Re: [PATCH RFC v2 00/20] mm: optimize unnecessary loads due to ptep_get() and friends out

[Yeoreum Yun <yeoreum.yun@arm.com>]

On Mon, Jul 27, 2026 at 05:18:00PM +0200, David Hildenbrand (Arm) wrote:
> On 7/22/26 19:30, Yeoreum Yun wrote:
> >> One of my nits about v1 was that it didn't really frame the series
> >> precisely. It talks about "optimization" instead of fundamentally
> >> changing the way that the kernel deals with folded page table levels.
> >>
> >> The optimization that this *wants* to talk about kinda falls out of that
> >> refactoring. It's not really the core of the series, IMNHO.
> >>
> >> This series effectively changes the rules about where and how folded
> >> page table levels can be manipulated. In doing that, the door is opened
> >> to optimizing out the unnecessary loads.
> > 
> > I see. might the title and the cover letter should be reparsed.
> > Thanks to remind it again :)
> 
> While v1 changed "where and how folded page table levels can be manipulated"
> more drastically, v2 leaves most of set_pgd/... working for now, and really only
> changes how pgd_get() operates.
> 
> With v2, there is mostly only a change for page table walkers that use
> pgd_get(): while the existing model already made all these levels be skipped in
> most places except in ptdump code, we now also have to skip them in ptdump code.
> 
> So yes, that should be clearly spelled out: as long as pgd_get() is not
> involved, set_pgd()/pgd_page_vaddr()... will keep working as is.
> 
> The moment someone switches to pgd_get() with folded page tables (like riscv
> already does), set_pgd() is disallowed, and modifications on the actual level
> must be performed. As discussed, a common helper for vmalloc faults would
> already minimize the pain quite a lot in that scenario.
> 
> While we should make that clearer in the patch description, I am wondering
> whether we should also add some basic MM documentation for folded page tables,
> where we at least explain the current state-of-the-art (what's ignored when
> traversing, why does "no_p4d" actually mean "pgd helpers are dummies and p4d
> helpers implement the real logic").

I see, while I'm trying to spell out those, let me see the MM doc
to check whether we need to explain more about this.

-- 
Sincerely,
Yeoreum Yun