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Date:   Fri, 12 Feb 2021 17:44:05 -0500
From:   Peter Xu <>
To:     Matthew Wilcox <>
Cc:     Axel Rasmussen <>,
        Alexander Viro <>,
        Alexey Dobriyan <>,
        Andrea Arcangeli <>,
        Andrew Morton <>,
        Anshuman Khandual <>,
        Catalin Marinas <>,
        Chinwen Chang <>,
        Huang Ying <>,
        Ingo Molnar <>, Jann Horn <>,
        Jerome Glisse <>,
        Lokesh Gidra <>,
        Michael Ellerman <>,
        Michal Koutný <>,
        Michel Lespinasse <>,
        Mike Kravetz <>,
        Mike Rapoport <>,
        Nicholas Piggin <>, Shaohua Li <>,
        Shawn Anastasio <>,
        Steven Rostedt <>,
        Steven Price <>,
        Vlastimil Babka <>,
        LKML <>,, Linux MM <>,
        Adam Ruprecht <>,
        Cannon Matthews <>,
        "Dr . David Alan Gilbert" <>,
        David Rientjes <>,
        Mina Almasry <>,
        Oliver Upton <>
Subject: Re: [PATCH v5 05/10] userfaultfd: add minor fault registration mode

On Fri, Feb 12, 2021 at 10:21:45PM +0000, Matthew Wilcox wrote:
> On Thu, Feb 11, 2021 at 11:28:09AM -0800, Axel Rasmussen wrote:
> > Ah, I had added this just after VM_UFFD_WP, without noticing that this
> > would be sharing a bit with VM_LOCKED. That seems like not such a
> > great idea.
> > 
> > I don't see another unused bit, and I don't see some other obvious
> > candidate to share with. So, the solution that comes to mind is
> it'd be even better if you didn't use the last unused bit for UFFD_WP.
> not sure how feasible that is, but you can see we're really short on
> bits here.

UFFD_WP is used now for anonymouse already.. And the support for hugetlbfs and
shmem is in rfc stage on the list.

Is it possible to use CONFIG_ARCH_USES_HIGH_VMA_FLAGS here?  So far uffd-wp is
only working for 64 bit x86 too due to enlarged pte space.  Maybe we can also
let minor mode to only support 64 bit hosts.


Peter Xu

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