Re: [Qemu-devel] [PATCH V10 4/4] docs: Added MAP_SYNC documentation

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

On 2019-01-23 at 12:50:50 -0200, Eduardo Habkost wrote:
> On Wed, Jan 23, 2019 at 11:00:02AM +0800, Zhang, Yi wrote:
> > From: Zhang Yi <yi.z.zhang@linux.intel.com>
> > 
> > Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> > ---
> >  docs/nvdimm.txt | 29 ++++++++++++++++++++++++++++-
> >  qemu-options.hx |  4 ++++
> >  2 files changed, 32 insertions(+), 1 deletion(-)
> > 
> > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> > index 5f158a6..166c395 100644
> > --- a/docs/nvdimm.txt
> > +++ b/docs/nvdimm.txt
> > @@ -142,11 +142,38 @@ backend of vNVDIMM:
> >  Guest Data Persistence
> >  ----------------------
> >  
> > +vNVDIMM is designed and implemented to guarantee the guest data
> > +persistence on the backends in case of host crash or a power 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 dax backend files with MAP_SYNC, which
> > +ensures filesystem metadata consistency in case of a host crash or a power
> > +failure. Enabling MAP_SYNC in QEMU requires below conditions
> > +
> > + - 'pmem' option of memory-backend-file is 'on':
> > +   The backend is a file supporting DAX, e.g., a file on an ext4 or
> > +   xfs file system mounted with '-o dax'. if your pmem=on ,but the backend is
> > +   not a file supporting DAX, mapping with this flag results in an EOPNOTSUPP
> > +   error.
> 
> Won't this break existing configurations that work today on QEMU
> 3.1.0?  Why exactly it is OK to break compatibility here?
won't, pmem option default is off, if people who start VM don't know what
backend file is, it is suggested and *default to set pmem=off,
if people well know the backend file have dax capbility. it is suggest
to set pmem=on. 

For a special case that we use /dev/dax as backend, we already have a
patch to add MAP_SYNC falg mapiing from device dax mode.
see https://lkml.org/lkml/2018/4/22/524 

So, if people force set pmem=on, mapping a regular file, it will results
in an EOPNOTSUPP error. 

see http://man7.org/linux/man-pages/man2/mmap.2.html 
> 
> > +
> > + - 'share' option of memory-backend-file is 'on':
> > +   MAP_SYNC flag available only with the MAP_SHARED_VALIDATE mapping type.
> 
> I don't understand what this paragraph means.
see http://man7.org/linux/man-pages/man2/mmap.2.html 
> 
> > +
> > + - 'MAP_SYNC' is supported on linux kernel.(default opened since Linux 4.15)
> > +
> 
> I don't understand why you are making the semantics of
> command-line options change depending on the host kernel.
the option pmem=on do not dependent the host kernel. MAP_SYNC will be ignore
if the kernel don't support. the "pmem=on" have another meaning
see https://patchwork.kernel.org/patch/10459407/
> 
> > +Otherwise, We will ignore the MAP_SYNC flag.
> > +
> 
> See the questions I sent about supported use cases at
> <https://www.mail-archive.com/qemu-devel@nongnu.org/msg588822.html>.
> I still don't see those questions answered:
> 
> ] 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
> ] 

Sorry for my poor understanding, I don't know what these mean? 
pmem=on will force flag the MAP_SYNC while it capable on current kernel.
As we talk with Micheal if we set pmem=on , MAP_SYNC is always desired.

Means, if pmem=on, there is no option to close MAP_SYNC seprately.


> ] 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?
> 
> It's not clear to me yet if you want to support use cases (1) and (2).
> 
> Also, you seem to be choosing between use case (1) or (3) depending on
> the build environment instead of command-line options.  The
> meaning of command-line options must be predictable and
> unambiguous, and not depend on build time variables.
so you are asking?
1) pmem=on, MAP_SYNC not supported kernel
- MAP_SYNC will be defined 0 and will be ignored in this case. see 2/4.
2) pmem=on, MAP_SYNC is supported but have a option to pass to mmap2()
- v7 send-out for a option sync to open/close MAP_SYNC seprately.
After talking with Micheal, we give up on a bit of flexibility, and
just say pmem=on forces MAP_SYNC. on a MAP_SYNC capable configrations(kernel+
backend dax)
3) pmem=on, ?
> 
> 
> > +For more details, please reference mmap(2) man page:
> > +http://man7.org/linux/man-pages/man2/mmap.2.html.
> > +
> >  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..0cd41f4 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} in case of host crash
> > +or a power failure. MAP_SYNC requires support from both the host kernel
> > +(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
> > 
> 
> -- 
> Eduardo