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

On Mon, 11 Jan 2021 15:33:32 -0500 Igor Mammedov <imammedo(a)redhat.com&gt; 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(a)redhat.com&gt; > Reviewed-by: Philippe Mathieu-Daudé <philmd(a)redhat.com&gt; > --- > v2: > rephrase deprecation comment andwarning message > (Philippe Mathieu-Daudé <philmd(a)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,