On Thu, Jan 04, 2018 at 09:23:28AM +0800, Haozhong Zhang wrote: > On 01/03/18 11:45 -0200, Eduardo Habkost wrote: > > On Wed, Jan 03, 2018 at 11:16:39AM +0800, Haozhong Zhang wrote: > > > On 01/02/18 18:02 +0200, Michael S. Tsirkin wrote: > > > > On Wed, Dec 27, 2017 at 02:56:20PM +0800, Haozhong Zhang wrote: > > > > > When a file supporting DAX is used as vNVDIMM backend, mmap it with > > > > > MAP_SYNC flag in addition can guarantee the persistence of guest write > > > > > to the backend file without other QEMU actions (e.g., periodic fsync() > > > > > by QEMU). > > > > > > > > > > By using MAP_SHARED_VALIDATE flag with MAP_SYNC, we can ensure mmap > > > > > with MAP_SYNC fails if MAP_SYNC is not supported by the kernel or the > > > > > backend file. On such failures, QEMU retries mmap without MAP_SYNC and > > > > > MAP_SHARED_VALIDATE. > > > > > > > > > > Signed-off-by: Haozhong Zhang <haozhong.zh...@intel.com> > > > > > > > > If users rely on MAP_SYNC then don't you need to fail allocation > > > > if you can't use it? > > > > > > MAP_SYNC is supported since Linux kernel 4.15 and only needed for mmap > > > files on nvdimm. qemu_ram_mmap() has no way to check whether its > > > parameter 'fd' points to files on nvdimm, except by looking up > > > sysfs. However, accessing sysfs may be denied by certain SELinux > > > policies. > > > > > > The missing of MAP_SYNC should not affect the primary functionality of > > > vNVDIMM when using files on host nvdimm as backend, except the > > > guarantee of write persistence in case of qemu/host crash. > > > > > > We may check the kernel support of MAP_SYNC and the type of vNVDIMM > > > backend in some management utility (e.g., libvirt?), and deny to > > > launch QEMU if MAP_SYNC is not supported while files on host NVDIMM > > > are in use. > > > > Instead of making libvirt check if MAP_SYNC is supported and just > > hope it won't fail, it would be safer to let libvirt tell QEMU > > that MAP_SYNC must never fail. > > For example, add an option "sync" to memory-backend-file, and pass the > it to qemu_ram_mmap()?
Yes. It could be a OnOffAuto option, "auto" would make QEMU try to use MAP_SYNC but not fail if it's unavailable. "on" would make QEMU ensure MAP_SYNC is really enabled. > > > > > However, it looks like kernel 4.14 won't even fail if MAP_SYNC is > > specified. How exactly can userspace detect if MAP_SYNC is > > really supported? > > Use MAP_SYNC with MAP_SHARED_VALIDATE (both introduced in 4.15 > kernel). Linux kernel 4.15 and later validate whether the MAP_SYNC is > supported. Because MAP_SHARED_VALIDATE is defined equally to > (MAP_SHARED | MAP_PRIVATE), it always fails on older kernels which do > not support MAP_SYNC as well. Nice. > > If we agree to introduce an option "sync" or likelihood, we can do the > above check in qemu_ram_mmap(). Sounds good to me. Thanks! -- Eduardo