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

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Message-ID: <76d906e8-4c2b-0832-dc43-5fb49b5c6cc5@redhat.com>
Date:   Thu, 25 Aug 2022 10:22:19 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.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 25.08.22 04:34, Kefeng Wang wrote:
> 
> 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;
>   }

I'd much rather just use a dedicated lock that does not involve memory
hotplug.


-- 
Thanks,

David / dhildenb