Re: [PATCH 07/10 -v2r1] mm: vmscan: Block kswapd if it isencountering pages under writeback

[Michal Hocko]

Here is what you have in your mm-vmscan-limit-reclaim-v2r1 branch:
> commit 0dae7d4be56e6a7fe3f128284679f5efc0cc2383
> Author: Mel Gorman <mgorman@xxxxxxx>
> Date:   Tue Mar 12 10:33:31 2013 +0000
> 
>     mm: vmscan: Block kswapd if it is encountering pages under writeback
>     
>     Historically, kswapd used to congestion_wait() at higher priorities if it
>     was not making forward progress. This made no sense as the failure to make
>     progress could be completely independent of IO. It was later replaced by
>     wait_iff_congested() and removed entirely by commit 258401a6 (mm: don't
>     wait on congested zones in balance_pgdat()) as it was duplicating logic
>     in shrink_inactive_list().
>     
>     This is problematic. If kswapd encounters many pages under writeback and
>     it continues to scan until it reaches the high watermark then it will
>     quickly skip over the pages under writeback and reclaim clean young
>     pages or push applications out to swap.
>     
>     The use of wait_iff_congested() is not suited to kswapd as it will only
>     stall if the underlying BDI is really congested or a direct reclaimer was
>     unable to write to the underlying BDI. kswapd bypasses the BDI congestion
>     as it sets PF_SWAPWRITE but even if this was taken into account then it
>     would cause direct reclaimers to stall on writeback which is not desirable.
>     
>     This patch sets a ZONE_WRITEBACK flag if direct reclaim or kswapd is
>     encountering too many pages under writeback. If this flag is set and
>     kswapd encounters a PageReclaim page under writeback then it'll assume
>     that the LRU lists are being recycled too quickly before IO can complete
>     and block waiting for some IO to complete.
>     
>     Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

Looks reasonable to me.
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index afedd1d..dd0d266 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -499,6 +499,9 @@ typedef enum {
>  					 * many dirty file pages at the tail
>  					 * of the LRU.
>  					 */
> +	ZONE_WRITEBACK,			/* reclaim scanning has recently found
> +					 * many pages under writeback
> +					 */
>  } zone_flags_t;
>  
>  static inline void zone_set_flag(struct zone *zone, zone_flags_t flag)
> @@ -526,6 +529,11 @@ static inline int zone_is_reclaim_dirty(const struct zone *zone)
>  	return test_bit(ZONE_TAIL_LRU_DIRTY, &zone->flags);
>  }
>  
> +static inline int zone_is_reclaim_writeback(const struct zone *zone)
> +{
> +	return test_bit(ZONE_WRITEBACK, &zone->flags);
> +}
> +
>  static inline int zone_is_reclaim_locked(const struct zone *zone)
>  {
>  	return test_bit(ZONE_RECLAIM_LOCKED, &zone->flags);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a8b94fa..e87de90 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -723,25 +723,51 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
>  			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
>  
> +		/*
> +		 * If a page at the tail of the LRU is under writeback, there
> +		 * are three cases to consider.
> +		 *
> +		 * 1) If reclaim is encountering an excessive number of pages
> +		 *    under writeback and this page is both under writeback and
> +		 *    PageReclaim then it indicates that pages are being queued
> +		 *    for IO but are being recycled through the LRU before the
> +		 *    IO can complete. In this case, wait on the IO to complete
> +		 *    and then clear the ZONE_WRITEBACK flag to recheck if the
> +		 *    condition exists.
> +		 *
> +		 * 2) Global reclaim encounters a page, memcg encounters a
> +		 *    page that is not marked for immediate reclaim or
> +		 *    the caller does not have __GFP_IO. In this case mark
> +		 *    the page for immediate reclaim and continue scanning.
> +		 *
> +		 *    __GFP_IO is checked  because a loop driver thread might
> +		 *    enter reclaim, and deadlock if it waits on a page for
> +		 *    which it is needed to do the write (loop masks off
> +		 *    __GFP_IO|__GFP_FS for this reason); but more thought
> +		 *    would probably show more reasons.
> +		 *
> +		 *    Don't require __GFP_FS, since we're not going into the
> +		 *    FS, just waiting on its writeback completion. Worryingly,
> +		 *    ext4 gfs2 and xfs allocate pages with
> +		 *    grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so testing
> +		 *    may_enter_fs here is liable to OOM on them.
> +		 *
> +		 * 3) memcg encounters a page that is not already marked
> +		 *    PageReclaim. memcg does not have any dirty pages
> +		 *    throttling so we could easily OOM just because too many
> +		 *    pages are in writeback and there is nothing else to
> +		 *    reclaim. Wait for the writeback to complete.
> +		 */
>  		if (PageWriteback(page)) {
> -			/*
> -			 * memcg doesn't have any dirty pages throttling so we
> -			 * could easily OOM just because too many pages are in
> -			 * writeback and there is nothing else to reclaim.
> -			 *
> -			 * Check __GFP_IO, certainly because a loop driver
> -			 * thread might enter reclaim, and deadlock if it waits
> -			 * on a page for which it is needed to do the write
> -			 * (loop masks off __GFP_IO|__GFP_FS for this reason);
> -			 * but more thought would probably show more reasons.
> -			 *
> -			 * Don't require __GFP_FS, since we're not going into
> -			 * the FS, just waiting on its writeback completion.
> -			 * Worryingly, ext4 gfs2 and xfs allocate pages with
> -			 * grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so
> -			 * testing may_enter_fs here is liable to OOM on them.
> -			 */
> -			if (global_reclaim(sc) ||
> +			/* Case 1 above */
> +			if (current_is_kswapd() &&
> +			    PageReclaim(page) &&
> +			    zone_is_reclaim_writeback(zone)) {
> +				wait_on_page_writeback(page);
> +				zone_clear_flag(zone, ZONE_WRITEBACK);
> +
> +			/* Case 2 above */
> +			} else if (global_reclaim(sc) ||
>  			    !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
>  				/*
>  				 * This is slightly racy - end_page_writeback()
> @@ -756,9 +782,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  				 */
>  				SetPageReclaim(page);
>  				nr_writeback++;
> +
>  				goto keep_locked;
> +
> +			/* Case 3 above */
> +			} else {
> +				wait_on_page_writeback(page);
>  			}
> -			wait_on_page_writeback(page);
>  		}
>  
>  		if (!force_reclaim)
> @@ -1373,8 +1403,10 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	 *                     isolated page is PageWriteback
>  	 */
>  	if (nr_writeback && nr_writeback >=
> -			(nr_taken >> (DEF_PRIORITY - sc->priority)))
> +			(nr_taken >> (DEF_PRIORITY - sc->priority))) {
>  		wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
> +		zone_set_flag(zone, ZONE_WRITEBACK);
> +	}
>  
>  	/*
>  	 * Similarly, if many dirty pages are encountered that are not
> @@ -2639,8 +2671,8 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
>   * kswapd shrinks the zone by the number of pages required to reach
>   * the high watermark.
>   *
> - * Returns true if kswapd scanned at least the requested number of
> - * pages to reclaim.
> + * Returns true if kswapd scanned at least the requested number of pages to
> + * reclaim or if the lack of process was due to pages under writeback.
>   */
>  static bool kswapd_shrink_zone(struct zone *zone,
>  			       struct scan_control *sc,
> @@ -2663,6 +2695,8 @@ static bool kswapd_shrink_zone(struct zone *zone,
>  	if (nr_slab == 0 && !zone_reclaimable(zone))
>  		zone->all_unreclaimable = 1;
>  
> +	zone_clear_flag(zone, ZONE_WRITEBACK);
> +
>  	return sc->nr_scanned >= sc->nr_to_reclaim;
>  }
 
-- 
Michal Hocko
SUSE Labs
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