On Thu, Nov 15, 2012 at 01:47:33PM -0800, David Rientjes wrote: > On Thu, 15 Nov 2012, Kirill A. Shutemov wrote: > > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > > > index d767a7c..05490b3 100644 > > > > --- a/mm/huge_memory.c > > > > +++ b/mm/huge_memory.c > > > > @@ -1259,6 +1259,8 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, > > > > pmd_t entry; > > > > entry = pmdp_get_and_clear(mm, addr, pmd); > > > > entry = pmd_modify(entry, newprot); > > > > + if (is_huge_zero_pmd(entry)) > > > > + entry = pmd_wrprotect(entry); > > > > set_pmd_at(mm, addr, pmd, entry); > > > > spin_unlock(&vma->vm_mm->page_table_lock); > > > > ret = 1; > > > > > > Nack, this should be handled in pmd_modify(). > > > > I disagree. It means we will have to enable hzp per arch. Bad idea. > > > > pmd_modify() only exists for those architectures with thp support already, > so you've already implicitly enabled for all the necessary architectures > with your patchset. Now we have huge zero page fully implemented inside mm/huge_memory.c. Push this logic to pmd_modify() means we expose hzp implementation details to arch code. Looks ugly for me. > > What's wrong with the check? > > > > Anybody using pmd_modify() to set new protections in the future perhaps > without knowledge of huge zero page can incorrectly make the huge zero > page writable, which can never be allowed to happen. It's better to make > sure it can never happen with the usual interface to modify protections. I haven't found where we check if the page is a 4k zero page, but it's definitely not pte_modify(). -- Kirill A. Shutemov
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