mm/mglru: only clear kswapd_failures if reclaimable

BugLink: https://bugs.launchpad.net/bugs/2087886

lru_gen_shrink_node() unconditionally clears kswapd_failures, which can
prevent kswapd from sleeping and cause 100% kswapd cpu usage even when
kswapd repeatedly fails to make progress in reclaim.

Only clear kswap_failures in lru_gen_shrink_node() if reclaim makes some
progress, similar to shrink_node().

I happened to run into this problem in one of my tests recently.  It
requires a combination of several conditions: The allocator needs to
allocate a right amount of pages such that it can wake up kswapd
without itself being OOM killed; there is no memory for kswapd to
reclaim (My test disables swap and cleans page cache first); no other
process frees enough memory at the same time.

Link: https://lkml.kernel.org/r/20241014221211.832591-1-weixugc@google.com
Fixes: e4dde56cd2 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
Signed-off-by: Wei Xu <weixugc@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Jan Alexander Steffens <heftig@archlinux.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

(cherry picked from commit b130ba4a6259f6b64d8af15e9e7ab1e912bcb7ad)
Signed-off-by: Matthew Ruffell <matthew.ruffell@canonical.com>
Acked-by: Koichiro Den <koichiro.den@canonical.com>
Acked-by: Agathe Porte <agathe.porte@canonical.com>
Acked-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

mm/vmscan.c

@@ -4925,8 +4925,8 @@ static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *
 
 	blk_finish_plug(&plug);
 done:
-	/* kswapd should never fail */
-	pgdat->kswapd_failures = 0;
+	if (sc->nr_reclaimed > reclaimed)
+		pgdat->kswapd_failures = 0;
 }
 
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