Re: [RFC PATCH v2 5/5] mm: Split move_pages_to_lru into 3 separate passes

Alex Shi
2020-08-19T07:56:43Z

在 2020/8/19 下午12:27, Alexander Duyck 写道:
> From: Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
>
> The current code for move_pages_to_lru is meant to release the LRU lock
> every time it encounters an unevictable page or a compound page that must
> be freed. This results in a fair amount of code bulk because the lruvec has
> to be reacquired every time the lock is released and reacquired.
>
> Instead of doing this I believe we can break the code up into 3 passes. The
> first pass will identify the pages we can move to LRU and move those. In
> addition it will sort the list out leaving the unevictable pages in the
> list and moving those pages that have dropped to a reference count of 0 to
> pages_to_free. The second pass will return the unevictable pages to the
> LRU. The final pass will free any compound pages we have in the
> pages_to_free list before we merge it back with the original list and
> return from the function.
>
> The advantage of doing it this way is that we only have to release the lock
> between pass 1 and 2, and then we reacquire the lock after pass 3 after we
> merge the pages_to_free back into the original list. As such we only have
> to release the lock at most once in an entire call instead of having to
> test to see if we need to relock with each page.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> ---
> mm/vmscan.c | 68 ++++++++++++++++++++++++++++++++++-------------------------
> 1 file changed, 39 insertions(+), 29 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 3ebe3f9b653b..6a2bdbc1a9eb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1850,22 +1850,21 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> {
> int nr_pages, nr_moved = 0;
> LIST_HEAD(pages_to_free);
> - struct page *page;
> - struct lruvec *orig_lruvec = lruvec;
> + struct page *page, *next;
> enum lru_list lru;
>
> - while (!list_empty(list)) {
> - page = lru_to_page(list);
> + list_for_each_entry_safe(page, next, list, lru) {
> VM_BUG_ON_PAGE(PageLRU(page), page);
> - list_del(&page->lru);
> - if (unlikely(!page_evictable(page))) {
> - if (lruvec) {
> - spin_unlock_irq(&lruvec->lru_lock);
> - lruvec = NULL;
> - }
> - putback_lru_page(page);
> +
> + /*
> + * if page is unevictable leave it on the list to be returned
> + * to the LRU after we have finished processing the other
> + * entries in the list.
> + */
> + if (unlikely(!page_evictable(page)))
> continue;
> - }
> +
> + list_del(&page->lru);
>
> /*
> * The SetPageLRU needs to be kept here for list intergrity.
> @@ -1878,20 +1877,14 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> * list_add(&page->lru,)
> * list_add(&page->lru,)
> */
> - lruvec = relock_page_lruvec_irq(page, lruvec);

It's actually changed the meaning from current func. which I had seen a bug if no relock.
but after move to 5.9 kernel, I can not reprodce the bug any more. I am not sure if 5.9 fixed
the problem, and we don't need relock here.

For the rest of this patch.
Reviewed-by: Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>

> SetPageLRU(page);
>
> if (unlikely(put_page_testzero(page))) {
> __ClearPageLRU(page);
> __ClearPageActive(page);
>
> - if (unlikely(PageCompound(page))) {
> - spin_unlock_irq(&lruvec->lru_lock);
> - lruvec = NULL;
> - destroy_compound_page(page);
> - } else
> - list_add(&page->lru, &pages_to_free);
> -
> + /* defer freeing until we can release lru_lock */
> + list_add(&page->lru, &pages_to_free);
> continue;
> }
>
> @@ -1904,16 +1897,33 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> if (PageActive(page))
> workingset_age_nonresident(lruvec, nr_pages);
> }
> - if (orig_lruvec != lruvec) {
> - if (lruvec)
> - spin_unlock_irq(&lruvec->lru_lock);
> - spin_lock_irq(&orig_lruvec->lru_lock);
> - }
>
> - /*
> - * To save our caller's stack, now use input list for pages to free.
> - */
> - list_splice(&pages_to_free, list);
> + if (unlikely(!list_empty(list) || !list_empty(&pages_to_free))) {
> + spin_unlock_irq(&lruvec->lru_lock);
> +
> + /* return any unevictable pages to the LRU list */
> + while (!list_empty(list)) {
> + page = lru_to_page(list);
> + list_del(&page->lru);
> + putback_lru_page(page);
> + }
> +
> + /*
> + * To save our caller's stack use input
> + * list for pages to free.
> + */
> + list_splice(&pages_to_free, list);
> +
> + /* free any compound pages we have in the list */
> + list_for_each_entry_safe(page, next, list, lru) {
> + if (likely(!PageCompound(page)))
> + continue;
> + list_del(&page->lru);
> + destroy_compound_page(page);
> + }
> +
> + spin_lock_irq(&lruvec->lru_lock);
> + }
>
> return nr_moved;
> }
>