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Date:   Wed, 30 Mar 2022 22:44:50 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: WARNING: mm/workingset.c:567 shadow_lru_isolate
Message-ID: <YkTPUmMitEDHfjeH@casper.infradead.org>
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On Mon, Mar 28, 2022 at 09:24:00PM -0700, Hugh Dickins wrote:
> I'm not familiar with workingset.c, I'll make no guesses, over to you!

I haven't been able to reproduce it, but I applied Feynman's Problem
Solving Algorithm, and I think this will do the trick:

diff --git a/lib/xarray.c b/lib/xarray.c
index b95e92598b9c..d3b168f619b1 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1079,6 +1079,7 @@ void xas_split(struct xa_state *xas, void *entry, unsigned int order)
 					xa_mk_node(child));
 			if (xa_is_value(curr))
 				values--;
+			xas_update(xas, child);
 		} else {
 			unsigned int canon = offset - xas->xa_sibs;
 
@@ -1093,6 +1094,7 @@ void xas_split(struct xa_state *xas, void *entry, unsigned int order)
 	} while (offset-- > xas->xa_offset);
 
 	node->nr_values += values;
+	xas_update(xas, node);
 }
 EXPORT_SYMBOL_GPL(xas_split);
 #endif

(the key insight is that the update function should have removed the
node from the list; the WARN_ON really means "this node should not be on
the list", so there's somewhere that we're forgetting to call
xas_update(), or somewhere that we're forgetting to call
mapping_set_update().  I didn't find any of the latter, but I did find
one of the former)