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

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Message-ID: <YrYGwVf8qbbBA62z@casper.infradead.org>
Date:   Fri, 24 Jun 2022 19:47:29 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     James Houghton <jthoughton@...gle.com>
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 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?