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

[Vlastimil Babka]

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@xxxxxxxxxxxxxxx>
> ---
>   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);

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