Re: [Qemu-devel] [PATCH] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

[Eduardo Habkost <ehabkost@redhat.com>]

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.zhang@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.

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?

-- 
Eduardo