Re: [Xen-devel] [PATCH] xen/p2m: Don't call get_balloon_scratch_page() twice, keep interrupts disabled for multicalls

[konrad wilk <konrad.wilk@xxxxxxxxxx>]

On 9/6/2013 11:09 AM, Stefano Stabellini wrote:
> On Fri, 6 Sep 2013, Boris Ostrovsky wrote:
> > m2p_remove_override() calls get_balloon_scratch_page() in
> > MULTI_update_va_mapping() even though it already has pointer to this page from
> > the earlier call (in scratch_page). This second call doesn't have a matching
> > put_balloon_scratch_page() thus not restoring preempt count back. (Also, there
> > is no put_balloon_scratch_page() in the error path.)
> >
> > In addition, the second multicall uses __xen_mc_entry() which does not disable
> > interrupts. Rearrange xen_mc_* calls to keep interrupts off while performing
> > multicalls.
> >
> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
You also might want to mention that this fixes the regression introduced 
by ..
commit id <"title"
> Reviewed-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>
>
> >   arch/x86/xen/p2m.c | 10 ++++++----
> >   1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> > index 0d4ec35..8b901e8 100644
> > --- a/arch/x86/xen/p2m.c
> > +++ b/arch/x86/xen/p2m.c
> > @@ -990,10 +990,13 @@ int m2p_remove_override(struct page *page,
> >                                 printk(KERN_WARNING "m2p_remove_override: "
> >                                                 "pfn %lx mfn %lx, failed to modify 
> > kernel mappings",
> >                                                 pfn, mfn);
> > +                               put_balloon_scratch_page();
> >                                 return -1;
> >                         }
> >   
> > -			mcs = xen_mc_entry(
> > +                       xen_mc_batch();
> > +
> > +                       mcs = __xen_mc_entry(
> >                                         sizeof(struct 
> > gnttab_unmap_and_replace));
> >                         unmap_op = mcs.args;
> >                         unmap_op->host_addr = kmap_op->host_addr;
> > @@ -1003,12 +1006,11 @@ int m2p_remove_override(struct page *page,
> >                         MULTI_grant_table_op(mcs.mc,
> >                                         GNTTABOP_unmap_and_replace, unmap_op, 
> > 1);
> >   
> > -			xen_mc_issue(PARAVIRT_LAZY_MMU);
> > -
> >                         mcs = __xen_mc_entry(0);
> >                         MULTI_update_va_mapping(mcs.mc, scratch_page_address,
> > -                                       
> > pfn_pte(page_to_pfn(get_balloon_scratch_page()),
> > +                                       pfn_pte(page_to_pfn(scratch_page),
> >                                         PAGE_KERNEL_RO), 0);
> > +
> >                         xen_mc_issue(PARAVIRT_LAZY_MMU);
> >   
> >   			kmap_op->host_addr = 0;
> > --
> > 1.8.3.1

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