Re: [PATCH 1/3] mm: honor FOLL_GET flag in follow_hugetlb_page

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Tue, May 07, 2013 at 04:45:54PM -0400, j.glisse@gmail.com wrote:
> From: Jerome Glisse <jglisse@redhat.com>
> 
> Do not increase page count if FOLL_GET is not set.
> 
> Signed-off-by: Jerome Glisse <jglisse@redhat.com>
> ---
>  mm/hugetlb.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 1a12f5b..5d1e46b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2991,7 +2991,9 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  same_page:
>  		if (pages) {
>  			pages[i] = mem_map_offset(page, pfn_offset);
> -			get_page(pages[i]);
> +			if (flags & FOLL_GET) {
> +				get_page_foll(pages[i]);
> +			}
>  		}
>  
>  		if (vmas)
> -- 

Hi Jerome,

I think that we need to be careful in handling tail pages, because
__get_page_tail_foll() uses page->_mapcount as refcount.
When you get refcount on a tail page and free the hugepage without
putting the *mapcount*, you will hit BUG_ON() in free_huge_page().
Yes, this is a very tricky workaround for thp, so to avoid making
things too complicated, I think either of the following is better:
 - to get refcount only for head pages, or
 - to introduce a hugetlbfs variant of get_page_foll().

BTW, who do you expect is the caller of follow_hugetlb_page()
with FOLL_GET (I can't find your subsequent patches 2/3 or 3/3)?
I'm interested in this change because in my project it's necessary
to implement this for hugepage migration
(see https://lkml.org/lkml/2013/3/22/553).

Thanks,
Naoya Horiguchi

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