Re: [PATCH v2] mm/page_alloc: don't wake kswapd from rmqueue() unless __GFP_KSWAPD_RECLAIM is specified

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2023/05/16 10:44, Huang, Ying wrote:
> Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> writes:
> 
>> On 2023/05/15 16:38, Mel Gorman wrote:
>>> On Sun, May 14, 2023 at 09:28:56AM +0900, Tetsuo Handa wrote:
>>>> Commit 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with zone lock
>>>> held") moved wakeup_kswapd() from steal_suitable_fallback() to rmqueue()
>>>> using ZONE_BOOSTED_WATERMARK flag.
>>>>
>>>> Only allocation contexts that include ALLOC_KSWAPD (which corresponds to
>>>> __GFP_KSWAPD_RECLAIM) should wake kswapd, for callers are supposed to
>>>> remove __GFP_KSWAPD_RECLAIM if trying to hold pgdat->kswapd_wait has a
>>>> risk of deadlock.
>>>
>>> kswapd_wait is a waitqueue so what is being held? It's safe for kswapd
>>> to try wake itself as the waitqueue will be active when wakeup_kswapd()
>>> is called so no wakeup occurs. If there is a deadlock, it needs a better
>>> explanation. I believe I already stated why this patch is fixing a bug
>>> but it wasn't deadlock related.
>>>
>>
>> I noticed this problem ( pgdat->kswapd_wait might be held without
>> __GFP_KSWAPD_RECLAIM ) when analyzing a different problem ( debugobject code
>> is holding pgdat->kswapd_wait due to __GFP_KSWAPD_RECLAIM ) reported at
>> https://syzkaller.appspot.com/bug?extid=fe0c72f0ccbb93786380 .
>>
>> I'm calling pgdat->kswapd_wait a lock, as lockdep uses it as a lock name.
> 
> This has confused me much before.  IIUC, the deadlock is unrelated with
> kswapd wakeup itself.  pgdat->kswapd_wait is the lock name and the lock
> in fact is the spinlock: pgdat->kswapd_wait.lock.  So the deadlock is,
> 
> pgdat->kswapd_wait.lock holders take the pi lock
> pi lock holders take the rq lock
> rq lock holders take the timer base lock
> timer base lock holders take the pgdat->kswapd_wait.lock (missing __GFP_KSWAPD_RECLAIM check)

Yes, thank you for explanation.

> 
> The above is based on analysis in
> 
> https://lore.kernel.org/all/20190107204627.GA25526@cmpxchg.org/
> https://lore.kernel.org/linux-mm/d642e597-cf7d-b410-16ce-22dff483fd8e@I-love.SAKURA.ne.jp/
> 
> Tetsuo's patch avoids to take pgdat->kswapd_wait.lock when timer base
> lock is held via adding check for __GFP_KSWAPD_RECLAIM, so breaks the
> circular dependency chain.

Yes. Mel, any questions on this patch?

Thomas Gleixner described this lock as kswapd_wait::lock at
https://lkml.kernel.org/r/168476016890.404.6911447269153588182.tip-bot2@tip-bot2 .
Should I resubmit this patch with s/pgdat->kswapd_wait/pgdat->kswapd_wait.lock/ or
s/pgdat->kswapd_wait/kswapd_wait::lock/ ?

> 
>> The latter was explained at
>> https://lore.kernel.org/linux-mm/d642e597-cf7d-b410-16ce-22dff483fd8e@I-love.SAKURA.ne.jp/
>> and we agreed that debugobject code needs to drop __GFP_KSWAPD_RECLAIM at
>> https://lore.kernel.org/linux-mm/87fs809fwk.ffs@tglx/ .
>>
>> This patch is for making sure that debugobject code will not try to hold
>> pgdat->kswapd_wait after debugobject code dropped __GFP_KSWAPD_RECLAIM.
>>
>> Thus, the problem this patch will fix is a deadlock related, isn't it?
> 
> Best Regards,
> Huang, Ying