From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:33928) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1glUb6-0004gx-Lw for qemu-devel@nongnu.org; Mon, 21 Jan 2019 03:06:25 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1glUS7-0002sh-TQ for qemu-devel@nongnu.org; Mon, 21 Jan 2019 02:57:04 -0500 Received: from mga06.intel.com ([134.134.136.31]:21203) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1glUS7-0002rj-LK for qemu-devel@nongnu.org; Mon, 21 Jan 2019 02:57:03 -0500 Date: Mon, 21 Jan 2019 15:57:18 +0800 From: Yi Zhang Message-ID: <20190121075717.GA24848@tiger-server> References: <20190116103615-mutt-send-email-mst@kernel.org> <20190121061143.GB21562@tiger-server> <790023026.65652877.1548055299697.JavaMail.zimbra@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <790023026.65652877.1548055299697.JavaMail.zimbra@redhat.com> Subject: Re: [Qemu-devel] [PATCH V9 6/6] docs: Added MAP_SYNC documentation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Pankaj Gupta Cc: "Michael S. Tsirkin" , xiaoguangrong eric , stefanha@redhat.com, pbonzini@redhat.com, yu c zhang , ehabkost@redhat.com, qemu-devel@nongnu.org, imammedo@redhat.com, dan j williams On 2019-01-21 at 02:21:39 -0500, Pankaj Gupta wrote: > > > > > --- > > > > docs/nvdimm.txt | 21 ++++++++++++++++++++- > > > > qemu-options.hx | 4 ++++ > > > > 2 files changed, 24 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt > > > > index 5f158a6..565ba73 100644 > > > > --- a/docs/nvdimm.txt > > > > +++ b/docs/nvdimm.txt > > > > @@ -142,11 +142,30 @@ backend of vNVDIMM: > > > > Guest Data Persistence > > > > ---------------------- > > > > > > > > +vNVDIMM is designed and implemented to guarantee the guest data > > > > +persistence on the backends even on the host crash and power > > > > > > in case of a host crash or a power failure > > > > > > (be consistent) > > > > > > > +failures. However, there are still some requirements and limitations > > > > +as explained below. > > > > + > > > > Though QEMU supports multiple types of vNVDIMM backends on Linux, > > > > -currently the only one that can guarantee the guest write persistence > > > > +if MAP_SYNC is not supported by the host kernel and the backends, > > > > +the only backend that can guarantee the guest write persistence > > > > is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to > > > > which all guest access do not involve any host-side kernel cache. > > > > > > > > +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such > > > > +systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure > > > > > > which ensures > > > > > > > +filesystem metadata consistent > > > > > > consistency > > > > > > > even after a system crash or power > > > > > > a power > > > > > > > +failure. Besides the host kernel support, enabling MAP_SYNC in QEMU > > > > +also requires: > > > > + > > > > + - the backend is a file supporting DAX, e.g., a file on an ext4 or > > > > + xfs file system mounted with '-o dax', > > > > + > > > > + - 'share' option of memory-backend-file is 'on'. > > > > > > why does share need to be on? > > According to mmap(2) man page, > > MAP_SYNC is available only with the MAP_SHARED_VALIDATE map‐ping type. > > I will add this to documentation, Thanks. > > MAP_SHARE_VALIDATE should be used to validate MAP_SYNC with MAP_SHARED. > > #define MAP_SHARED_VALIDATE 0x03 > > In contrast, MAP_SHARED is: > #define MAP_SHARED 0x01 /* Share changes */ > > I think we should use MAP_SHARED_VALIDATE with MAP_SYNC here? > > Thanks, > Pankaj Yes, Pankaj, Thanks point that out, I will add the MAP_SHARED_VALIDATE. > > > > > > > > + > > > > + - 'pmem' option of memory-backend-file is 'on' > > > > + > > > > When using other types of backends, it's suggested to set 'unarmed' > > > > option of '-device nvdimm' to 'on', which sets the unarmed flag of the > > > > guest NVDIMM region mapping structure. This unarmed flag indicates > > > > diff --git a/qemu-options.hx b/qemu-options.hx > > > > index 08f8516..545cb8a 100644 > > > > --- a/qemu-options.hx > > > > +++ b/qemu-options.hx > > > > @@ -4002,6 +4002,10 @@ using the SNIA NVM programming model (e.g. Intel > > > > NVDIMM). > > > > If @option{pmem} is set to 'on', QEMU will take necessary operations to > > > > guarantee the persistence of its own writes to @option{mem-path} > > > > (e.g. in vNVDIMM label emulation and live migration). > > > > +Also, we will map the backend-file with MAP_SYNC flag, which can ensure > > > > +the file metadata is in sync to @option{mem-path} even on the host crash > > > > +and power failures. MAP_SYNC requires supports from both the host kernel > > > > > > requires support > > > > > > > +(since Linux kernel 4.15) and @option{mem-path} (only files supporting > > > > DAX). > > > > > > > > @item -object > > > > memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave} > > > > > > > > -- > > > > 2.7.4 > >