Re: [RFC 3/3] mm: use watermak checks for __GFP_REPEAT high order allocations

[Michal Hocko <mhocko@suse.cz>]

On Wed 02-12-15 15:07:26, Hillf Danton wrote:
> > From: Michal Hocko <mhocko@suse.com>
> > 
> > __alloc_pages_slowpath retries costly allocations until at least
> > order worth of pages were reclaimed or the watermark check for at least
> > one zone would succeed after all reclaiming all pages if the reclaim
> > hasn't made any progress.
> > 
> > The first condition was added by a41f24ea9fd6 ("page allocator: smarter
> > retry of costly-order allocations) and it assumed that lumpy reclaim
> > could have created a page of the sufficient order. Lumpy reclaim,
> > has been removed quite some time ago so the assumption doesn't hold
> > anymore. It would be more appropriate to check the compaction progress
> > instead but this patch simply removes the check and relies solely
> > on the watermark check.
> > 
> > To prevent from too many retries the stall_backoff is not reseted after
> > a reclaim which made progress because we cannot assume it helped high
> > order situation.
> > 
> > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > ---
> >  mm/page_alloc.c | 20 ++++++++------------
> >  1 file changed, 8 insertions(+), 12 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 168a675e9116..45de14cd62f4 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2998,7 +2998,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
> >  	struct page *page = NULL;
> >  	int alloc_flags;
> > -	unsigned long pages_reclaimed = 0;
> >  	unsigned long did_some_progress;
> >  	enum migrate_mode migration_mode = MIGRATE_ASYNC;
> >  	bool deferred_compaction = false;
> > @@ -3167,24 +3166,21 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> > 
> >  	/*
> >  	 * Do not retry high order allocations unless they are __GFP_REPEAT
> > -	 * and even then do not retry endlessly unless explicitly told so
> > +	 * unless explicitly told so.
> 
> s/unless/or/

Fixed
 
> Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>

Thanks!

> 
> >  	 */
> > -	pages_reclaimed += did_some_progress;
> > -	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> > -		if (!(gfp_mask & __GFP_NOFAIL) &&
> > -		   (!(gfp_mask & __GFP_REPEAT) || pages_reclaimed >= (1<<order)))
> > -			goto noretry;
> > -
> > -		if (did_some_progress)
> > -			goto retry;
> > -	}
> > +	if (order > PAGE_ALLOC_COSTLY_ORDER &&
> > +			!(gfp_mask & (__GFP_REPEAT|__GFP_NOFAIL)))
> > +		goto noretry;
> > 
> >  	/*
> >  	 * Be optimistic and consider all pages on reclaimable LRUs as usable
> >  	 * but make sure we converge to OOM if we cannot make any progress after
> >  	 * multiple consecutive failed attempts.
> > +	 * Costly __GFP_REPEAT allocations might have made a progress but this
> > +	 * doesn't mean their order will become available due to high fragmentation
> > +	 * so do not reset the backoff for them
> >  	 */
> > -	if (did_some_progress)
> > +	if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER)
> >  		stall_backoff = 0;
> >  	else
> >  		stall_backoff = min(stall_backoff+1, MAX_STALL_BACKOFF);
> > --
> > 2.6.2
> 

-- 
Michal Hocko
SUSE Labs

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