Re: [RFC 1/3] mm, oom: refactor oom detection

[Michal Hocko]

On Fri 30-10-15 14:23:59, KAMEZAWA Hiroyuki wrote:
> On 2015/10/30 0:17, mhocko@xxxxxxxxxx wrote:
[...]
> > @@ -3135,13 +3145,56 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >   	if (gfp_mask & __GFP_NORETRY)
> >   		goto noretry;
> >   
> > -	/* Keep reclaiming pages as long as there is reasonable progress */
> > +	/*
> > +	 * Do not retry high order allocations unless they are __GFP_REPEAT
> > +	 * and even then do not retry endlessly.
> > +	 */
> >   	pages_reclaimed += did_some_progress;
> > -	if ((did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER) ||
> > -	    ((gfp_mask & __GFP_REPEAT) && pages_reclaimed < (1 << order))) {
> > -		/* Wait for some write requests to complete then retry */
> > -		wait_iff_congested(ac->preferred_zone, BLK_RW_ASYNC, HZ/50);
> > -		goto retry;
> > +	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> > +		if (!(gfp_mask & __GFP_REPEAT) || pages_reclaimed >= (1<<order))
> > +			goto noretry;
> > +
> > +		if (did_some_progress)
> > +			goto retry;
> 
> why directly retry here ?

Because I wanted to preserve the previous logic for GFP_REPEAT as much
as possible here and do an incremental change in the later patch.

[...]

> > @@ -3150,8 +3203,10 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >   		goto got_pg;
> >   
> >   	/* Retry as long as the OOM killer is making progress */
> > -	if (did_some_progress)
> > +	if (did_some_progress) {
> > +		stall_backoff = 0;
> >   		goto retry;
> > +	}
> 
> Umm ? I'm sorry that I didn't notice page allocation may fail even
> if order < PAGE_ALLOC_COSTLY_ORDER.  I thought old logic ignores
> did_some_progress. It seems a big change.

__alloc_pages_may_oom will set did_some_progress

> So, now, 0-order page allocation may fail in a OOM situation ?

No they don't normally and this patch doesn't change the logic here.
-- 
Michal Hocko
SUSE Labs
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