On Thu, Jun 02, 2011 at 02:03:52AM +0100, Mel Gorman wrote: > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 2d29c9a..65fa251 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -631,12 +631,14 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm, > entry = mk_pmd(page, vma->vm_page_prot); > entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma); > entry = pmd_mkhuge(entry); > + > /* > - * The spinlocking to take the lru_lock inside > - * page_add_new_anon_rmap() acts as a full memory > - * barrier to be sure clear_huge_page writes become > - * visible after the set_pmd_at() write. > + * Need a write barrier to ensure the writes from > + * clear_huge_page become visible before the > + * set_pmd_at > */ > + smp_wmb(); > + On x86 at least this is noop because of the spin_lock(&page_table_lock) after clear_huge_page. But I'm not against adding this in case other archs supports THP later. But smp_wmb() is optimized away at build time by cpp so this can't possibly help if you're reproducing !SMP. > page_add_new_anon_rmap(page, vma, haddr); > set_pmd_at(mm, haddr, pmd, entry); > prepare_pmd_huge_pte(pgtable, mm); > @@ -753,6 +755,13 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, > > pmdp_set_wrprotect(src_mm, addr, src_pmd); > pmd = pmd_mkold(pmd_wrprotect(pmd)); > + > + /* > + * Write barrier to make sure the setup for the PMD is fully visible > + * before the set_pmd_at > + */ > + smp_wmb(); > + > set_pmd_at(dst_mm, addr, dst_pmd, pmd); > prepare_pmd_huge_pte(pgtable, dst_mm); This part seems superfluous to me, it's also noop for !SMP. Only wmb() would stay. the pmd is perfectly fine to stay in a register, not even a compiler barrier is needed, even less a smp serialization. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>