[libvirt] [Qemu-devel] [PATCH] deprecate -mem-path fallback to anonymous RAM

[Igor Mammedov]

On Mon, 24 Jun 2019 10:36:55 +0100
Daniel P. Berrangé <berrange at redhat.com> wrote:

> On Mon, Jun 24, 2019 at 10:17:33AM +0200, Markus Armbruster wrote:
> > Igor Mammedov <imammedo at redhat.com> writes:
> >   
> > > Fallback might affect guest or worse whole host performance
> > > or functionality if backing file were used to share guest RAM
> > > with another process.
> > >
> > > Patch deprecates fallback so that we could remove it in future
> > > and ensure that QEMU will provide expected behavior and fail if
> > > it can't use user provided backing file.
> > >
> > > Signed-off-by: Igor Mammedov <imammedo at redhat.com>
> > > ---
> > > PS:
> > > Patch is written on top of
> > >   [PATCH v4 0/3] numa: deprecate '-numa node,  mem' and default memory distribution
> > > to avoid conflicts in qemu-deprecated.texi
> > >
> > >  numa.c               | 4 ++--
> > >  qemu-deprecated.texi | 8 ++++++++
> > >  2 files changed, 10 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/numa.c b/numa.c
> > > index 91a29138a2..53d67b8ad9 100644
> > > --- a/numa.c
> > > +++ b/numa.c
> > > @@ -494,8 +494,8 @@ static void allocate_system_memory_nonnuma(MemoryRegion *mr, Object *owner,
> > >              if (mem_prealloc) {
> > >                  exit(1);
> > >              }
> > > -            error_report("falling back to regular RAM allocation.");
> > > -
> > > +            warn_report("falling back to regular RAM allocation. "
> > > +                        "Fallback to RAM allocation is deprecated.");  
> > 
> > Can we give the user clues on how to avoid the deprecated fallback?  
> 
> There's nothing a user can do aside from ensuring they have sufficient
> free memory before launching QEMU to satisfy the huge pag request.
> 
> Probably just needs changing to do.
> 
>     "This is deprecated, future QEMU releases will exit when
>      huge pages cannot be allocated"

Also it could be that users might use other than hugepages backing
storage, that's why I completely left concrete advice out from
suggestion.

User should know what he/she is doing when providing mem-path,
if user supplies mis-configured path QEMU will print error
from memory-backend-file if/when allocation fails.


> Regards,
> Daniel