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Date: Fri, 31 May 2024 16:16:37 +0800
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Subject: Re: [PATCH v4 4/4] blk-flush: reuse rq queuelist in flush state
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To: Christoph Hellwig <hch@lst.de>
Cc: Friedrich Weber <f.weber@proxmox.com>, axboe@kernel.dk,
 ming.lei@redhat.com, bvanassche@acm.org, linux-block@vger.kernel.org,
 linux-kernel@vger.kernel.org, zhouchengming@bytedance.com
References: <14b89dfb-505c-49f7-aebb-01c54451db40@proxmox.com>
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 <87f495c2-7504-4d22-b355-608b13c456cd@linux.dev>
 <cb0fab18-ba27-420f-ace3-b099c74398c6@proxmox.com>
 <09be2bc6-d16a-4740-908a-f157dcd97ca8@linux.dev>
 <acc28f2c-0e72-409d-bb61-791ef62ddfd4@proxmox.com>
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 <20240531061708.GB18075@lst.de>
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From: Chengming Zhou <chengming.zhou@linux.dev>
In-Reply-To: <20240531061708.GB18075@lst.de>
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On 2024/5/31 14:17, Christoph Hellwig wrote:
> On Wed, May 29, 2024 at 04:50:02PM +0800, Chengming Zhou wrote:
>> Yes, because we use list_move_tail() in the flush sequences. Maybe we can
>> just use list_add_tail() so we don't need the queuelist initialized. It
>> should be ok since rq can't be on any list when PREFLUSH or POSTFLUSH,
>> so there isn't any move actually.
> 
> Sounds good.

Ok, I could send a fix that changes to use list_add_tail() later.

> 
>> But now I'm concerned that rq->queuelist maybe changed by driver after
>> request end?
> 
> How could the driver change it?

I don't know much about drivers. Normally, they will detach rq->queuelist
from their internal list and do blk_mq_end_request(), in which we reuse
this queuelist to add rq to the post-flush list.

Strictly speaking, that rq's ownership still belongs to the drivers until
they call blk_mq_free_request(), right? So I'm not sure if the drivers
would touch rq->queuelist after blk_mq_end_request(). If the drivers don't
have such behaviors, then we are good.

> 
>>> Also, just out of interest: Can you estimate whether this issue is
>>> specific to software RAID setups, or could similar NULL pointer
>>> dereferences also happen in setups without software RAID?
>>
>> I think it can also happen without software RAID.
> 
> Seems to be about batch allocation.  So you either need a plug in
> the stacking device, or io_uring.  I guess people aren't using the
> io_uring high performance options on devices with a write cache
> all that much, as that should immediately reproduce the problem.
>