[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> 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(+) 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, -- 2.27.0
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,
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,
On Mon, Jan 11, 2021 at 03:33:32PM -0500, 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> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
I'm queueing this for 6.1, after changing "since 6.0" to "since 6.1". Sorry for letting it fall through the cracks. -- Eduardo
On Tue, Apr 27, 2021 at 04:48:48PM -0400, Eduardo Habkost wrote:
On Mon, Jan 11, 2021 at 03:33:32PM -0500, 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> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
I'm queueing this for 6.1, after changing "since 6.0" to "since 6.1".
Sorry for letting it fall through the cracks.
This caused build failures[1] and I had to apply the following fixup. [1] https://gitlab.com/ehabkost/qemu/-/jobs/1216917482#L3444 Signed-off-by: Eduardo Habkost <ehabkost@redhat.com> --- docs/system/deprecated.rst | 1 + 1 file changed, 1 insertion(+) diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst index cc8d810be1a..c55c4bceb00 100644 --- a/docs/system/deprecated.rst +++ b/docs/system/deprecated.rst @@ -257,6 +257,7 @@ 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`` -- 2.30.2 -- Eduardo
On Wed, 28 Apr 2021 12:29:30 -0400 Eduardo Habkost <ehabkost@redhat.com> wrote:
On Tue, Apr 27, 2021 at 04:48:48PM -0400, Eduardo Habkost wrote:
On Mon, Jan 11, 2021 at 03:33:32PM -0500, 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> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
I'm queueing this for 6.1, after changing "since 6.0" to "since 6.1".
Sorry for letting it fall through the cracks.
This caused build failures[1] and I had to apply the following fixup. Thanks!
[1] https://gitlab.com/ehabkost/qemu/-/jobs/1216917482#L3444
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com> --- docs/system/deprecated.rst | 1 + 1 file changed, 1 insertion(+)
diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst index cc8d810be1a..c55c4bceb00 100644 --- a/docs/system/deprecated.rst +++ b/docs/system/deprecated.rst @@ -257,6 +257,7 @@ 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``
participants (3)
-
Eduardo Habkost -
Igor Mammedov -
Philippe Mathieu-Daudé