linux-kernel - Re: [PATCH RFC md-6.16 v3 15/19] md/md-llbitmap: implement APIs to dirty bits and clear bits

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Message-ID: <d5ae7af0-dd73-7d6b-f520-c25e411f8f06@huaweicloud.com>
Date: Tue, 13 May 2025 17:32:13 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Christoph Hellwig <hch@....de>, Yu Kuai <yukuai1@...weicloud.com>
Cc: xni@...hat.com, colyli@...nel.org, agk@...hat.com, snitzer@...nel.org,
 mpatocka@...hat.com, song@...nel.org, linux-kernel@...r.kernel.org,
 dm-devel@...ts.linux.dev, linux-raid@...r.kernel.org, yi.zhang@...wei.com,
 yangerkun@...wei.com, johnny.chenyi@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC md-6.16 v3 15/19] md/md-llbitmap: implement APIs to
 dirty bits and clear bits

Hi,

在 2025/05/13 15:43, Christoph Hellwig 写道:
> On Tue, May 13, 2025 at 03:14:03PM +0800, Yu Kuai wrote:
>> Yes, following change can work as well.
>>
>> Just wonder, if the array is created by another array, and which is
>> created by another array ...  In this case, the stack depth can be
>> huge. :(  This is super weird case, however, should we keep the old code
>> in this case?
> 
> Yeah, that's a good question.  Stacking multiple arrays using bitmaps
> on top of each other is weird.  But what if other block remappers
> starting to use this for other remapping and they are stacked?  That
> seems much more likely unfotunately, so maybe we can't go down this
> route after all, sorry for leading you to it.

I was thinking about record a stack dev depth in mddev to handle the
weird case inside raid. Is there other stack device have the same
problem? AFAIK, some dm targets like dm-crypt are using workqueue
to handle all IO.

I'm still interested because this can improve first write latency.

> 
> So instead just write a comment documenting why you switch to a
> different stack using the workqueue.

Ok, I'll add comment if we keep using the workqueue.

Thanks,
Kuai

> .
>