linux-kernel - Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb

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Message-ID: <CADrL8HV6wuwJ+yPV2S4YkOT=V7Wmi_=Jk_pZE0NO6ZFVY1PgJw@mail.gmail.com>
Date:   Thu, 12 Jan 2023 11:55:51 -0500
From:   James Houghton <jthoughton@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        "Zach O'Keefe" <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb_range

> The original approach was implemented in RFC v1, but the
> implementation was broken: the way refcount was handled was wrong; it
> was incremented once for each new page table mapping. (How?
> find_lock_page(), called once per hugetlb_no_page/UFFDIO_CONTINUE
> would increment refcount and we wouldn't drop it, and in
> __unmap_hugepage_range(), the mmu_gather bits would decrement the
> refcount once per mapping.)
>
> At the time, I figured the complexity of handling mapcount AND
> refcount correctly in the original approach would be quite complex, so
> I switched to the new one.

Sorry I didn't make this clear... the following steps are how we could
correctly implement the original approach.

> 1. In places that already change the mapcount, check that we're
> installing the hstate-level PTE, not a high-granularity PTE. Adjust
> mapcount AND refcount appropriately.
> 2. In the HGM walking bits, to the caller if we made the hstate-level
> PTE present. (hugetlb_[pmd,pte]_alloc is the source of truth.) Need to
> keep track of this until we figure out which page we're allocating
> PTEs for, then change mapcount/refcount appropriately.
> 3. In unmapping bits, change mmu_gather/tlb bits to drop refcount only
> once per hugepage. (This is probably the hardest of these three things
> to get right.)