linux-kernel - Re: [PATCH v6 md-6.18 11/11] md/md-llbitmap: introduce new lockless bitmap

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Message-ID: <791992fe-3b98-1d00-6276-56fa1b45b2c8@huaweicloud.com>
Date: Thu, 28 Aug 2025 15:10:52 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Paul Menzel <pmenzel@...gen.mpg.de>, Yu Kuai <yukuai1@...weicloud.com>
Cc: hch@...radead.org, corbet@....net, agk@...hat.com, snitzer@...nel.org,
 mpatocka@...hat.com, song@...nel.org, xni@...hat.com, hare@...e.de,
 linan122@...wei.com, colyli@...nel.org, linux-doc@...r.kernel.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 v6 md-6.18 11/11] md/md-llbitmap: introduce new lockless
 bitmap

Hi,

在 2025/08/27 14:07, Paul Menzel 写道:
> Dear Kuai,
> 
> 
> Thank you for your reply.
> 
> Am 27.08.25 um 05:44 schrieb Yu Kuai:
> 
>> 在 2025/08/26 17:52, Paul Menzel 写道:
>>> It’d be great if you could motivate, why a lockless bitmap is needed 
>>> > compared to the current implemention.
>>
>> Se the performance test, old bitmap have global spinlock and is bad with
>> fast disk.
> 
> Yes, but it’s at the end, and not explicitly stated. Should you resend, 
> it’d be great if you could add that.

If there is no suggestions about functionality, I can add following in
the beginning when I apply this:

Due to known performance issues with md-bitmap and the unreasonable
implementations:

  - self-managed IO submitting like filemap_write_page();
  - global spin_lock

I have decided not to continue optimizing based on the current bitmap
implementation.

And the same as fixing those typos.

Thanks,
Kuai

> 
>> [snip the typo part]
>>
>>> How can/should this patch be tested/benchmarked?
>>
>> There is pending mdadm patch, rfc verion can be used. Will work on
>> formal version after this set is applied.
> 
> Understood. Maybe add an URL to the mdadm patch. (Sorry, should I have 
> missed it.)
> 
>>> --- a/drivers/md/md-bitmap.h
>>> +++ b/drivers/md/md-bitmap.h
>>> @@ -9,10 +9,26 @@
>>>    #define BITMAP_MAGIC 0x6d746962
>>> +/*
>>> + * version 3 is host-endian order, this is deprecated and not used 
>>> for new
>>> + * array
>>> + */
>>> +#define BITMAP_MAJOR_LO        3
>>> +#define BITMAP_MAJOR_HOSTENDIAN    3
>>> +/* version 4 is little-endian order, the default value */
>>> +#define BITMAP_MAJOR_HI        4
>>> +/* version 5 is only used for cluster */
>>> +#define BITMAP_MAJOR_CLUSTERED    5
> 
>>> Move this to the header in a separate patch?
>>
>> I prefer not, old bitmap use this as well.
> Hmm, I do not understand the answer, as it’s moved in this patch, why 
> can’t it be moved in another? But it’s not that important.
> 
> 
> Kind regards,
> 
> Paul
> .
>