Re: [PATCH] powerpc/book3s_hv_uvmem: Check for failed page migration

[Ram Pai <linuxram@us.ibm.com>]

On Fri, Dec 04, 2020 at 03:48:41PM +0530, Bharata B Rao wrote:
> On Thu, Dec 03, 2020 at 04:08:12PM +1100, Alistair Popple wrote:
> > migrate_vma_pages() may still clear MIGRATE_PFN_MIGRATE on pages which
> > are not able to be migrated. Drivers may safely copy data prior to
> > calling migrate_vma_pages() however a remote mapping must not be
> > established until after migrate_vma_pages() has returned as the
> > migration could still fail.
> > 
> > UV_PAGE_IN_in both copies and maps the data page, therefore it should
> > only be called after checking the results of migrate_vma_pages().
> > 
> > Signed-off-by: Alistair Popple <alistair@popple.id.au>
> > ---
> >  arch/powerpc/kvm/book3s_hv_uvmem.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > index 84e5a2dc8be5..08aa6a90c525 100644
> > --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> > +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > @@ -762,7 +762,10 @@ static int kvmppc_svm_page_in(struct vm_area_struct *vma,
> >  		goto out_finalize;
> >  	}
> >  
> > -	if (pagein) {
> > +	*mig.dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
> > +	migrate_vma_pages(&mig);
> > +
> > +	if ((*mig.src & MIGRATE_PFN_MIGRATE) && pagein) {
> >  		pfn = *mig.src >> MIGRATE_PFN_SHIFT;
> >  		spage = migrate_pfn_to_page(*mig.src);
> >  		if (spage) {
> > @@ -773,8 +776,6 @@ static int kvmppc_svm_page_in(struct vm_area_struct *vma,
> >  		}
> >  	}
> >  
> > -	*mig.dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
> > -	migrate_vma_pages(&mig);
> >  out_finalize:
> >  	migrate_vma_finalize(&mig);
> >  	return ret;

This patch certainly looks like the problem, that has been hurting
us for a while.  Let me run this patch through my SVM tests.  Looks very
promising.

BTW: The code does a similar thing while paging out.  It pages out from the
UV, and then does the migration. Is there a bug there aswell?

RP