diff mbox series

Deprecate pmem=on with non-DAX capable backend file

Message ID 20210111203332.740815-1-imammedo@redhat.com
State New
Headers show
Series Deprecate pmem=on with non-DAX capable backend file | expand

Commit Message

Igor Mammedov Jan. 11, 2021, 8:33 p.m. UTC
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>)
---
 docs/system/deprecated.rst | 17 +++++++++++++++++
 util/mmap-alloc.c          |  3 +++
 2 files changed, 20 insertions(+)

Comments

Igor Mammedov Jan. 20, 2021, 7:31 p.m. UTC | #1
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,
Philippe Mathieu-Daudé Jan. 20, 2021, 9:35 p.m. UTC | #2
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,
>
Igor Mammedov Feb. 9, 2021, 4:09 p.m. UTC | #3
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>)

Paolo,

can you pick it up via your tree?

> ---
>  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,
diff mbox series

Patch

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,