From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759754AbdEVNpA (ORCPT ); Mon, 22 May 2017 09:45:00 -0400 Received: from mail-wm0-f50.google.com ([74.125.82.50]:37644 "EHLO mail-wm0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757929AbdEVNo7 (ORCPT ); Mon, 22 May 2017 09:44:59 -0400 Date: Mon, 22 May 2017 16:44:56 +0300 From: "Kirill A. Shutemov" To: Mike Rapoport Cc: Andrew Morton , Arnd Bergmann , "Kirill A. Shutemov" , Andrea Arcangeli , Pavel Emelyanov , linux-mm , lkml Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Message-ID: <20170522134456.ig2tgf2spbuq55ig@node.shutemov.name> References: <1495433562-26625-1-git-send-email-rppt@linux.vnet.ibm.com> <20170522114243.2wrdbncilozygbpl@node.shutemov.name> <20170522133559.GE27382@rapoport-lnx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170522133559.GE27382@rapoport-lnx> User-Agent: NeoMutt/20170428 (1.8.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 22, 2017 at 04:36:00PM +0300, Mike Rapoport wrote: > On Mon, May 22, 2017 at 02:42:43PM +0300, Kirill A. Shutemov wrote: > > On Mon, May 22, 2017 at 09:12:42AM +0300, Mike Rapoport wrote: > > > Currently applications can explicitly enable or disable THP for a memory > > > region using MADV_HUGEPAGE or MADV_NOHUGEPAGE. However, once either of > > > these advises is used, the region will always have > > > VM_HUGEPAGE/VM_NOHUGEPAGE flag set in vma->vm_flags. > > > The MADV_CLR_HUGEPAGE resets both these flags and allows managing THP in > > > the region according to system-wide settings. > > > > Seems reasonable. But could you describe an use-case when it's useful in > > real world. > > My use-case was combination of pre- and post-copy migration of containers > with CRIU. > In this case we populate a part of a memory region with data that was saved > during the pre-copy stage. Afterwards, the region is registered with > userfaultfd and we expect to get page faults for the parts of the region > that were not yet populated. However, khugepaged collapses the pages and > the page faults we would expect do not occur. > > We could have used MADV_NOHUGEPAGE before populating the region with the > pre-copy data, but then, in the end, the restored application will be resumed > with vma->vm_flags different from the ones it had when it was frozen. > > Another possibility I've considered was to register the region with > userfaultfd before populating it with data, but in that case we get the > overhead of UFFD_EVENT_PAGEFAULT + UFFDIO_{COPY,ZEROPAGE} for nothing :( Okay. Makes sense. Feel free to use my Acked-by (with change to RESET). -- Kirill A. Shutemov