Re: [RFC 1/2] lib/vsprintf: Add support for pte_t

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 19/06/25 6:38 PM, Petr Mladek wrote:
> On Wed 2025-06-18 19:16:00, Pedro Falcato wrote:
>> On Wed, Jun 18, 2025 at 10:44:20AM +0200, David Hildenbrand wrote:
>>> On 18.06.25 10:37, Anshuman Khandual wrote:
>>>>
>>>>
>>>> On 18/06/25 1:48 PM, David Hildenbrand wrote:
>>>>> On 18.06.25 06:12, Anshuman Khandual wrote:
>>>>>> Add a new format for printing page table entries.
>>>>>>
>>>>>> Cc: Petr Mladek <pmladek@suse.com>
>>>>>> Cc: Steven Rostedt <rostedt@goodmis.org>
>>>>>> Cc: Jonathan Corbet <corbet@lwn.net>
>>>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>>>> Cc: David Hildenbrand <david@redhat.com>
>>>>>> Cc: linux-doc@vger.kernel.org
>>>>>> Cc: linux-kernel@vger.kernel.org
>>>>>> Cc: linux-mm@kvack.org
>>>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>>>>>> ---
>>>>>>    Documentation/core-api/printk-formats.rst | 14 ++++++++++++++
>>>>>>    lib/vsprintf.c                            | 20 ++++++++++++++++++++
>>>>>>    mm/memory.c                               |  5 ++---
>>>>>>    scripts/checkpatch.pl                     |  2 +-
>>>>>>    4 files changed, 37 insertions(+), 4 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
>>>>>> index 4b7f3646ec6ce..75a110b059ee1 100644
>>>>>> --- a/Documentation/core-api/printk-formats.rst
>>>>>> +++ b/Documentation/core-api/printk-formats.rst
>>>>>> @@ -689,6 +689,20 @@ Rust
>>>>>>    Only intended to be used from Rust code to format ``core::fmt::Arguments``.
>>>>>>    Do *not* use it from C.
>>>>>>    +Page Table Entry
>>>>>> +----------------
>>>>>> +
>>>>>> +::
>>>>>> +        %ppte
>>>>>> +
>>>>>> +Print standard page table entry pte_t.
>>>>>> +
>>>>>> +Passed by reference.
>>>>>
>>>>> Curious, why the decision to pass by reference?
>>>>
>>>> Just to make this via %p<> based address mechanism. But wondering
>>>> will it be better for the pte to be represented via value instead
>>>> of reference ?
>>>
>>> We commonly pass ptes to functions through value, not reference, that's why
>>> I am asking.
>>
>>
>> All printf/printk extensions in the kernel follow %p<some letters> and use
>> pointers because %p takes pointers, so it lets us use -Wformat with no issues.
>>
>> So yes, taking a pte_t * is required.
> 
> Correct. But the pointer is usually needed because the %pxx format
> need to access a structure.

Right.

> 
> Passing a pointer is another potential source of errors. I mean that
> the callers might pass an invalid pointer by mistake...

Agreed - could be a source of error when not used properly.

> 
> Another aspect is performance. It is likely not a big deal for classic
> printk() which is a slow path. But trace_printk() tries to optimize
> the speed by deferred formatting where possible, see vbin_printf()
> and bstr_printf().
> 
> I think that this is not a blocker for this patchset. But you should
> know that using %pxx has a cost.
Got it - thanks for the explanation.