):u32' > /sys/kernel/tracing/dynamic_events and now you have an event that shows the mm_count of the mm structure. -- Steve
