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

[Yu Kuai]

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

> .