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Message-ID: <1e74c6d8-ae74-49c2-bdc4-d9880110ab57@roeck-us.net>
Date: Tue, 11 Jun 2024 16:53:43 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Vincent Donnefort <vdonnefort@...gle.com>,
Joel Fernandes <joel@...lfernandes.org>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
suleiman@...gle.com, Thomas Gleixner <tglx@...utronix.de>,
Vineeth Pillai <vineeth@...byteword.org>,
Youssef Esmat <youssefesmat@...gle.com>,
Beau Belgrave <beaub@...ux.microsoft.com>, Alexander Graf <graf@...zon.com>,
Baoquan He <bhe@...hat.com>, Borislav Petkov <bp@...en8.de>,
"Paul E. McKenney" <paulmck@...nel.org>, David Howells
<dhowells@...hat.com>, Mike Rapoport <rppt@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>, Tony Luck <tony.luck@...el.com>,
Ross Zwisler <zwisler@...gle.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v4 01/13] ring-buffer: Allow mapped field to be set
without mapping
On 6/11/24 15:53, Steven Rostedt wrote:
> On Tue, 11 Jun 2024 15:43:59 -0700
> Guenter Roeck <linux@...ck-us.net> wrote:
>
>> On 6/11/24 12:28, Steven Rostedt wrote:
>>> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
>>>
>>> In preparation for having the ring buffer mapped to a dedicated location,
>>> which will have the same restrictions as user space memory mapped buffers,
>>> allow it to use the "mapped" field of the ring_buffer_per_cpu structure
>>> without having the user space meta page mapping.
>>>
>>> When this starts using the mapped field, it will need to handle adding a
>>> user space mapping (and removing it) from a ring buffer that is using a
>>> dedicated memory range.
>>>
>>> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
>>> ---
>>> kernel/trace/ring_buffer.c | 11 ++++++++---
>>> 1 file changed, 8 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
>>> index 28853966aa9a..78beaccf9c8c 100644
>>> --- a/kernel/trace/ring_buffer.c
>>> +++ b/kernel/trace/ring_buffer.c
>>> @@ -5224,6 +5224,9 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
>>> {
>>> struct trace_buffer_meta *meta = cpu_buffer->meta_page;
>>>
>>> + if (!meta)
>>> + return;
>>> +
>>> meta->reader.read = cpu_buffer->reader_page->read;
>>> meta->reader.id = cpu_buffer->reader_page->id;
>>> meta->reader.lost_events = cpu_buffer->lost_events;
>>> @@ -6167,7 +6170,7 @@ rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)
>>>
>>> mutex_lock(&cpu_buffer->mapping_lock);
>>>
>>> - if (!cpu_buffer->mapped) {
>>> + if (!cpu_buffer->mapped || !cpu_buffer->meta_page) {
>>> mutex_unlock(&cpu_buffer->mapping_lock);
>>> return ERR_PTR(-ENODEV);
>>> }
>>> @@ -6359,12 +6362,13 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
>>> */
>>> raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>>> rb_setup_ids_meta_page(cpu_buffer, subbuf_ids);
>>> +
>>
>> Picky again. Is that a leftover from something ? I don't see an immediate reason
>> for the added newline.
>
> Hmm, I could remove it.
>
>>
>>> raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>>>
>>> err = __rb_map_vma(cpu_buffer, vma);
>>> if (!err) {
>>> raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>>> - cpu_buffer->mapped = 1;
>>> + cpu_buffer->mapped++;
>>> raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>>> } else {
>>> kfree(cpu_buffer->subbuf_ids);
>>> @@ -6403,7 +6407,8 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
>>> mutex_lock(&buffer->mutex);
>>> raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>>>
>>> - cpu_buffer->mapped = 0;
>>> + WARN_ON_ONCE(!cpu_buffer->mapped);
>>> + cpu_buffer->mapped--;
>>
>> This will wrap to UINT_MAX if it was 0. Is that intentional ?
>
> If mapped is non zero, it limits what it can do. If it enters here as zero,
> we are really in a unknown state, so yeah, wrapping will just keep it
> limited. Which is a good thing.
>
> Do you want me to add a comment there?
>
Maybe. I just wondered if something like
if (!WARN_ON_ONCE(!cpu_buffer->mapped))
cpu_buffer->mapped--;
would be better than wrapping because 'mapped' is used as flag elsewhere,
but then I can see that it is also manipulated in __rb_inc_dec_mapped(),
and that it is checked against UINT_MAX there (and not decremented if it is 0).
Maybe explain why sometimes __rb_inc_dec_mapped() is called to increment
or decrement ->mapped, and sometimes it id done directly ? I can see that
the function also acquires the buffer mutex, which isn't needed at the places
where mapped is incremented/decremented directly, but common code would
still be nice, and it is odd to see over/underflows handled sometimes but
not always.
Thanks,
Guenter
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