From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3D7EFC3526F for ; Wed, 15 Dec 2021 16:01:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244038AbhLOPnl (ORCPT ); Wed, 15 Dec 2021 10:43:41 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:30871 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244022AbhLOPnl (ORCPT ); Wed, 15 Dec 2021 10:43:41 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1639583020; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=PUFi5HYw/QsmVtr4Iq57nJgL1HfI/Nq8EZnbybSwxkw=; b=gk9QuNhMW/5ovQepZdMW4q+C4GT2eVPTpAt+3KtmSeBwZTW3BWBOpi3Mj8QnM8kp8lYkGQ NszC9t9vQg7ZCyQ/vxB7XBa7KQd/9gQjUtFvQFe1taM39rwHlBqB7DSppvpC8fXljcxGz/ ygjHqHoinKeotP5OJUDbGGZM5rbNqGs= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-303-8jIwBgHyM02sDIafyuGRpg-1; Wed, 15 Dec 2021 10:43:37 -0500 X-MC-Unique: 8jIwBgHyM02sDIafyuGRpg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 742DB81426A; Wed, 15 Dec 2021 15:43:34 +0000 (UTC) Received: from horse.redhat.com (unknown [10.22.16.227]) by smtp.corp.redhat.com (Postfix) with ESMTP id CCC3E196F1; Wed, 15 Dec 2021 15:43:33 +0000 (UTC) Received: by horse.redhat.com (Postfix, from userid 10451) id 194412206B8; Wed, 15 Dec 2021 10:43:33 -0500 (EST) Date: Wed, 15 Dec 2021 10:43:33 -0500 From: Vivek Goyal To: Stefan Hajnoczi Cc: Dan Williams , Christoph Hellwig , Vishal Verma , Dave Jiang , Alasdair Kergon , Mike Snitzer , Ira Weiny , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Miklos Szeredi , Matthew Wilcox , device-mapper development , Linux NVDIMM , linux-s390 , linux-fsdevel , virtualization@lists.linux-foundation.org Subject: Re: [PATCH 4/5] dax: remove the copy_from_iter and copy_to_iter methods Message-ID: References: <20211209063828.18944-1-hch@lst.de> <20211209063828.18944-5-hch@lst.de> <20211213082318.GB21462@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Wed, Dec 15, 2021 at 10:30:50AM +0000, Stefan Hajnoczi wrote: > On Tue, Dec 14, 2021 at 03:32:43PM -0500, Vivek Goyal wrote: > > On Tue, Dec 14, 2021 at 08:41:30AM -0800, Dan Williams wrote: > > > On Tue, Dec 14, 2021 at 6:23 AM Vivek Goyal wrote: > > > > > > > > On Mon, Dec 13, 2021 at 09:23:18AM +0100, Christoph Hellwig wrote: > > > > > On Sun, Dec 12, 2021 at 06:44:26AM -0800, Dan Williams wrote: > > > > > > On Fri, Dec 10, 2021 at 6:17 AM Vivek Goyal wrote: > > > > > > > Going forward, I am wondering should virtiofs use flushcache version as > > > > > > > well. What if host filesystem is using DAX and mapping persistent memory > > > > > > > pfn directly into qemu address space. I have never tested that. > > > > > > > > > > > > > > Right now we are relying on applications to do fsync/msync on virtiofs > > > > > > > for data persistence. > > > > > > > > > > > > This sounds like it would need coordination with a paravirtualized > > > > > > driver that can indicate whether the host side is pmem or not, like > > > > > > the virtio_pmem driver. However, if the guest sends any fsync/msync > > > > > > you would still need to go explicitly cache flush any dirty page > > > > > > because you can't necessarily trust that the guest did that already. > > > > > > > > > > Do we? The application can't really know what backend it is on, so > > > > > it sounds like the current virtiofs implementation doesn't really, does it? > > > > > > > > Agreed that application does not know what backend it is on. So virtiofs > > > > just offers regular posix API where applications have to do fsync/msync > > > > for data persistence. No support for mmap(MAP_SYNC). We don't offer persistent > > > > memory programming model on virtiofs. That's not the expectation. DAX > > > > is used only to bypass guest page cache. > > > > > > > > With this assumption, I think we might not have to use flushcache version > > > > at all even if shared filesystem is on persistent memory on host. > > > > > > > > - We mmap() host files into qemu address space. So any dax store in virtiofs > > > > should make corresponding pages dirty in page cache on host and when > > > > and fsync()/msync() comes later, it should flush all the data to PMEM. > > > > > > > > - In case of file extending writes, virtiofs falls back to regular > > > > FUSE_WRITE path (and not use DAX), and in that case host pmem driver > > > > should make sure writes are flushed to pmem immediately. > > > > > > > > Are there any other path I am missing. If not, looks like we might not > > > > have to use flushcache version in virtiofs at all as long as we are not > > > > offering guest applications user space flushes and MAP_SYNC support. > > > > > > > > We still might have to use machine check safe variant though as loads > > > > might generate synchronous machine check. What's not clear to me is > > > > that if this MC safe variant should be used only in case of PMEM or > > > > should it be used in case of non-PMEM as well. > > > > > > It should be used on any memory address that can throw exception on > > > load, which is any physical address, in paths that can tolerate > > > memcpy() returning an error code, most I/O paths, and can tolerate > > > slower copy performance on older platforms that do not support MC > > > recovery with fast string operations, to date that's only PMEM users. > > > > Ok, So basically latest cpus can do fast string operations with MC > > recovery so that using MC safe variant is not a problem. > > > > Then there is range of cpus which can do MC recovery but do slower > > versions of memcpy and that's where the issue is. > > > > So if we knew that virtiofs dax window is backed by a pmem device > > then we should always use MC safe variant. Even if it means paying > > the price of slow version for the sake of correctness. > > > > But if we are not using pmem on host, then there is no point in > > using MC safe variant. > > > > IOW. > > > > if (virtiofs_backed_by_pmem) { > > use_mc_safe_version > > else > > use_non_mc_safe_version > > } > > > > Now question is, how do we know if virtiofs dax window is backed by > > a pmem or not. I checked virtio_pmem driver and that does not seem > > to communicate anything like that. It just communicates start of the > > range and size of range, nothing else. > > > > I don't have full handle on stack of modules of virtio_pmem, but my guess > > is it probably is using MC safe version always (because it does not > > know anthing about the backing storage). > > > > /me will definitely like to pay penalty of slower memcpy if virtiofs > > device is not backed by a pmem. > > Reads from the page cache handle machine checks (filemap_read() -> > raw_copy_to_user()). I think virtiofs should therefore always handle > machine checks when reading from the DAX Window. IIUC, raw_copy_to_user() does not handle recovery from machine check. For example, it can call copy_user_enhanced_fast_string() if cpu supports X86_FEATURE_ERMS. But equivalent machine check safe version is copy_mc_enhanced_fast_string() instead. Hence, I don't think reading from page cache is using machine check safe variants by default. This copy_mc_to_user() path has to be taken explicitly for machine check safe variants. And currently only pmem driver seems to take it by calling _copy_mc_to_iter(). Thanks Vivek