linux-kernel - Re: [PATCH v3] userfaultfd/shmem: fix MCOPY_ATOMIC

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Date:   Wed, 31 Mar 2021 08:54:09 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.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 v3] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTNUE behavior

Axel,

On Tue, Mar 30, 2021 at 04:30:13PM -0700, Axel Rasmussen wrote:
> Yes, a refactor like that is promising. It's hard to say for certain
> without actually looking at the result - I'll spend some time tomorrow
> on a few options, and send along the cleanest version I come up with.

Before you move onto a new version...  See this commit:

5b51072e97d5 ("userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem", 2018-11-30)

I found it when I was thinking why not move the whole continue logic directly
into mfill_atomic_pte(), if we can have the pte installation helper, because
that's all we need.

So previously I got the semantics a bit mixed up: for private shmem mappings,
UFFDIO_COPY won't fill in page cache at all, but it's all private.  We keep the
page cache empty even after UFFDIO_COPY for a private mapping.

UFFDIO_CONTINUE is slightly different, since we _know_ the page cache is
there..  So I'm thinking maybe you need to handle the continue request in
mfill_atomic_pte() before the VM_SHARED check so as to cover both cases.

-- 
Peter Xu