Re: [PATCH RESEND] mm/pagewalk: split walk_page_range_novma() into kernel/user parts

[David Hildenbrand <david@redhat.com>]

On 04.06.25 10:07, Mike Rapoport wrote:
> On Wed, Jun 04, 2025 at 09:39:30AM +0200, David Hildenbrand wrote:
>> On 03.06.25 21:22, Lorenzo Stoakes wrote:
>>> The walk_page_range_novma() function is rather confusing - it supports two
>>> modes, one used often, the other used only for debugging.
>>>
>>> The first mode is the common case of traversal of kernel page tables, which
>>> is what nearly all callers use this for.
>>
>> ... and what people should be using it for 🙂
>>
>>>
>>> Secondly it provides an unusual debugging interface that allows for the
>>> traversal of page tables in a userland range of memory even for that memory
>>> which is not described by a VMA.
>>>
>>> This is highly unusual and it is far from certain that such page tables
>>> should even exist, but perhaps this is precisely why it is useful as a
>>> debugging mechanism.
>>>
>>> As a result, this is utilised by ptdump only. Historically, things were
>>> reversed - ptdump was the only user, and other parts of the kernel evolved
>>> to use the kernel page table walking here.
>>>
>>> Since we have some complicated and confusing locking rules for the novma
>>> case, it makes sense to separate the two usages into their own functions.
>>>
>>> Doing this also provide self-documentation as to the intent of the caller -
>>> are they doing something rather unusual or are they simply doing a standard
>>> kernel page table walk?
>>>
>>> We therefore maintain walk_page_range_novma() for this single usage, and
>>> document the function as such.
>>
>> If we have to keep this dangerous interface, it should probably be
>>
>> walk_page_range_debug() or walk_page_range_dump()
> 
> We can also move it from include/linux/pagewalk.h to mm/internal.h

Agreed.

-- 
Cheers,

David / dhildenb