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Date: Tue, 7 Oct 2014 13:36:45 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
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Subject: Re: [Qemu-devel] [PATCH 08/17] mm: madvise MADV_USERFAULT
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Robert Love <rlove@google.com>, Dave Hansen <dave@sr71.net>, Jan Kara <jack@suse.cz>, kvm@vger.kernel.org, Neil Brown <neilb@suse.de>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, linux-mm@kvack.org, KOSAKI Motohiro <kosaki.motohiro@gmail.com>, Michel Lespinasse <walken@google.com>, Taras Glek <tglek@mozilla.com>, Andrew Jones <drjones@redhat.com>, Juan Quintela <quintela@redhat.com>, Hugh Dickins <hughd@google.com>, Isaku Yamahata <yamahata@valinux.co.jp>, Mel Gorman <mgorman@suse.de>, Sasha Levin <sasha.levin@oracle.com>, Android Kernel Team <kernel-team@android.com>, "\\\"Dr. David Alan Gilbert\\\"" <dgilbert@redhat.com>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, Andres Lagar-Cavilla <andreslc@google.com>, Christopher Covington <cov@codeaurora.org>, Anthony Liguori <anthony@codemonkey.ws>, Mike Hommey <mh@glandium.org>, Keith Packard <keithp@keithp.com>, Wenchao Xia <wenchaoqemu@gmail.com>, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>, Minchan Kim <minchan@kernel.org>, Dmitry Adamushko <dmitry.adamushko@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Paolo Bonzini <pbonzini@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>, Peter Feiner <pfeiner@google.com>

On Fri, Oct 03, 2014 at 07:07:58PM +0200, Andrea Arcangeli wrote:
> MADV_USERFAULT is a new madvise flag that will set VM_USERFAULT in the
> vma flags. Whenever VM_USERFAULT is set in an anonymous vma, if
> userland touches a still unmapped virtual address, a sigbus signal is
> sent instead of allocating a new page. The sigbus signal handler will
> then resolve the page fault in userland by calling the
> remap_anon_pages syscall.

Hm. I wounder if this functionality really fits madvise(2) interface: as
far as I understand it, it provides a way to give a *hint* to kernel which
may or may not trigger an action from kernel side. I don't think an
application will behaive reasonably if kernel ignore the *advise* and will
not send SIGBUS, but allocate memory.

I would suggest to consider to use some other interface for the
functionality: a new syscall or, perhaps, mprotect().

-- 
 Kirill A. Shutemov