Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception path for isolate_lru_pages

[Matthew Wilcox]

On Mon, Aug 24, 2020 at 08:55:04PM +0800, Alex Shi wrote:
> +++ 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));
>  				goto busy;

I read Hugh's review and that led me to take a look at this.  We don't
do it like this.  Use the same pattern as elsewhere in mm:

        page_ref_sub(page, nr);
        VM_BUG_ON_PAGE(page_count(page) <= 0, page);