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SubjectRe: [PATCH 3/7] khugepaged: Drain LRU add pagevec to get rid of extra pins
On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
<kirill@shutemov.name> wrote:
>
> __collapse_huge_page_isolate() may fail due to extra pin in the LRU add
> pagevec. It's petty common for swapin case: we swap in pages just to
> fail due to the extra pin.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> mm/khugepaged.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 14d7afc90786..39e0994abeb8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -585,11 +585,19 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> * The page must only be referenced by the scanned process
> * and page swap cache.
> */
> + if (page_count(page) != 1 + PageSwapCache(page)) {
> + /*
> + * Drain pagevec and retry just in case we can get rid
> + * of the extra pin, like in swapin case.
> + */
> + lru_add_drain();

This is definitely correct.

I'm wondering if we need one more lru_add_drain() before PageLRU check
in khugepaged_scan_pmd() or not? The page might be in lru cache then
get skipped. This would improve the success rate.

Reviewed-by: Yang Shi <yang.shi@linux.alibaba.com>

> + }
> if (page_count(page) != 1 + PageSwapCache(page)) {
> unlock_page(page);
> result = SCAN_PAGE_COUNT;
> goto out;
> }
> +
> if (pte_write(pteval)) {
> writable = true;
> } else {
> --
> 2.26.0
>
>

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