lkml.org 
[lkml]   [2020]   [Apr]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork
From
Date

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).
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> 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.

s/additinal/an additional

> + *
> + * 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)) {
> 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;
> }
>

\
 
 \ /
  Last update: 2020-04-06 22:16    [W:0.335 / U:0.276 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site