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

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Message-ID: <8eb5acff-4c21-4be8-8d3c-b98bd258ef99@molgen.mpg.de>
Date: Wed, 27 Aug 2025 08:07:25 +0200
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: 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

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.

> [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