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Date: Fri, 22 May 2015 14:18:30 -0700
From: Andrew Morton <akpm@linux-foundation.org>
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Subject: Re: [Qemu-devel] [PATCH 22/23] userfaultfd: avoid mmap_sem read
 recursion in mcopy_atomic
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>, zhang.zhanghailiang@huawei.com, kvm@vger.kernel.org, Pavel Emelyanov <xemul@parallels.com>, linux-api@vger.kernel.org, Johannes Weiner <hannes@cmpxchg.org>, Dave Hansen <dave.hansen@intel.com>, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, linux-mm@kvack.org, Andres Lagar-Cavilla <andreslc@google.com>, Mel Gorman <mgorman@suse.de>, Paolo Bonzini <pbonzini@redhat.com>, "Kirill A. Shutemov" <kirill@shutemov.name>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, Sanidhya Kashyap <sanidhya.gatech@gmail.com>, Linus Torvalds <torvalds@linux-foundation.org>, Andy Lutomirski <luto@amacapital.net>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Peter Feiner <pfeiner@google.com>


There's a more serious failure with i386 allmodconfig:

fs/userfaultfd.c:145:2: note: in expansion of macro 'BUILD_BUG_ON'
  BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);

I'm surprised the feature is even reachable on i386 builds?