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SubjectRe: [PATCH] mm: compaction: Abort compaction if too many pages are isolated and caller is asynchronous
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.


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