linux-kernel - Re: [PATCH 0/9] userfaultfd: add minor fault handling

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Message-ID: <20210121223703.GH260413@xz-x1>
Date:   Thu, 21 Jan 2021 17:37:03 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     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>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        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>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH 0/9] userfaultfd: add minor fault handling

On Thu, Jan 21, 2021 at 02:13:50PM -0800, Axel Rasmussen wrote:
> When I wrote this, my thinking was that users of this feature would
> have two mappings, one of which is not UFFD registered at all. So, to
> replace the existing page contents, userspace would just write to the
> non-UFFD mapping (with memcpy() or whatever else, or we could get
> fancy and imagine using some RDMA technology to copy the page over the
> network from the live migration source directly in place). After
> performing the write, we just UFFDIO_CONTINUE.
> 
> I believe FALLOC_FL_PUNCH_HOLE / MADV_REMOVE doesn't work with
> hugetlbfs? Once shmem support is implemented, I would expect
> FALLOC_FL_PUNCH_HOLE + UFFDIO_COPY to work, but I wonder if such an
> operation would be more expensive than just copying using the other
> side of the shared mapping?

IIUC hugetlb supports that (hugetlbfs_punch_hole()).  But I agree with you on
what you said should be good enough.  Thanks,

-- 
Peter Xu