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

From: Kirill A. Shutemov
Date: Thu Feb 21 2019 - 09:57:13 EST

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.

> 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.

Kirill A. Shutemov