From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:48534) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gj85L-00076G-3m for qemu-devel@nongnu.org; Mon, 14 Jan 2019 14:39:47 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gj85K-0001kj-4m for qemu-devel@nongnu.org; Mon, 14 Jan 2019 14:39:47 -0500 Received: from mx1.redhat.com ([209.132.183.28]:59070) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gj85J-0001kC-SJ for qemu-devel@nongnu.org; Mon, 14 Jan 2019 14:39:46 -0500 Date: Mon, 14 Jan 2019 17:39:38 -0200 From: Eduardo Habkost Message-ID: <20190114193938.GG28115@habkost.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH V8 5/5] hostmem-file: add 'sync' option List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Zhang Yi Cc: xiaoguangrong.eric@gmail.com, stefanha@redhat.com, pbonzini@redhat.com, pagupta@redhat.com, yu.c.zhang@linux.intel.com, mst@redhat.com, imammedo@redhat.com, dan.j.williams@intel.com, qemu-devel@nongnu.org On Wed, Jan 02, 2019 at 01:26:34PM +0800, Zhang Yi wrote: > This option controls will mmap the memory backend file with MAP_SYNC flag, > which can ensure filesystem metadata consistent even after a system crash > or power failure, if MAP_SYNC flag is supported by the host kernel(Linux > kernel 4.15 and later) and the backend is a file supporting DAX (e.g., > file on ext4/xfs file system mounted with '-o dax'). > > It can take one of following values: > - on: try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or > 'share=off' or 'pmem!=on', QEMU will not pass this flags to > mmap(2) > - off: default, never pass MAP_SYNC to mmap(2) > > Signed-off-by: Haozhong Zhang > Signed-off-by: Zhang Yi > --- [...] > +vNVDIMM is designed and implemented to guarantee the guest data > +persistence on the backends even on the host crash and power > +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 > +filesystem metadata consistent even after a system crash or 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', > + > + - 'sync' option of memory-backend-file is on, and > + > + - 'share' option of memory-backend-file is 'on'. > + > + - 'pmem' option of memory-backend-file is 'on' I miss one piece of information here: are there any negative side-effects of enabling MAP_SYNC on a pmem=on backend? Could it affect performance? If it has no negative effects, why don't we try to always enable it whenever possible? > + > 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/util/mmap-alloc.c b/util/mmap-alloc.c > index a9d5e56..33a7639 100644 > --- a/util/mmap-alloc.c > +++ b/util/mmap-alloc.c > @@ -99,7 +99,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags) > void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); > #endif > bool shared = flags & RAM_SHARED; > - bool is_pmem = flags & RAM_PMEM; > + bool is_pmemsync = (flags & RAM_PMEM) && (flags & RAM_SYNC); You seem to be reverting what you did on patch 3/5. In patch 3/5, you were setting MAP_SYNC automatically on all pmem=on backends. Now, you are only setting MAP_SYNC only if sync=on is set explicitly. I don't know which behavior is better (see question above), but it's better to start with the right behavior in the first place. Also, I don't think we should clear MAP_SYNC silently if sync=on was explicitly requested in the command-line. If sync=on was set, we should do exactly as told, and require MAP_SYNC. If we still want to support use cases where MAP_SYNC is desired but optional (do we?), we can make 'sync' a OnOffAuto option. > int mmap_xflags = 0; > size_t offset; > void *ptr1; > @@ -111,7 +111,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags) > assert(is_power_of_2(align)); > /* Always align to host page size */ > assert(align >= getpagesize()); > - if (shared && is_pmem) { > + if (shared && is_pmemsync) { > mmap_xflags |= MAP_SYNC; > } > > -- > 2.7.4 > > -- Eduardo