linux-kernel - Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC

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Date:   Wed, 7 Apr 2021 15:50:54 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Peter Xu <peterx@...hat.com>, Shaohua Li <shli@...com>,
        Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Wang Qing <wangqing@...o.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        linux-kselftest@...r.kernel.org, Brian Geffon <bgeffon@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE
 behavior

On Wed, 7 Apr 2021, Axel Rasmussen wrote:
> Agreed about taking one direction or the other further.
> 
> I get the sense that Peter prefers the mcopy_atomic_install_ptes()
> version, and would thus prefer to just expose that and let
> shmem_mcopy_atomic_pte() use it.
> 
> But, I get the sense that you (Hugh) slightly prefer the other way -
> just letting shmem_mcopy_atomic_pte() deal with both the VM_SHARED and
> !VM_SHARED cases.

No, either direction seems plausible to me: start from whichever
end you prefer.

> 
> I was planning to write "I prefer option X because (reasons), and
> objections?" but I'm realizing that it isn't really clear to me which
> route would end up being cleaner. I think I have to just pick one,
> write it out, and see where I end up. If it ends up gross, I don't
> mind backtracking and taking the other route. :) To that end, I'll
> proceed by having shmem_mcopy_atomic_pte() call the new
> mcopy_atomic_install_ptes() helper, and see how it looks (unless there
> are objections).

I am pleased to read that: it's exactly how I would approach it -
so it must be right :-)

Hugh