confusion about kvm_mmu_pte_write

["cs-jerry**" <jerry.wzy@xxxxxx>]

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?
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