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
