Re: [PATCH] mm: put the page into the correct list when shrink_page_list fails to reclaim.

[Yang Shi <shy828301@gmail.com>]

On Mon, Oct 28, 2019 at 6:37 AM zhong jiang <zhongjiang@huawei.com> wrote:
>
> Recently, I notice an race case between mlock syscall and shrink_page_list.
>
> one cpu run mlock syscall to make an range of the vma locked in memory. And
> The specified pages will escaped from evictable list from unevictable.
> Meanwhile, another cpu scan and isolate the specified pages to reclaim.
> shrink_page_list hold the page lock to shrink the page and follow_page_pte
> will fails to get the page lock, hence we fails to mlock the page to make
> it Unevictabled.
>
> shrink_page_list fails to reclaim the page due to some reason. it will putback
> the page to evictable lru. But the page actually belongs to an locked range of
> the vma. it is unreasonable to do that. It is better to put the page to unevictable
> lru.

Yes, there is definitely race between mlock() and vmscan, and in the
above case it might stay in evictable LRUs one more round, but it
should be not harmful since try_to_unmap() would move the page to
unevictable list eventually.

>
> The patch set PageMlocked when mlock fails to get the page locked. shrink_page_list
> fails to reclaim the page will putback to the correct list. if it success to reclaim
> the page, we should ClearPageMlocked in time to prevent the warning from free_pages_prepare.
>
> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
> ---
>  mm/gup.c    | 28 ++++++++++++++++++----------
>  mm/vmscan.c |  9 ++++++++-
>  2 files changed, 26 insertions(+), 11 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index c2b3e11..c26d28c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -283,16 +283,24 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>                  * handle it now - vmscan will handle it later if and
>                  * when it attempts to reclaim the page.
>                  */
> -               if (page->mapping && trylock_page(page)) {
> -                       lru_add_drain();  /* push cached pages to LRU */
> -                       /*
> -                        * Because we lock page here, and migration is
> -                        * blocked by the pte's page reference, and we
> -                        * know the page is still mapped, we don't even
> -                        * need to check for file-cache page truncation.
> -                        */
> -                       mlock_vma_page(page);
> -                       unlock_page(page);
> +               if (page->mapping) {
> +                       if (trylock_page(page)) {
> +                               lru_add_drain();  /* push cached pages to LRU */
> +                               /*
> +                                * Because we lock page here, and migration is
> +                                * blocked by the pte's page reference, and we
> +                                * know the page is still mapped, we don't even
> +                                * need to check for file-cache page truncation.
> +                                */
> +                               mlock_vma_page(page);
> +                               unlock_page(page);
> +                       } else {
> +                               /*
> +                                * Avoid putback the page to evictable list when
> +                                * the page is in the locked vma.
> +                                */
> +                               SetPageMlocked(page);
> +                       }
>                 }
>         }
>  out:
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 1154b3a..f7d1301 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1488,8 +1488,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>                  */
>                 if (unlikely(PageTransHuge(page)))
>                         (*get_compound_page_dtor(page))(page);
> -               else
> +               else {
> +                       /*
> +                        * There is an race between mlock and shrink_page_list
> +                        * when mlock fails to get the PageLocked().
> +                        */
> +                       if (unlikely(PageMlocked(page)))
> +                               ClearPageMlocked(page);
>                         list_add(&page->lru, &free_pages);
> +               }
>                 continue;
>
>  activate_locked_split:
> --
> 1.7.12.4
>
>