Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753434AbbBRAxt (ORCPT ); Tue, 17 Feb 2015 19:53:49 -0500 Received: from mail-yh0-f54.google.com ([209.85.213.54]:35448 "EHLO mail-yh0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753107AbbBRAxq (ORCPT ); Tue, 17 Feb 2015 19:53:46 -0500 MIME-Version: 1.0 In-Reply-To: <20150217123212.GA6362@redhat.com> References: <1423842599-5174-1-git-send-email-imammedo@redhat.com> <20150217090242.GA20254@redhat.com> <54E31F24.1060705@redhat.com> <20150217123212.GA6362@redhat.com> Date: Tue, 17 Feb 2015 16:53:45 -0800 Message-ID: Subject: Re: [PATCH] vhost: support upto 509 memory regions From: Eric Northup To: "Michael S. Tsirkin" Cc: Paolo Bonzini , Igor Mammedov , Linux Kernel Mailing List , KVM , netdev@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1789 Lines: 57 On Tue, Feb 17, 2015 at 4:32 AM, Michael S. Tsirkin wrote: > On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: >> >> >> On 17/02/2015 10:02, Michael S. Tsirkin wrote: >> > > Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 >> > > to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. >> > > >> > > Signed-off-by: Igor Mammedov >> > >> > This scares me a bit: each region is 32byte, we are talking >> > a 16K allocation that userspace can trigger. >> >> What's bad with a 16K allocation? > > It fails when memory is fragmented. > >> > How does kvm handle this issue? >> >> It doesn't. >> >> Paolo > > I'm guessing kvm doesn't do memory scans on data path, > vhost does. > > qemu is just doing things that kernel didn't expect it to need. > > Instead, I suggest reducing number of GPA<->HVA mappings: > > you have GPA 1,5,7 > map them at HVA 11,15,17 > then you can have 1 slot: 1->11 > > To avoid libc reusing the memory holes, reserve them with MAP_NORESERVE > or something like this. This works beautifully when host virtual address bits are more plentiful than guest physical address bits. Not all architectures have that property, though. > We can discuss smarter lookup algorithms but I'd rather > userspace didn't do things that we then have to > work around in kernel. > > > -- > MST > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/