Re: [PATCHv6 33/36] migrate_pages: try to split pages on qeueuing

Vlastimil Babka
2015-06-11T09:27:33Z

On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> We are not able to migrate THPs. It means it's not enough to split only
> PMD on migration -- we need to split compound page under it too.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> mm/mempolicy.c | 37 +++++++++++++++++++++++++++++++++----
> 1 file changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 528f6c467cf1..0b1499c2f890 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -489,14 +489,31 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
> struct page *page;
> struct queue_pages *qp = walk->private;
> unsigned long flags = qp->flags;
> - int nid;
> + int nid, ret;
> pte_t *pte;
> spinlock_t *ptl;
>
> - split_huge_pmd(vma, pmd, addr);
> - if (pmd_trans_unstable(pmd))
> - return 0;
> + if (pmd_trans_huge(*pmd)) {
> + ptl = pmd_lock(walk->mm, pmd);
> + if (pmd_trans_huge(*pmd)) {
> + page = pmd_page(*pmd);
> + if (is_huge_zero_page(page)) {
> + spin_unlock(ptl);
> + split_huge_pmd(vma, pmd, addr);
> + } else {
> + get_page(page);
> + spin_unlock(ptl);
> + lock_page(page);
> + ret = split_huge_page(page);
> + unlock_page(page);
> + put_page(page);
> + if (ret)
> + return 0;
> + }
> + }
> + }
>
> +retry:
> pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> for (; addr != end; pte++, addr += PAGE_SIZE) {
> if (!pte_present(*pte))
> @@ -513,6 +530,18 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
> nid = page_to_nid(page);
> if (node_isset(nid, *qp->nmask) == !!(flags & MPOL_MF_INVERT))
> continue;
> + if (PageTail(page) && PageAnon(page)) {

Hm, can it really happen that we stumble upon THP tail page here,
without first stumbling upon it in the previous hunk above? If so, when?

> + get_page(page);
> + pte_unmap_unlock(pte - 1, ptl);
> + lock_page(page);
> + ret = split_huge_page(page);
> + unlock_page(page);
> + put_page(page);
> + /* Failed to split -- skip. */
> + if (ret)
> + continue;
> + goto retry;
> + }
>
> if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
> migrate_page_add(page, qp->pagelist, flags);
>