Re: [Qemu-devel] [PATCH] hostmem-file: warn when memory-backend-file, share=on and in incoming migration

[Catherine Ho]

Hi Peter Xu

On Mon, 8 Apr 2019 at 11:25, Peter Xu <address@hidden> wrote:

> On Sun, Apr 07, 2019 at 10:19:05PM -0400, Catherine Ho wrote:
> > Currently it is not forbidden to use "-object
> memory-backend-file,share=on"
> > and together with "-incoming". But after incoming migration is finished,
> > the memory-backend-file will be definitely written if share=on. So the
> > memory-backend-file can only be used once, but failed in the 2nd time
> > incoming.
> >
> > Thus it gives a warning and the users can run the qemu if they really
> > want to do it.
> >
> > Signed-off-by: Catherine Ho <address@hidden>
> > ---
> >  backends/hostmem-file.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> > index 37ac6445d2..59429ee0b4 100644
> > --- a/backends/hostmem-file.c
> > +++ b/backends/hostmem-file.c
> > @@ -16,6 +16,7 @@
> >  #include "sysemu/hostmem.h"
> >  #include "sysemu/sysemu.h"
> >  #include "qom/object_interfaces.h"
> > +#include "migration/migration.h"
> >
> >  /* hostmem-file.c */
> >  /**
> > @@ -79,6 +80,16 @@ file_backend_memory_alloc(HostMemoryBackend *backend,
> Error **errp)
> >          }
> >      }
> >
> > +    /*
> > +     * In ignore shared case, if share=on for host memory backend file,
> > +     * the ram might be written after incoming process is finished. Thus
> > +     * the memory backend can't be reused for 2nd/3rd... incoming
> > +     */
> > +    if (backend->share && migrate_ignore_shared()
> > +                       && runstate_check(RUN_STATE_INMIGRATE))
> > +        warn_report("share=on for memory backend file might be "
> > +                        "conflicted with incoming in ignore shared
> case");
>
> I feel like this message wasn't really clear to me...  you want to
> warn people these data might not match with each other, right?  How
> about simply state it:
>
>   NOTE: Please make sure the data on the shared memory backend file
>   and the data from the incoming migration stream contains matching
>   contents, otherwise...
>

Sorry for my vague expression.
The background is [1]
I happened to use "-object memory-backend-file,share=on" and together with
"-incoming".
It worked fine in 1st incoming migration but failed in 2nd, 3rd incoming
migration.
Because qemu uses qemu_ram_mmap(..., MAP_SHARED,...) when share=on, the
memory-backend-file will be written after 1st incoming migration.
Finally I realized that this was caused by "share=on" flag, and after I
removed it,
the memory-backend-file will not be changed any more.

So do you think it will be better that qemu gives the user a clear warning
that
incoming migration will change the data in  memory-backend-file  with
"share=on" ?

[1] http://lists.nongnu.org/archive/html/qemu-devel/2019-04/msg00350.html


> Regards,
>
> --
> Peter Xu
>