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

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 24 Jun 2019 16:01:49 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> Igor Mammedov <imammedo@redhat.com> writes:
> 
> > On Mon, 24 Jun 2019 10:17:33 +0200
> > Markus Armbruster <armbru@redhat.com> wrote:
> >  
> >> Igor Mammedov <imammedo@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@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?  
> >
> > I've intentionally left it out for a lack of clear enough advise.
> > Something like:
> >   "Make sure that host has resources to map file pointed by -mem-path"
> > would be pretty useless.  
> 
> I see.
> 
> > I think describing how host should be configured in various ways
> > depending on type of backing storage is well out of scope of any
> > QEMU documentation. But if you have an idea to what to put there
> > (or what to put in deprecation doc and refer to from here),
> > I'll add it on respin.
> >  
> >> Warning message nitpick: the message should be a single phrase, with no
> >> newline or trailing punctuation.  Suggest something like
> >> 
> >>                warn_report("falling back to regular RAM allocation");
> >>                error_printf("This is deprecated.  <Advice on what\n"
> >>                             "to do goes here>\n");
> >>   
> >> >              /* Legacy behavior: if allocation failed, fall back to
> >> >               * regular RAM allocation.
> >> >               */
> >> > diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
> >> > index 2fe9b72121..2193705644 100644
> >> > --- a/qemu-deprecated.texi
> >> > +++ b/qemu-deprecated.texi
> >> > @@ -112,6 +112,14 @@ QEMU using implicit generic or board specific splitting rule.
> >> >  Use @option{memdev} with @var{memory-backend-ram} backend or @option{mem} (if
> >> >  it's supported by used machine type) to define mapping explictly instead.
> >> >  
> >> > +@subsection -mem-path fallback to RAM (since 4.1)
> >> > +Currently if system memory allocation from file pointed by @option{mem-path}
> >> > +fails, QEMU fallbacks to allocating from anonymous RAM. Which might result
> >> > +in unpredictable behavior since provided backing file wasn't used.    
> >> 
> >> 
> >> Noch such verb "to fallback", obvious fix "QEMU falls back to"
> >> 
> >> Suggest "RAM, which might".
> >> 
> >> Better: "since the backing file specified by the user is ignored".
> >>   
> >> >                                                                     In future
> >> > +QEMU will not fallback and fail to start up, so user could fix his/her QEMU/host
> >> > +configuration or explicitly use -m without -mem-path if system memory allocated
> >> > +from anonymous RAM suits usecase.    
> >> 
> >> What's "system memory allocation"?  
> > Using man page language, would be
> >  'guest startup RAM size'
> > acceptable?
> >
> >  
> >> Perhaps: "In the future, QEMU will not fall back, but fail instead.
> >> Adjust either the host configuration [FIXME how?] or the QEMU
> >> configuration [FIXME how?]."  
> >
> > Maybe
> > "
> >  In the future, QEMU will not fall back, but fail instead.
> >  Adjust either the QEMU configuration by removing @option{-mem-path} so
> >  QEMU will use only anonymous or host configuration to make sure that  
> 
> Do you mean "only anonymous memory"?
yep

> >  there are sufficient resources on backing storage pointed by -mem-path
> >  to allocate amount specified by @option{-m}.
> > "  
> 
> Perhaps this could suffice: "Users will be responsible for making sure
> the backing storage specified with -mem-path can actually provide the
> guest RAM configured with -m."
amended hunk will look like:
"
@subsection -mem-path fallback to RAM (since 4.1)                                
Currently if guest RAM allocation from file pointed by @option{mem-path}         
fails, QEMU falls back to allocating from RAM, which might result                
in unpredictable behavior since the backing file specified by the user           
is ignored. In the future, users will be responsible for making sure             
the backing storage specified with @option{-mem-path} can actually provide       
the guest RAM configured with @option{-m} and fail to start up if RAM allocation 
is unsuccessful.   
"
> 
> >> > +
> >> >  @section QEMU Machine Protocol (QMP) commands
> >> >  
> >> >  @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)    
> >>