Re: [Qemu-devel] [PATCH 08/17] mm: madvise MADV_USERFAULT

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: "Dr. David Alan Gilbert" <dgilbert@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>,
	Andrea Arcangeli <aarcange@redhat.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>,
	"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>
Subject: Re: [Qemu-devel] [PATCH 08/17] mm: madvise MADV_USERFAULT
Date: Tue, 7 Oct 2014 13:52:45 +0300	[thread overview]
Message-ID: <20141007105245.GC30762@node.dhcp.inet.fi> (raw)
In-Reply-To: <20141007104603.GI2404@work-vm>

On Tue, Oct 07, 2014 at 11:46:04AM +0100, Dr. David Alan Gilbert wrote:
> * Kirill A. Shutemov (kirill@shutemov.name) wrote:
> > 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.
> 
> Aren't DONTNEED and DONTDUMP  similar cases of madvise operations that are
> expected to do what they say ?

No. If kernel would ignore MADV_DONTNEED or MADV_DONTDUMP it will not
affect correctness, just behaviour will be suboptimal: more than needed
memory used or wasted space in coredump.

-- 
 Kirill A. Shutemov