Re: [RFC PATCH v2 3/5] mm: Add explicit page decrement in exception path for isolate_lru_pages

Alex Shi
2020-08-19T07:50:50Z

在 2020/8/19 下午12:27, Alexander Duyck 写道:
> From: Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
>
> In isolate_lru_pages we have an exception path where if we call
> get_page_unless_zero and that succeeds, but TestClearPageLRU fails we call
> put_page. Normally this would be problematic but due to the way that the
> calls are ordered and the fact that we are holding the LRU lock we know
> that the caller must be holding another reference for the page. Since we
> can assume that we can replace the put_page with a call to
> put_page_testzero contained within a WARN_ON. By doing this we should see
> if we ever leak a page as a result of the reference count somehow hitting
> zero when it shouldn't, and can avoid the overhead and confusion of using
> the full put_page call.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> ---
> mm/vmscan.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5bc0c2322043..3ebe3f9b653b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1688,10 +1688,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>
> if (!TestClearPageLRU(page)) {
> /*
> - * This page may in other isolation path,
> - * but we still hold lru_lock.
> + * This page is being isolated in another
> + * thread, but we still hold lru_lock. The
> + * other thread must be holding a reference
> + * to the page so this should never hit a
> + * reference count of 0.
> */
> - put_page(page);
> + WARN_ON(put_page_testzero(page));

seems WARN_ON is always enabled.

Reviewed-by: Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>

> goto busy;
> }
>
>