mm/vmscan: skip increasing kswapd_failures when reclaim was boosted

We have a colocation cluster used for deploying both offline and online
services simultaneously. In this environment, we encountered a
scenario where direct memory reclamation was triggered due to kswapd
not running.

1. When applications start up, rapidly consume memory, or experience
network traffic bursts, the kernel reaches steal_suitable_fallback(),
which sets watermark_boost and subsequently wakes kswapd.

2. In the core logic of kswapd thread (balance_pgdat()), when reclaim is
triggered by watermark_boost, the maximum priority is 10. Higher
priority values mean less aggressive LRU scanning, which can result in
no pages being reclaimed during a single scan cycle:

if (nr_boost_reclaim && sc.priority == DEF_PRIORITY - 2)
raise_priority = false;

3. Additionally, many of our pods are configured with memory.low, which
prevents memory reclamation in certain cgroups, further increasing the
chance of failing to reclaim memory.

4. This eventually causes pgdat->kswapd_failures to continuously
accumulate, exceeding MAX_RECLAIM_RETRIES, and consequently kswapd
stops working. At this point, the system's available memory is still
significantly above the high watermark -- it's inappropriate for kswapd
to stop under these conditions.

The final observable issue is that a brief period of rapid memory
allocation causes kswapd to stop running, ultimately triggering direct
reclaim and making the applications unresponsive.

This problem leading to direct memory reclamation has been a
long-standing issue in our production environment. We initially held
the simple assumption that it was caused by applications allocating
memory too rapidly for kswapd to keep up with reclamation. However,
after we began monitoring kswapd's runtime behavior, we discovered a
different pattern:

kswapd initially exhibits very aggressive activity even when there is
still considerable free memory, but it subsequently stops running
entirely, even as memory levels approach the low watermark.

In summary, both boosted watermarks and memory.low increase the
probability of kswapd operation failures.

This patch specifically addresses the scenario involving boosted
watermarks by not incrementing kswapd_failures when reclamation fails.
A more general solution, potentially addressing memory.low or other
cases, requires further discussion.

Link: https://lkml.kernel.org/r/53de0b3ee0b822418e909db29bfa6513faff9d36@linux.dev
Link: https://lkml.kernel.org/r/20251024022711.382238-1-jiayuan.chen@linux.dev
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>