Re: [Qemu-devel] [PATCH V9 6/6] docs: Added MAP_SYNC documentation

[Yi Zhang <yi.z.zhang@linux.intel.com>]

On 2019-01-16 at 10:40:23 -0500, Michael S. Tsirkin wrote:
> On Wed, Jan 16, 2019 at 04:11:16PM +0800, Zhang Yi wrote:
> > Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> > ---
> >  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.
> 
> > +
> > + - '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