[PATCH] Deprecate pmem=on with non-DAX capable backend file
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 +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. +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. + 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"); } /* * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC, -- 2.27.0
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"
+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.
+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``
+ 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"? Beside the nitpicking comments, Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
} /* * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
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,
participants (2)
-
Igor Mammedov -
Philippe Mathieu-Daudé