linux-kernel - Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped

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Date:   Tue, 22 Aug 2023 16:22:50 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
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Subject: Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp()
 versus uffd

On Tue, Aug 22, 2023 at 04:39:43PM +0200, Jann Horn wrote:
> > Perhaps something else will want that same behaviour in future (it's
> > tempting, but difficult to guarantee correctness); for now, it is just
> > userfaultfd (but by saying "_armed" rather than "_missing", I'm half-
> > expecting uffd to add more such exceptional modes in future).
> 
> Hm, yeah, sounds okay. (I guess we'd also run into this if we ever
> wanted to make it possible to reliably install PTE markers with
> madvise() or something like that, which might be nice for allowing
> userspace to create guard pages without unnecessary extra VMAs...)

I don't know what a userspace API for this would look like, but I have
a dream of creating guard VMAs which only live in the maple tree and
don't require the allocation of a struct VMA.  Use some magic reserved
pointer value like XA_ZERO_ENTRY to represent them ... seems more
robust than putting a PTE marker in the page tables?