Message-ID: <f1f290fc-b2f0-483b-96d5-5995362e5a8b@redhat.com>
Date: Mon, 25 Aug 2025 15:20:13 +0200
From: David Hildenbrand <david@...hat.com>
To: Eugen Hristev <eugen.hristev@...aro.org>, Michal Hocko <mhocko@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
 linux-arch@...r.kernel.org, linux-mm@...ck.org, tglx@...utronix.de,
 andersson@...nel.org, pmladek@...e.com,
 linux-arm-kernel@...ts.infradead.org, linux-hardening@...r.kernel.org,
 corbet@....net, mojha@....qualcomm.com, rostedt@...dmis.org,
 jonechou@...gle.com, tudor.ambarus@...aro.org,
 Christoph Hellwig <hch@...radead.org>,
 Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [RFC][PATCH v2 22/29] mm/numa: Register information into Kmemdump


>>
>> IIRC, kernel/vmcore_info.c is never built as a module, as it also
>> accesses non-exported symbols.
> 
> Hello David,
> 
> I am looking again into this, and there are some things which in my
> opinion would be difficult to achieve.
> For example I looked into my patch #11 , which adds the `runqueues` into
> kmemdump.
> 
> The runqueues is a variable of `struct rq` which is defined in
> kernel/sched/sched.h , which is not supposed to be included outside of
> sched.
> Now moving all the struct definition outside of sched.h into another
> public header would be rather painful and I don't think it's a really
> good option (The struct would be needed to compute the sizeof inside
> vmcoreinfo). Secondly, it would also imply moving all the nested struct
> definitions outside as well. I doubt this is something that we want for
> the sched subsys. How the subsys is designed, out of my understanding,
> is to keep these internal structs opaque outside of it.

All the kmemdump module needs is a start and a length, correct? So the 
only tricky part is getting the length.

One could just add a const variable that holds this information, or even 
better, a simple helper function to calculate that.

Maybe someone else reading along has a better idea.

Interestingly, runqueues is a percpu variable, which makes me wonder if 
what you had would work as intended (maybe it does, not sure).

> 
>  From my perspective it's much simpler and cleaner to just add the
> kmemdump annotation macro inside the sched/core.c as it's done in my
> patch. This macro translates to a noop if kmemdump is not selected.

I really don't like how we are spreading kmemdump all over the kernel, 
and adding complexity with __section when really, all we need is a place 
to obtain a start and a length.

So we should explore if there is anything easier possible.

>>
>>>
>>> So I am unsure whether just removing the static and adding them into
>>> header files would be more acceptable.
>>>
>>> Added in CC Cristoph Hellwig and Sergey Senozhatsky maybe they could
>>> tell us directly whether they like or dislike this approach, as kmemdump
>>> would be builtin and would not require exports.
>>>
>>> One other thing to mention is the fact that the printk code dynamically
>>> allocates memory that would need to be registered. There is no mechanism
>>> for kmemdump to know when this process has been completed (or even if it
>>> was at all, because it happens on demand in certain conditions).
>>
>> If we are talking about memblock allocations, they sure are finished at
>> the time ... the buddy is up.
>>
>> So it's just a matter of placing yourself late in the init stage where
>> the buddy is already up and running.
>>
>> I assume dumping any dynamically allocated stuff through the buddy is
>> out of the picture for now.
>>
> 
> The dumping mechanism needs to work for dynamically allocated stuff, and
> right now, it works for e.g. printk, if the buffer is dynamically
> allocated later on in the boot process.

You are talking about the memblock_alloc() result, correct? Like

new_log_buf = memblock_alloc(new_log_buf_len, LOG_ALIGN);

The current version is always stored in

static char *log_buf = __log_buf;


Once early boot is done and memblock gets torn down, you can just use 
log_buf and be sure that it will not change anymore.

> 
> To have this working outside of printk, it would be required to walk
> through all the printk structs/allocations and select the required info.
> Is this something that we want to do outside of printk ?

I don't follow, please elaborate.

How is e.g., log_buf_len_get() + log_buf_addr_get() not sufficient, 
given that you run your initialization after setup_log_buf() ?


-- 
Cheers

David / dhildenb

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