Re: [PATCH 11/11] mm: Isolate pages for immediate reclaim on theirown LRU

[Rik van Riel]

On 12/14/2011 10:41 AM, Mel Gorman wrote:
> It was observed that scan rates from direct reclaim during tests
> writing to both fast and slow storage were extraordinarily high. The
> problem was that while pages were being marked for immediate reclaim
> when writeback completed, the same pages were being encountered over
> and over again during LRU scanning.
>
> This patch isolates file-backed pages that are to be reclaimed when
> clean on their own LRU list.

The idea makes total sense to me.  This is very similar
to the inactive_laundry list in the early 2.4 kernel.

One potential issue is that the page cannot be moved
back to the active list by mark_page_accessed(), which
would have to be taught about the immediate LRU.

> @@ -255,24 +256,80 @@ static void pagevec_move_tail(struct pagevec *pvec)
>   }
>
>   /*
> + * Similar pair of functions to pagevec_move_tail except it is called when
> + * moving a page from the LRU_IMMEDIATE to one of the [in]active_[file|anon]
> + * lists
> + */
> +static void pagevec_putback_immediate_fn(struct page *page, void *arg)
> +{
> +	struct zone *zone = page_zone(page);
> +
> +	if (PageLRU(page)) {
> +		enum lru_list lru = page_lru(page);
> +		list_move(&page->lru,&zone->lru[lru].list);
> +	}
> +}

Should this not put the page at the reclaim end of the
inactive list, since we want to try evicting it?

> +	/*
> +	 * There is a potential race that if a page is set PageReclaim
> +	 * and moved to the LRU_IMMEDIATE list after writeback completed,
> +	 * it can be left on the LRU_IMMEDATE list with no way for
> +	 * reclaim to find it.
> +	 *
> +	 * This race should be very rare but count how often it happens.
> +	 * If it is a continual race, then it's very unsatisfactory as there
> +	 * is no guarantee that rotate_reclaimable_page() will be called
> +	 * to rescue these pages but finding them in page reclaim is also
> +	 * problematic due to the problem of deciding when the right time
> +	 * to scan this list is.
> +	 */

Would it be an idea for the pageout code to check whether the
page at the head of the LRU_IMMEDIATE list is freeable, and
then take that page?

Of course, that does mean adding a check to rotate_reclaimable_page
to make sure the page is still on the LRU_IMMEDIATE list, and did
not get moved by somebody else...

Also, it looks like your debugging check can trigger even when the
bug does not happen (on the last LRU_IMMEDIATE page), because you
decrement NR_IMMEDIATE before you get to this check.

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