Re: [PATCH v9 10/21] mm: Add generic p?d_leaf() macros

From: Steven Price
Date: Thu Aug 01 2019 - 08:22:41 EST

On 01/08/2019 07:09, Anshuman Khandual wrote:
> On 07/29/2019 05:08 PM, Steven Price wrote:
>> On 28/07/2019 12:44, Anshuman Khandual wrote:
>>> On 07/23/2019 03:11 PM, Mark Rutland wrote:
>>>> On Mon, Jul 22, 2019 at 04:41:59PM +0100, Steven Price wrote:
>>>>> Exposing the pud/pgd levels of the page tables to walk_page_range() means
>>>>> we may come across the exotic large mappings that come with large areas
>>>>> of contiguous memory (such as the kernel's linear map).
>>>>> For architectures that don't provide all p?d_leaf() macros, provide
>>>>> generic do nothing default that are suitable where there cannot be leaf
>>>>> pages that that level.
>>>>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>>>> Not a big deal, but it would probably make sense for this to be patch 1
>>>> in the series, given it defines the semantic of p?d_leaf(), and they're
>>>> not used until we provide all the architectural implemetnations anyway.
>>> Agreed.
>>>> It might also be worth pointing out the reasons for this naming, e.g.
>>>> p?d_large() aren't currently generic, and this name minimizes potential
>>>> confusion between p?d_{large,huge}().
>>> Agreed. But these fallback also need to first check non-availability of large
>>> pages. I am not sure whether CONFIG_HUGETLB_PAGE config being clear indicates
>>> that conclusively or not. Being a page table leaf entry has a broader meaning
>>> than a large page but that is really not the case today. All leaf entries here
>>> are large page entries from MMU perspective. This dependency can definitely be
>>> removed when there are other types of leaf entries but for now IMHO it feels
>>> bit problematic not to directly associate leaf entries with large pages in
>>> config restriction while doing exactly the same.
>> The intention here is that the page walkers are able to walk any type of
>> page table entry which the kernel may use. CONFIG_HUGETLB_PAGE only
>> controls whether "huge TLB pages" are used by user space processes. It's
>> quite possible that option to not be selected but the linear mapping to
>> have been mapped using "large pages" (i.e. leaf entries further up the
>> tree than normal).
> I understand that kernel page table might use large pages where as user space
> never enabled HugeTLB. The point to make here was CONFIG_HUGETLB approximately
> indicates the presence of large pages though the absence of same does not
> conclusively indicate that large pages are really absent on the MMU. Perhaps it
> will requires something new like MMU_[LARGE|HUGE]_PAGES.

CONFIG_HUGETLB doesn't necessarily mean leaf entries can appear anywhere
other than PTE. Some architectures always have a full tree of page
tables, but can program their TLBs with larger entries - I think all the
architectures I've come across have software page table walking, but in
theory the arm64 contiguous hint bit could be considered similar.