From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753634AbaJGLas (ORCPT ); Tue, 7 Oct 2014 07:30:48 -0400 Received: from mta-out1.inet.fi ([62.71.2.197]:48185 "EHLO jenni2.inet.fi" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753216AbaJGLap (ORCPT ); Tue, 7 Oct 2014 07:30:45 -0400 Date: Tue, 7 Oct 2014 14:30:02 +0300 From: "Kirill A. Shutemov" To: "Dr. David Alan Gilbert" Cc: Robert Love , Dave Hansen , Jan Kara , kvm@vger.kernel.org, Neil Brown , Stefan Hajnoczi , qemu-devel@nongnu.org, linux-mm@kvack.org, KOSAKI Motohiro , Michel Lespinasse , Andrea Arcangeli , Taras Glek , Juan Quintela , Hugh Dickins , Isaku Yamahata , Mel Gorman , Sasha Levin , Android Kernel Team , Andrew Jones , "Huangpeng (Peter)" , Andres Lagar-Cavilla , Christopher Covington , Anthony Liguori , Paolo Bonzini , Keith Packard , Wenchao Xia , linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, Andy Lutomirski , Minchan Kim , Dmitry Adamushko , Johannes Weiner , Mike Hommey , Andrew Morton , Linus Torvalds , Peter Feiner Subject: Re: [Qemu-devel] [PATCH 08/17] mm: madvise MADV_USERFAULT Message-ID: <20141007113002.GE30762@node.dhcp.inet.fi> References: <1412356087-16115-1-git-send-email-aarcange@redhat.com> <1412356087-16115-9-git-send-email-aarcange@redhat.com> <20141007103645.GB30762@node.dhcp.inet.fi> <20141007104603.GI2404@work-vm> <20141007105245.GC30762@node.dhcp.inet.fi> <20141007110102.GJ2404@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141007110102.GJ2404@work-vm> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 07, 2014 at 12:01:02PM +0100, Dr. David Alan Gilbert wrote: > * Kirill A. Shutemov (kirill@shutemov.name) wrote: > > 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. > > That's not how the manpage reads for DONTNEED; it calls it out as a special > case near the top, and explicitly says what will happen if you read the > area marked as DONTNEED. Your are right. MADV_DONTNEED doesn't fit the interface too. That's bad and we can't fix it. But it's not a reason to make this mistake again. Read the next sentence: "The kernel is free to ignore the advice." Note, POSIX_MADV_DONTNEED has totally different semantics. > It looks like there are openssl patches that use DONTDUMP to explicitly > make sure keys etc don't land in cores. That's nice to have. But openssl works on systems without the interface, meaning it's not essential for functionality. -- Kirill A. Shutemov