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

[Pankaj Gupta <pagupta@redhat.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.

MAP_SHARE_VALIDATE should be used to validate MAP_SYNC with MAP_SHARED.

#define MAP_SHARED_VALIDATE 0x03

In contrast, MAP_SHARED is:
#define MAP_SHARED      0x01            /* Share changes */

I think we should use MAP_SHARED_VALIDATE with MAP_SYNC here?

Thanks,
Pankaj 

> > 
> > > +
> > > + - '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
>