Re: [PATCH] mm, oom: protect !costly allocations some more

[Michal Hocko <mhocko@kernel.org>]

On Tue 08-03-16 12:12:20, Vlastimil Babka wrote:
> On 03/08/2016 11:10 AM, Michal Hocko wrote:
> > On Tue 08-03-16 10:52:15, Vlastimil Babka wrote:
> >> On 03/08/2016 10:46 AM, Michal Hocko wrote:
> > [...]
> >>>>> @@ -3294,6 +3289,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >>>>>  				 did_some_progress > 0, no_progress_loops))
> >>>>>  		goto retry;
> >>>>>  
> >>>>> +	/*
> >>>>> +	 * !costly allocations are really important and we have to make sure
> >>>>> +	 * the compaction wasn't deferred or didn't bail out early due to locks
> >>>>> +	 * contention before we go OOM.
> >>>>> +	 */
> >>>>> +	if (order && order <= PAGE_ALLOC_COSTLY_ORDER) {
> >>>>> +		if (compact_result <= COMPACT_CONTINUE)
> >>>>
> >>>> Same here.
> >>>> I was going to say that this didn't have effect on Sergey's test, but
> >>>> turns out it did :)
> >>>
> >>> This should work as expected because compact_result is unsigned long
> >>> and so this is the unsigned arithmetic. I can make
> >>> #define COMPACT_NONE            -1UL
> >>>
> >>> to make the intention more obvious if you prefer, though.
> >>
> >> Well, what wasn't obvious to me is actually that here (unlike in the
> >> test above) it was actually intended that COMPACT_NONE doesn't result in
> >> a retry. But it makes sense, otherwise we would retry endlessly if
> >> reclaim couldn't form a higher-order page, right.
> > 
> > Yeah, that was the whole point. An alternative would be moving the test
> > into should_compact_retry(order, compact_result, contended_compaction)
> > which would be CONFIG_COMPACTION specific so we can get rid of the
> > COMPACT_NONE altogether. Something like the following. We would lose the
> > always initialized compact_result but this would matter only for
> > order==0 and we check for that. Even gcc doesn't complain.
> 
> Yeah I like this version better, you can add my Acked-By.

OK, patch updated and I will post it as a reply to the original email.
 
> Thanks.
> 
> > A more important question is whether the criteria I have chosen are
> > reasonable and reasonably independent on the particular implementation
> > of the compaction. I still cannot convince myself about the convergence
> > here. Is it possible that the compaction would keep returning 
> > compact_result <= COMPACT_CONTINUE while not making any progress at all?
> 
> Theoretically, if reclaim/compaction suitability decisions and
> allocation attempts didn't match the watermark checks, including the
> alloc_flags and classzone_idx parameters. Possible scenarios:
> 
> - reclaim thinks compaction has enough to proceed, but compaction thinks
> otherwise and returns COMPACT_SKIPPED
> - compaction thinks it succeeded and returns COMPACT_PARTIAL, but
> allocation attempt fails
> - and perhaps some other combinations

But that might happen right now as well so it wouldn't be a regression,
right?
-- 
Michal Hocko
SUSE Labs