linux-kernel - Re: [PATCH v2 1/3] brd: protect page with rcu

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Message-ID: <20250428125207.GB27794@lst.de>
Date: Mon, 28 Apr 2025 14:52:07 +0200
From: Christoph Hellwig <hch@....de>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: hch@....de, axboe@...nel.dk, kbusch@...nel.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com,
	johnny.chenyi@...wei.com
Subject: Re: [PATCH v2 1/3] brd: protect page with rcu

On Sun, Apr 27, 2025 at 04:39:23PM +0800, Yu Kuai wrote:
>  	page = brd_lookup_page(brd, sector);
>  
>  	kaddr = bvec_kmap_local(&bv);
>  	if (op_is_write(opf)) {
> -		BUG_ON(!page);
> -		memcpy_to_page(page, offset, kaddr, bv.bv_len);
> +		if (page)
> +			memcpy_to_page(page, offset, kaddr, bv.bv_len);

This could use a comment on why page can be NULL here.

>  	} else if (page) {
>  		memcpy_from_page(kaddr, page, offset, bv.bv_len);
>  	} else {
>  		memset(kaddr, 0, bv.bv_len);

And why the above change my if/else cascade doesn't really make much sense
any more and this should become:

	} else {
		if (page)
	 		memcpy_from_page(kaddr, page, offset, bv.bv_len);
		else
	 		memset(kaddr, 0, bv.bv_len);
 	}

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@....de>