Re: [Qemu-devel] [PATCH 2/2] migration/postcopy: Explicitly disallow huge pages

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrange (berrange@redhat.com) wrote:
> On Thu, Sep 29, 2016 at 08:09:38PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > At the moment postcopy will fail as soon as qemu tries to register
> > userfault on the RAMBlock pages that are backed by hugepages.
> > However, the kernel is going to get userfault support for hugepage
> > at some point, and we've not got the rest of the QEMU code to support
> > it yet, so fail neatly with an error like:
> > 
> > Postcopy doesn't support hugetlbfs yet (/objects/mem1)
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  migration/postcopy-ram.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> > 
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index 9b04778..9723593 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -85,6 +85,23 @@ static bool ufd_version_check(int ufd)
> >  }
> >  
> >  /*
> > + * Check for things that postcopy won't support; returns 0 if the block
> > + * is fine.
> > + */
> > +static int check_range(const char *block_name, void *host_addr,
> > +                      ram_addr_t offset, ram_addr_t length, void *opaque)
> > +{
> > +    RAMBlock *rb = qemu_ram_block_by_name(block_name);
> > +
> > +    if (qemu_ram_pagesize(rb) > getpagesize()) {
> > +        error_report("Postcopy doesn't support hugetlbfs yet (%s)", block_name);
> 
> A small nitpick - I'd suggest  s/hugetlbfs/large page sizes/ as this error
> will ultimately bubble up to users, and 'large page sizes' is the conceptual
> feature

Interesting, I'd never heard it called 'large' before.
Everytime I just say 'huge page' to someone I then have to explain it's fine
on transparent huge page.

Dave

> 
> 
> Regards,
> Daniel
> -- 
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK