Re: [PATCH] Deprecate pmem=on with non-DAX capable backend file

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 29 Dec 2020 19:04:58 +0100
Philippe Mathieu-Daudé <philmd@redhat.com> wrote:

> On 12/29/20 6:29 PM, Igor Mammedov wrote:
> > It is not safe to pretend that emulated NVDIMM supports
> > persistence while backend actually failed to enable it
> > and used non-persistent mapping as fall back.
> > Instead of falling-back, QEMU should be more strict and
> > error out with clear message that it's not supported.
> > So if user asks for persistence (pmem=on), they should
> > store backing file on NVDIMM.
> > 
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > ---
> >  docs/system/deprecated.rst | 14 ++++++++++++++
> >  util/mmap-alloc.c          |  3 +++
> >  2 files changed, 17 insertions(+)
> > 
> > diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
> > index bacd76d7a5..ba4f6ed2fe 100644
> > --- a/docs/system/deprecated.rst
> > +++ b/docs/system/deprecated.rst
> > @@ -327,6 +327,20 @@ The Raspberry Pi machines come in various models (A, A+, B, B+). To be able
> >  to distinguish which model QEMU is implementing, the ``raspi2`` and ``raspi3``
> >  machines have been renamed ``raspi2b`` and ``raspi3b``.
> >  
> > +Backend options
> > +---------------
> > +
> > +Using non-persistent backing file with pmem=on (since 6.0)
> > +''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> > +
> > +This option is used when ``memory-backend-file`` is consumed by emulated NVDIMM
> > +device. However enabling ``memory-backend-file.pmem`` option, when backing file
> > +is not DAX capable or not on a filesystem that support direct mapping of persistent  
> 
> Maybe clearer enumerating? As:
> "is a) not DAX capable or b) not on a filesystem that support direct
> mapping of persistent"

will change it to your variant in v2

> 
> > +memory, is not safe and may lead to data loss or corruption in case of host crash.
> > +Using pmem=on option with such file will return error, instead of a warning.  
> 
> Not sure the difference between warn/err is important in the doc.
not many care about warnings until QEMU starts fine,
I've mentioned error here so that whomever reading this would know what to expect

> 
> > +Options are to move backing file to NVDIMM storage or modify VM configuration
> > +to set ``pmem=off`` to continue using fake NVDIMM without persistence guaranties.  
> 
> Maybe:
> 
> The possibilities to continue using fake NVDIMM (without persistence
> guaranties) are:
> - move backing file to NVDIMM storage
> - modify VM configuration to set ``pmem=off``

only the later is faking nvdimm, the first is properly emulated one with persistence guaranties.
Maybe:
 Options are:
    - modify VM configuration to set ``pmem=off`` to continue using fake NVDIMM
      (without persistence guaranties) on with backing file on non DAX storage
    - move backing file to NVDIMM storage and keep ``pmem=on``,
      to have NVDIMM with persistence guaranties.

> > +
> >  Device options
> >  --------------
> >  
> > diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> > index 27dcccd8ec..d226273a98 100644
> > --- a/util/mmap-alloc.c
> > +++ b/util/mmap-alloc.c
> > @@ -20,6 +20,7 @@
> >  #include "qemu/osdep.h"
> >  #include "qemu/mmap-alloc.h"
> >  #include "qemu/host-utils.h"
> > +#include "qemu/error-report.h"
> >  
> >  #define HUGETLBFS_MAGIC       0x958458f6
> >  
> > @@ -166,6 +167,8 @@ void *qemu_ram_mmap(int fd,
> >                      "crash.\n", file_name);
> >              g_free(proc_link);
> >              g_free(file_name);
> > +            warn_report("Deprecated using non DAX backing file with"
> > +                        " pmem=on option");  
> 
> Maybe "Using non DAX backing file with 'pmem=on' option is deprecated"?
ok

> 
> Beside the nitpicking comments,
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> 
> >          }
> >          /*
> >           * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
> >   
>