linux-kernel - Re: [PATCH 2/2] mm: slince possible data races about pgdat->kswapd

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Date:   Thu, 25 Aug 2022 10:34:18 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>
CC:     <muchun.song@...ux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: slince possible data races about pgdat->kswapd


On 2022/8/24 16:24, David Hildenbrand wrote:
> On 24.08.22 09:19, Kefeng Wang wrote:
>> The pgdat->kswapd could be accessed concurrently by kswapd_run() and
>> kcompactd(), it don't be protected by any lock, which could leads to
>> data races, adding READ/WRITE_ONCE() to slince it.
> Okay, I think this patch here makes it clearer that we really just want
> proper synchronization instead of hacking around it.
>
> What speaks against protecting pgdat->kswapd this using some proper
> locking primitive?

as comments about kswapd in struct pglist_data,  pgdat->kswapd should be

protected by mem_hotplug_begin/done(), how about this way?

diff --git a/mm/compaction.c b/mm/compaction.c
index 640fa76228dd..62018f35242a 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1983,7 +1983,13 @@ static inline bool is_via_compact_memory(int order)

  static bool kswapd_is_running(pg_data_t *pgdat)
  {
-       return pgdat->kswapd && task_is_running(pgdat->kswapd);
+       bool running;
+
+       mem_hotplug_begin();
+       running = pgdat->kswapd && task_is_running(pgdat->kswapd);
+       mem_hotplug_end();
+
+       return running;
  }