confusion about kvm_mmu_pte_write

[cs-jerry**]

hello all,
i am reading kvm code and confused about the mmu_pte_write process,
when update a gpte, function kvm_mmu_pte_write is called , and walk the shadow 
page for it.why condition is :
if (sp->gfn != gfn || sp->role.direct || sp->role.invalid)
   continue;
and not update those sp->gfn == gfn && sp->role.direct,what does it mean when 
role.direct is clear?
thanks.