linux-kernel - Re: [PATCH v5 05/10] userfaultfd: add minor fault registration mode

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Date:   Fri, 12 Feb 2021 17:44:05 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Koutný <mkoutny@...e.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>, Shaohua Li <shli@...com>,
        Shawn Anastasio <shawn@...stas.io>,
        Steven Rostedt <rostedt@...dmis.org>,
        Steven Price <steven.price@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Adam Ruprecht <ruprecht@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
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.

Thanks,

-- 
Peter Xu