Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork

[John Hubbard]

On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
> The page can be included into collapse as long as it doesn't have extra
> pins (from GUP or otherwise).

Hi Kirill,

s/callapse/collapse/ in the Subject line.

The commit message should mention that you're also removing a
VM_BUG_ON_PAGE().

> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>   mm/khugepaged.c | 25 ++++++++++++++-----------
>   1 file changed, 14 insertions(+), 11 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 57ff287caf6b..1e7e6543ebca 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -581,11 +581,18 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>   		}
>   
>   		/*
> -		 * cannot use mapcount: can't collapse if there's a gup pin.
> -		 * The page must only be referenced by the scanned process
> -		 * and page swap cache.
> +		 * Check if the page has any GUP (or other external) pins.
> +		 *
> +		 * The page table that maps the page has been already unlinked
> +		 * from the page table tree and this process cannot get
> +		 * additinal pin on the page.


I'd recommend this wording instead, for the last two lines:

		 * from the page table tree. Therefore, this page will not
		 * normally receive any additional pins.


> +		 *
> +		 * New pins can come later if the page is shared across fork,
> +		 * but not for the this process. It is fine. The other process
> +		 * cannot write to the page, only trigger CoW.
>   		 */
> -		if (page_count(page) != 1 + PageSwapCache(page)) {
> +		if (total_mapcount(page) + PageSwapCache(page) !=
> +				page_count(page)) {


I think it's time to put that logic ( "does this page have any extra references")
into a small function. It's already duplicated once below. And the documentation is
duplicated as well.

I took a quick peek at this patch because, after adding pin_user_pages*() APIs earlier
to complement get_user_pages*(), I had a moment of doubt here: what if I'd done  it in
a way that required additional logic here? Fortunately, that's not the case: all
pin_user_pages() calls on huge pages take a "primary/real" refcount, in addition
to scribbling into the compound_pincount_ptr() area. whew. :)



>   			unlock_page(page);
>   			result = SCAN_PAGE_COUNT;
>   			goto out;
> @@ -672,7 +679,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>   		} else {
>   			src_page = pte_page(pteval);
>   			copy_user_highpage(page, src_page, address, vma);
> -			VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
>   			release_pte_page(src_page);
>   			/*
>   			 * ptl mostly unnecessary, but preempt has to
> @@ -1206,12 +1212,9 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>   			goto out_unmap;
>   		}
>   
> -		/*
> -		 * cannot use mapcount: can't collapse if there's a gup pin.
> -		 * The page must only be referenced by the scanned process
> -		 * and page swap cache.
> -		 */
> -		if (page_count(page) != 1 + PageSwapCache(page)) {
> +		/* Check if the page has any GUP (or other external) pins */
> +		if (total_mapcount(page) + PageSwapCache(page) !=
> +				page_count(page)) {>   			result = SCAN_PAGE_COUNT;
>   			goto out_unmap;
>   		}



thanks,
--
John Hubbard
NVIDIA