Re: [PATCH v2 03/13] mm: Add generic p?d_large() macros

[Steven Price]

On 21/02/2019 14:57, Kirill A. Shutemov wrote:
> On Thu, Feb 21, 2019 at 02:46:18PM +0000, Steven Price wrote:
>> On 21/02/2019 14:28, Kirill A. Shutemov wrote:
>>> On Thu, Feb 21, 2019 at 11:34:52AM +0000, Steven Price wrote:
>>>> From: James Morse <james.morse@xxxxxxx>
>>>>
>>>> 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 p?d_large() macros, provided a
>>>> does nothing default.
>>>
>>> Nak, sorry.
>>>
>>> Power will get broken by the patch. It has pmd_large() inline function,
>>> that will be overwritten by the define from this patch.
>>>
>>> I believe it requires more ground work on arch side in general.
>>> All architectures that has huge page support has to provide these helpers
>>> (and matching defines) before you can use it in a generic code.
>>
>> Sorry about that, I had compile tested on power, but obviously not the
>> right config to actually see the breakage.
> 
> I don't think you'll catch it at compile-time. It would silently override
> the helper with always-false.

Ah, that might explain why I missed it.

>> I'll do some grepping - hopefully this is just a case of exposing the
>> functions/defines that already exist for those architectures.
> 
> I see the same type of breakage on s390 and sparc.
> 
>> Note that in terms of the new page walking code, these new defines are
>> only used when walking a page table without a VMA (which isn't currently
>> done), so architectures which don't use p?d_large currently will work
>> fine with the generic versions. They only need to provide meaningful
>> definitions when switching to use the walk-without-a-VMA functionality.
> 
> How other architectures would know that they need to provide the helpers
> to get walk-without-a-VMA functionality? This looks very fragile to me.

Yes, you've got a good point there. This would apply to the p?d_large
macros as well - any arch which (inadvertently) uses the generic version
is likely to be fragile/broken.

I think probably the best option here is to scrap the generic versions
altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which
would enable the new functionality to those arches that opt-in. Do you
think this would be less fragile?

Thanks,

Steve