Re: [PATCH 09/10] vmscan: Do not writeback filesystem pages in direct reclaim

[Wu Fengguang <fengguang.wu@intel.com>]

Mel,

Sorry for being late, I'm doing pretty much prework these days ;)

On Mon, Sep 06, 2010 at 06:47:32PM +0800, Mel Gorman wrote:
> When memory is under enough pressure, a process may enter direct
> reclaim to free pages in the same manner kswapd does. If a dirty page is
> encountered during the scan, this page is written to backing storage using
> mapping->writepage. This can result in very deep call stacks, particularly
> if the target storage or filesystem are complex. It has already been observed
> on XFS that the stack overflows but the problem is not XFS-specific.
> 
> This patch prevents direct reclaim writing back filesystem pages by checking
> if current is kswapd or the page is anonymous before writing back.  If the
> dirty pages cannot be written back, they are placed back on the LRU lists
> for either background writing by the BDI threads or kswapd. If in direct
> lumpy reclaim and dirty pages are encountered, the process will stall for
> the background flusher before trying to reclaim the pages again.
> 
> As the call-chain for writing anonymous pages is not expected to be deep
> and they are not cleaned by flusher threads, anonymous pages are still
> written back in direct reclaim.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Acked-by: Rik van Riel <riel@redhat.com>
> Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/vmscan.c |   49 ++++++++++++++++++++++++++++++++++++++++++++++---
>  1 files changed, 46 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ff52b46..408c101 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -145,6 +145,9 @@ static DECLARE_RWSEM(shrinker_rwsem);
>  #define scanning_global_lru(sc)	(1)
>  #endif
>  
> +/* Direct lumpy reclaim waits up to five seconds for background cleaning */
> +#define MAX_SWAP_CLEAN_WAIT 50
> +
>  static struct zone_reclaim_stat *get_reclaim_stat(struct zone *zone,
>  						  struct scan_control *sc)
>  {
> @@ -682,11 +685,13 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
>   * shrink_page_list() returns the number of reclaimed pages
>   */
>  static unsigned long shrink_page_list(struct list_head *page_list,
> -				      struct scan_control *sc)
> +					struct scan_control *sc,
> +					unsigned long *nr_still_dirty)
>  {
>  	LIST_HEAD(ret_pages);
>  	LIST_HEAD(free_pages);
>  	int pgactivate = 0;
> +	unsigned long nr_dirty = 0;
>  	unsigned long nr_reclaimed = 0;
>  
>  	cond_resched();
> @@ -785,6 +790,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		}
>  
>  		if (PageDirty(page)) {
> +			/*
> +			 * Only kswapd can writeback filesystem pages to
> +			 * avoid risk of stack overflow
> +			 */
> +			if (page_is_file_cache(page) && !current_is_kswapd()) {
> +				nr_dirty++;
> +				goto keep_locked;
> +			}
> +
>  			if (references == PAGEREF_RECLAIM_CLEAN)
>  				goto keep_locked;
>  			if (!may_enter_fs)
> @@ -908,6 +922,8 @@ keep_lumpy:
>  	free_page_list(&free_pages);
>  
>  	list_splice(&ret_pages, page_list);
> +
> +	*nr_still_dirty = nr_dirty;
>  	count_vm_events(PGACTIVATE, pgactivate);
>  	return nr_reclaimed;
>  }
> @@ -1312,6 +1328,10 @@ static inline bool should_reclaim_stall(unsigned long nr_taken,
>  	if (sc->lumpy_reclaim_mode == LUMPY_MODE_NONE)
>  		return false;
>  
> +	/* If we cannot writeback, there is no point stalling */
> +	if (!sc->may_writepage)
> +		return false;
> +
>  	/* If we have relaimed everything on the isolated list, no stall */
>  	if (nr_freed == nr_taken)
>  		return false;
> @@ -1339,11 +1359,13 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
>  			struct scan_control *sc, int priority, int file)
>  {
>  	LIST_HEAD(page_list);
> +	LIST_HEAD(putback_list);
>  	unsigned long nr_scanned;
>  	unsigned long nr_reclaimed = 0;
>  	unsigned long nr_taken;
>  	unsigned long nr_anon;
>  	unsigned long nr_file;
> +	unsigned long nr_dirty;
>  
>  	while (unlikely(too_many_isolated(zone, file, sc))) {
>  		congestion_wait(BLK_RW_ASYNC, HZ/10);
> @@ -1392,14 +1414,35 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
>  
>  	spin_unlock_irq(&zone->lru_lock);
>  
> -	nr_reclaimed = shrink_page_list(&page_list, sc);
> +	nr_reclaimed = shrink_page_list(&page_list, sc, &nr_dirty);
>  
>  	/* Check if we should syncronously wait for writeback */
>  	if (should_reclaim_stall(nr_taken, nr_reclaimed, priority, sc)) {

It is possible to OOM if the LRU list is small and/or the storage is slow, so
that the flusher cannot clean enough pages before the LRU is fully scanned.

So we may need do waits on dirty/writeback pages on *order-0*
direct reclaims, when priority goes rather low (such as < 3).

> +		int dirty_retry = MAX_SWAP_CLEAN_WAIT;
>  		set_lumpy_reclaim_mode(priority, sc, true);
> -		nr_reclaimed += shrink_page_list(&page_list, sc);
> +
> +		while (nr_reclaimed < nr_taken && nr_dirty && dirty_retry--) {
> +			struct page *page, *tmp;
> +

> +			/* Take off the clean pages marked for activation */
> +			list_for_each_entry_safe(page, tmp, &page_list, lru) {
> +				if (PageDirty(page) || PageWriteback(page))
> +					continue;
> +
> +				list_del(&page->lru);
> +				list_add(&page->lru, &putback_list);
> +			}

nitpick: I guess the above loop is optional code to avoid overheads
of shrink_page_list() repeatedly going through some unfreeable pages?
Considering this is the slow code path, I'd prefer to keep the code
simple than to do such optimizations.

> +			wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty);

how about 
                        if (!laptop_mode)
                                wakeup_flusher_threads(nr_dirty);

> +			wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
> +
> +			nr_reclaimed = shrink_page_list(&page_list, sc,
> +							&nr_dirty);
> +		}
>  	}
>  
> +	list_splice(&putback_list, &page_list);
> +
>  	local_irq_disable();
>  	if (current_is_kswapd())
>  		__count_vm_events(KSWAPD_STEAL, nr_reclaimed);
> -- 
> 1.7.1

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