linux-kernel - Re: [PATCH RFC v2 00/14] md: introduce a new lockless bitmap

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Message-ID: <c6c608e2-23e7-486f-100a-d1fb6cfff4f2@huaweicloud.com>
Date: Sat, 29 Mar 2025 09:11:13 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Christoph Hellwig <hch@...radead.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc: hch@....de, xni@...hat.com, colyli@...nel.org, axboe@...nel.dk,
 agk@...hat.com, snitzer@...nel.org, mpatocka@...hat.com, song@...nel.org,
 linux-block@...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, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC v2 00/14] md: introduce a new lockless bitmap

Hi,

在 2025/03/28 19:06, Christoph Hellwig 写道:
> On Fri, Mar 28, 2025 at 02:08:39PM +0800, Yu Kuai wrote:
>> A hidden disk, named mdxxx_bitmap, is created for bitmap, see details in
>> llbitmap_add_disk(). And a file is created as well to manage bitmap IO for
>> this disk, see details in llbitmap_open_disk(). Read/write bitmap is
>> converted to buffer IO to this file.
>>
>> IO fast path will set bits to dirty, and those dirty bits will be cleared
>> by daemon after IO is done. llbitmap_barrier is used to syncronize between
>> IO path and daemon;
> 
> Why do you need a separate gendisk?  I'll try to find some time to read
> the code to understand what it does, but it would also be really useful
> to explain the need for such an unusual concept here.

The purpose here is to hide the low level bitmap IO implementation to
the API disk->submit_bio(), and the bitmap IO can be converted to buffer
IO to the bdev_file. This is the easiest way that I can think of to
resue the pagecache, with natural ability for dirty page writeback. I do
think about creating a new anon file and implement a new
file_operations, this will be much more complicated.

Meanwhile, bitmap file for the old bitmap will be removed sooner or
later, and this bdev_file implementation will compatible with bitmap
file as well.

Thanks,
Kuai

> 
> .
>