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

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Aug 05, 2010 at 03:59:37PM +0900, KOSAKI Motohiro wrote:
> 
> again, very sorry for the delay.
> 

No problem.

> > 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 |   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?
> 

It's correct. When in lumpy mode, clean everything if the disk has to
spin up.

> 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)
> 

If IO is not started on the correct pages, the flusher threads will be
rekicked for more work and another attempt is made at shrink_page_list.

> 
> 
> > +			congestion_wait(BLK_RW_ASYNC, HZ/10);
> 
> As we discussed, congestion_wait() don't works find if slow strage device
> is connected.
> 

I currently support the removal of this congestion_wait(), but it belongs
in its own patch.

> 
> >  
> > -		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?
> 

Such a removal belongs in the series related to lower latency of lumpy
reclaim. This patch is just about preventing dirty file pages being written
back by direct reclaim.

> 
> > +		}
> >  	}
> >  
> > +	list_splice(&putback_list, &page_list);
> > +
> >  	local_irq_disable();
> >  	if (current_is_kswapd())
> >  		__count_vm_events(KSWAPD_STEAL, nr_reclaimed);
> > -- 
> > 1.7.1
> > 
> 
> 
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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