linux-kernel - Re: [PATCH 3/5] mm: page_alloc: defrag

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Message-ID: <20250323013405.GC1894930@cmpxchg.org>
Date: Sat, 22 Mar 2025 21:34:05 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>, Zi Yan <ziy@...dia.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] mm: page_alloc: defrag_mode

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.