Re: [PATCH 3/5] mm: page_alloc: defrag_mode

[Johannes Weiner]

On Sat, Mar 22, 2025 at 09:34:09PM -0400, Johannes Weiner wrote:
> On Sat, Mar 22, 2025 at 08:58:27PM -0400, Johannes Weiner wrote:
> > On Sat, Mar 22, 2025 at 04:05:52PM +0100, Brendan Jackman wrote:
> > > On Thu Mar 13, 2025 at 10:05 PM CET, Johannes Weiner wrote:
> > > > +	/* Reclaim/compaction failed to prevent the fallback */
> > > > +	if (defrag_mode) {
> > > > +		alloc_flags &= ALLOC_NOFRAGMENT;
> > > > +		goto retry;
> > > > +	}
> > > 
> > > I can't see where ALLOC_NOFRAGMENT gets cleared, is it supposed to be
> > > here (i.e. should this be ~ALLOC_NOFRAGMENT)?
> 
> Please ignore my previous email, this is actually a much more severe
> issue than I thought at first. The screwed up clearing is bad, but
> this will also not check the flag before retrying, which means the
> thread will retry reclaim/compaction and never reach OOM.
> 
> This code has weeks of load testing, with workloads fine-tuned to
> *avoid* OOM. A blatant OOM test shows this problem immediately.
> 
> A simple fix, but I'll put it through the wringer before sending it.

Ok, here is the patch. I verified this with intentional OOMing 100
times in a loop; this would previously lock up on first try in
defrag_mode, but kills and recovers reliably with this applied.

I also re-ran the full THP benchmarks, to verify that erroneous
looping here did not accidentally contribute to fragmentation
avoidance and thus THP success & latency rates. They were in fact not;
the improvements claimed for defrag_mode are unchanged with this fix:

                                                VANILLA    defrag_mode=1-OOMFIX
Hugealloc Time mean               52739.45 (    +0.00%)   27342.44 (   -48.15%)
Hugealloc Time stddev             56541.26 (    +0.00%)   33227.16 (   -41.23%)
Kbuild Real time                    197.47 (    +0.00%)     196.32 (    -0.58%)
Kbuild User time                   1240.49 (    +0.00%)    1231.89 (    -0.69%)
Kbuild System time                   70.08 (    +0.00%)      58.75 (   -15.95%)
THP fault alloc                   46727.07 (    +0.00%)   62669.93 (   +34.12%)
THP fault fallback                21910.60 (    +0.00%)    5966.40 (   -72.77%)
Direct compact fail                 195.80 (    +0.00%)      50.53 (   -73.81%)
Direct compact success                7.93 (    +0.00%)       4.07 (   -43.28%)
Compact daemon scanned migrate  3369601.27 (    +0.00%) 1588238.93 (   -52.87%)
Compact daemon scanned free     5075474.47 (    +0.00%) 1441944.27 (   -71.59%)
Compact direct scanned migrate   161787.27 (    +0.00%)   64838.53 (   -59.92%)
Compact direct scanned free      163467.53 (    +0.00%)   37243.00 (   -77.22%)
Compact total migrate scanned   3531388.53 (    +0.00%) 1653077.47 (   -53.19%)
Compact total free scanned      5238942.00 (    +0.00%) 1479187.27 (   -71.77%)
Alloc stall                        2371.07 (    +0.00%)     553.00 (   -76.64%)
Pages kswapd scanned            2160926.73 (    +0.00%) 4052539.93 (   +87.54%)
Pages kswapd reclaimed           533191.07 (    +0.00%)  765447.47 (   +43.56%)
Pages direct scanned             400450.33 (    +0.00%)  358933.93 (   -10.37%)
Pages direct reclaimed            94441.73 (    +0.00%)   26991.60 (   -71.42%)
Pages total scanned             2561377.07 (    +0.00%) 4411473.87 (   +72.23%)
Pages total reclaimed            627632.80 (    +0.00%)  792439.07 (   +26.26%)
Swap out                          47959.53 (    +0.00%)  128511.80 (  +167.96%)
Swap in                            7276.00 (    +0.00%)   27736.20 (  +281.16%)
File refaults                    138043.00 (    +0.00%)  206198.40 (   +49.37%)

Many thanks for your careful review, Brendan.

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