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

[Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>]

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;
>  			}
>  
>