Re: [PATCH 06/10] migrate: add hugepage migration code to move_pages()

[Michal Hocko <mhocko@suse.cz>]

On Fri 22-03-13 16:23:51, Naoya Horiguchi wrote:
> This patch extends move_pages() to handle vma with VM_HUGETLB set.
> We will be able to migrate hugepage with move_pages(2) after
> applying the enablement patch which comes later in this series.
> 
> We avoid getting refcount on tail pages of hugepage, because unlike thp,
> hugepage is not split and we need not care about races with splitting.
> 
> And migration of larger (1GB for x86_64) hugepage are not enabled.
> 
> ChangeLog v2:
>  - updated description and renamed patch title
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  mm/memory.c  |  6 ++++--
>  mm/migrate.c | 26 +++++++++++++++++++-------
>  2 files changed, 23 insertions(+), 9 deletions(-)
> 
> diff --git v3.9-rc3.orig/mm/memory.c v3.9-rc3/mm/memory.c
> index 494526a..3b6ad3d 100644
> --- v3.9-rc3.orig/mm/memory.c
> +++ v3.9-rc3/mm/memory.c
> @@ -1503,7 +1503,8 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
>  	if (pud_none(*pud))
>  		goto no_page_table;
>  	if (pud_huge(*pud) && vma->vm_flags & VM_HUGETLB) {
> -		BUG_ON(flags & FOLL_GET);
> +		if (flags & FOLL_GET)
> +			goto out;


>  		page = follow_huge_pud(mm, address, pud, flags & FOLL_WRITE);
>  		goto out;
>  	}
> @@ -1514,8 +1515,9 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
>  	if (pmd_none(*pmd))
>  		goto no_page_table;
>  	if (pmd_huge(*pmd) && vma->vm_flags & VM_HUGETLB) {
> -		BUG_ON(flags & FOLL_GET);
>  		page = follow_huge_pmd(mm, address, pmd, flags & FOLL_WRITE);
> +		if (flags & FOLL_GET && PageHead(page))
> +			get_page_foll(page);

Hmm, so the caller gets a non-null page without elevated ref counted
even when he asked for it. This means that all callers have to check
PageTail && hugetlb and put_page according to that. That is _really_
fragile.
I think that returning NULL would make more sense in this case.

>  		goto out;
>  	}
>  	if ((flags & FOLL_NUMA) && pmd_numa(*pmd))
> @@ -1164,6 +1175,12 @@ static int do_move_page_to_node_array(struct mm_struct *mm,
[...]
>  				!migrate_all)
>  			goto put_and_set;
>  
> +		if (PageHuge(page)) {
> +			get_page(page);
> +			list_move_tail(&page->lru, &pagelist);
> +			goto put_and_set;
> +		}

Why do you take an additional reference here? You have one from
follow_page already.

> +
>  		err = isolate_lru_page(page);
>  		if (!err) {
>  			list_add_tail(&page->lru, &pagelist);
[...]
-- 
Michal Hocko
SUSE Labs