Re: [PATCH 5/6] vmscan: Do not writeback filesystem pages in direct reclaim

[KOSAKI Motohiro]

again, very sorry for the delay.

> 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@xxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  mm/vmscan.c |   69 ++++++++++++++++++++++++++++++++++++++++++++++------------
>  1 files changed, 54 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d83812a..2d2b588 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -139,6 +139,9 @@ 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)
>  {
> @@ -645,11 +648,13 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
>   */
>  static unsigned long shrink_page_list(struct list_head *page_list,
>  					struct scan_control *sc,
> -					enum pageout_io sync_writeback)
> +					enum pageout_io sync_writeback,
> +					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();
> @@ -743,6 +748,15 @@ 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)
> @@ -860,6 +874,8 @@ keep:
>  	free_page_list(&free_pages);
>  
>  	list_splice(&ret_pages, page_list);
> +
> +	*nr_still_dirty = nr_dirty;
>  	count_vm_events(PGACTIVATE, pgactivate);
>  	return nr_reclaimed;
>  }
> @@ -1242,12 +1258,14 @@ 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_active;
>  	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);
> @@ -1296,28 +1314,49 @@ 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, PAGEOUT_IO_ASYNC);
> +	nr_reclaimed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC,
> +								&nr_dirty);
>  
>  	/*
> -	 * If we are direct reclaiming for contiguous pages and we do
> +	 * If specific pages are needed such as with direct reclaiming
> +	 * for contiguous pages or for memory containers and we do
>  	 * not reclaim everything in the list, try again and wait
> -	 * for IO to complete. This will stall high-order allocations
> -	 * but that should be acceptable to the caller
> +	 * for IO to complete. This will stall callers that require
> +	 * specific pages but it should be acceptable to the caller
>  	 */
> -	if (nr_reclaimed < nr_taken && !current_is_kswapd() &&
> -			sc->lumpy_reclaim_mode) {
> -		congestion_wait(BLK_RW_ASYNC, HZ/10);
> +	if (sc->may_writepage && !current_is_kswapd() &&
> +			(sc->lumpy_reclaim_mode || sc->mem_cgroup)) {
> +		int dirty_retry = MAX_SWAP_CLEAN_WAIT;
>  
> -		/*
> -		 * The attempt at page out may have made some
> -		 * of the pages active, mark them inactive again.
> -		 */
> -		nr_active = clear_active_flags(&page_list, NULL);
> -		count_vm_events(PGDEACTIVATE, nr_active);
> +		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);
> +			}
> +
> +			wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty);

ditto.
wakeup_flusher_threads(0) is not correct?

And, When flusher thread still don't start IO, this loop don't have proper
waiting. do we need wait_on_page_dirty() or something?
(similar wait_on_page_writeback)



> +			congestion_wait(BLK_RW_ASYNC, HZ/10);

As we discussed, congestion_wait() don't works find if slow strage device
is connected.


>  
> -		nr_reclaimed += shrink_page_list(&page_list, sc, PAGEOUT_IO_SYNC);
> +			/*
> +			 * The attempt at page out may have made some
> +			 * of the pages active, mark them inactive again.
> +			 */
> +			nr_active = clear_active_flags(&page_list, NULL);
> +			count_vm_events(PGDEACTIVATE, nr_active);
> +
> +			nr_reclaimed += shrink_page_list(&page_list, sc,
> +						PAGEOUT_IO_SYNC, &nr_dirty);

After my patch, when PAGEOUT_IO_SYNC failure, retry is no good idea.
can we remove this loop?


> +		}
>  	}
>  
> +	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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