From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:38541) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gkYcZ-0006ys-Me for qemu-devel@nongnu.org; Fri, 18 Jan 2019 13:12:01 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gkYcY-0001Eh-MA for qemu-devel@nongnu.org; Fri, 18 Jan 2019 13:11:59 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56312) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gkYcY-0001D5-DS for qemu-devel@nongnu.org; Fri, 18 Jan 2019 13:11:58 -0500 Date: Fri, 18 Jan 2019 16:11:47 -0200 From: Eduardo Habkost Message-ID: <20190118181147.GJ4136@habkost.net> References: <64bea1ff5f80647cc4592ee94d399d647bdd9862.1547624239.git.yi.z.zhang@linux.intel.com> <20190116104105-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190116104105-mutt-send-email-mst@kernel.org> Subject: Re: [Qemu-devel] [PATCH V9 4/6] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Zhang Yi , pagupta@redhat.com, qemu-devel@nongnu.org, yu.c.zhang@linux.intel.com, stefanha@redhat.com, imammedo@redhat.com, pbonzini@redhat.com, dan.j.williams@intel.com, xiaoguangrong.eric@gmail.com On Wed, Jan 16, 2019 at 10:58:44AM -0500, Michael S. Tsirkin wrote: > On Wed, Jan 16, 2019 at 04:10:58PM +0800, Zhang Yi wrote: > > When a file supporting DAX is used as vNVDIMM backend, mmap it with > > MAP_SYNC flag in addition which can ensure file system metadata > > synced in each guest writes to the backend file, without other QEMU > > actions (e.g., periodic fsync() by QEMU). > > > > Signed-off-by: Haozhong Zhang > > Signed-off-by: Zhang Yi > > --- > > include/qemu/mmap-alloc.h | 1 + > > include/qemu/osdep.h | 16 ++++++++++++++++ > > util/mmap-alloc.c | 7 ++++++- > > 3 files changed, 23 insertions(+), 1 deletion(-) > > > > diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h > > index 6fe6ed4..a95d91c 100644 > > --- a/include/qemu/mmap-alloc.h > > +++ b/include/qemu/mmap-alloc.h > > @@ -18,6 +18,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path); > > * @flags: specifies additional properties of the mapping, which can be one or > > * bit-or of following values > > * - RAM_SHARED: mmap with MAP_SHARED flag > > + * - RAM_PMEM: mmap with MAP_SYNC flag > > * Other bits are ignored. > > * > > * Return: > > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h > > index 457d24e..27a6bfe 100644 > > --- a/include/qemu/osdep.h > > +++ b/include/qemu/osdep.h > > @@ -419,6 +419,22 @@ void qemu_anon_ram_free(void *ptr, size_t size); > > # define QEMU_VMALLOC_ALIGN getpagesize() > > #endif > > > > +/* > > + * MAP_SHARED_VALIDATE and MAP_SYNC are introduced in Linux kernel > > + * 4.15, so they may not be defined when compiling on older kernels. > > + */ > > +#ifdef CONFIG_LINUX > > + > > +#include > > I suspect this is a wrong way to pull in this header. > > You are normally supposed to use > #include > > but see below. > > > > + > > +#ifndef MAP_SYNC > > +#define MAP_SYNC 0x0 > > +#endif > > Oh that's bad. > > So if you run with a new kernel but > your installed headers are old, you get MAP_SYNC 0 > and no persistence transparently with no warning. Yes. The semantics of the command-line to not change depending on build time circumstances. Anyway, I see a more fundamental problem in each version of this patch: the semantics of the command-line options are not clearly documented. We have at least 3 different possible use cases we might need to support: 1) pmem=on, MAP_SYNC not desired 2) pmem=on, MAP_SYNC desired but optional 3) pmem=on, MAP_SYNC required, not optional Which cases from the list above we need to support? >>From the cases above, what's the expected semantics of "pmem=on" with no extra options? If these questions are not answered (in the commit message and user documentation), we won't be able to review and discuss the code. > > > + > > +#else /* !CONFIG_LINUX */ > > +#define MAP_SYNC 0x0 > > +#endif /* CONFIG_LINUX */ > > + > > #ifdef CONFIG_POSIX > > struct qemu_signalfd_siginfo { > > uint32_t ssi_signo; /* Signal number */ > > diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c > > index 8f0a740..cba961c 100644 > > --- a/util/mmap-alloc.c > > +++ b/util/mmap-alloc.c > > @@ -99,6 +99,8 @@ 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; > > + int mmap_xflags = 0; > > size_t offset; > > void *ptr1; > > > > @@ -109,12 +111,15 @@ 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) { > > + mmap_xflags |= MAP_SYNC; > > + } > > > > offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr; > > ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE, > > MAP_FIXED | > > (fd == -1 ? MAP_ANONYMOUS : 0) | > > - (shared ? MAP_SHARED : MAP_PRIVATE), > > + (shared ? MAP_SHARED : MAP_PRIVATE) | mmap_xflags, > > fd, 0); > > if (ptr1 == MAP_FAILED) { > > munmap(ptr, total); > > -- > > 2.7.4 > -- Eduardo