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: <20250513064803.GA1508@lst.de>
Date: Tue, 13 May 2025 08:48:03 +0200
From: Christoph Hellwig <hch@....de>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: Christoph Hellwig <hch@....de>, 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

On Tue, May 13, 2025 at 02:32:04PM +0800, Yu Kuai wrote:
> However, for bitmap file case, bio is issued from submit_bh(), I'll have
> to change buffer_head code and I'm not sure if we want to do that.

Don't bother with the old code, I'm still hoping we can replace it with
your new code ASAP. 

> +       BIO_STACKED_META,       /* bio is metadata from stacked device */

I don't think that is the right name and description.  The whole point
of the recursion avoidance is to to supported stacked devices by
reducing the stack depth.  So we clearly need to document why that
is not desirable for some very specific cases like reading in metadata
needed to process a write I/O.  We should also ensure it doesn't
propagate to lover stacked devices.

In fact I wonder if a better interfaces would be one to stash away
current->bio_list and then restore it after the call, as that would
enforce all that.