linux-kernel - Re: [RFC PATCH 00/26] hugetlb: Introduce HugeTLB high-granularity mapping

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Message-ID: <CADrL8HWQsoBnPu+8u+inPvgf8QLjp5npnPAvUQR88FeEBkdJsA@mail.gmail.com>
Date:   Mon, 27 Jun 2022 09:48:28 -0700
From:   James Houghton <jthoughton@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Jue Wang <juew@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/26] hugetlb: Introduce HugeTLB high-granularity mapping

On Fri, Jun 24, 2022 at 11:47 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Fri, Jun 24, 2022 at 05:36:30PM +0000, James Houghton wrote:
> > - Page table walking and manipulation
> > A new function, hugetlb_walk_to, handles walking HugeTLB page tables for
> > high-granularity mappings. Eventually, it's possible to merge
> > hugetlb_walk_to with huge_pte_offset and huge_pte_alloc.
> >
> > We keep track of HugeTLB page table entries with a new struct, hugetlb_pte.
> > This is because we generally need to know the "size" of a PTE (previously
> > always just huge_page_size(hstate)).
> >
> > For every page table manipulation function that has a huge version (e.g.
> > huge_ptep_get and ptep_get), there is a wrapper for it (e.g.
> > hugetlb_ptep_get).  The correct version is used depending on if a HugeTLB
> > PTE really is "huge".
>
> I'm disappointed to hear that page table walking is going to become even
> more special.  I'd much prefer it if hugetlb walking were exactly the
> same as THP walking.  This seems like a good time to do at least some
> of that work.
>
> Was there a reason you chose the "more complexity" direction?

I chose this direction because it seemed to be the most
straightforward to get to a working prototype and then to an RFC. I
agree with your sentiment -- I'll see what I can do to reconcile THP
walking with HugeTLB(+HGM) walking.