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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Cc'ing MST.

On 1/20/21 8:31 PM, Igor Mammedov wrote:
> On Mon, 11 Jan 2021 15:33:32 -0500
> Igor Mammedov <imammedo@redhat.com> 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>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>> v2:
>>   rephrase deprecation comment andwarning message
>>   (Philippe Mathieu-Daudé <philmd@redhat.com>)
> 
> I've posted as v1 though it's v2 and it looks like it fell through cracks,
> 
> can someone pick it up if it looks fine, please?
> 
>> ---
>>  docs/system/deprecated.rst | 17 +++++++++++++++++
>>  util/mmap-alloc.c          |  3 +++
>>  2 files changed, 20 insertions(+)
>>
>> diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
>> index bacd76d7a5..e79fb02b3a 100644
>> --- a/docs/system/deprecated.rst
>> +++ b/docs/system/deprecated.rst
>> @@ -327,6 +327,23 @@ 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 (a) not DAX capable or (b) not on a filesystem that support direct mapping
>> +of persistent memory, is not safe and may lead to data loss or corruption in case
>> +of host crash.
>> +Options are:
>> +    - modify VM configuration to set ``pmem=off`` to continue using fake NVDIMM
>> +      (without persistence guaranties) 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..0388cc3be2 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("Using non DAX backing file with 'pmem=on' option"
>> +                        " is deprecated");
>>          }
>>          /*
>>           * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
>