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Date: Thu, 27 Aug 2020 09:35:52 +0100
From: Christoph Hellwig <hch@infradead.org>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Subject: Re: [PATCH 6/9] iomap: Convert read_count to byte count
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> @@ -269,20 +263,17 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  	if (ctx->bio && bio_end_sector(ctx->bio) == sector)
>  		is_contig = true;
>  
> -
>  	/*
> -	 * If we start a new segment we need to increase the read count, and we
> -	 * need to do so before submitting any previous full bio to make sure
> -	 * that we don't prematurely unlock the page.
> +	 * We need to increase the read count before submitting any
> +	 * previous bio to make sure that we don't prematurely unlock
> +	 * the page.
>  	 */
>  	if (iop)
> -		atomic_inc(&iop->read_count);
> +		atomic_add(plen, &iop->read_count);
> +
> +	if (is_contig &&
> +	    __bio_try_merge_page(ctx->bio, page, plen, poff, &same_page))
> +		goto done;
>  
>  	if (!ctx->bio || !is_contig || bio_full(ctx->bio, plen)) {

I think we can simplify this a bit by reordering the checks:

 	if (iop)
		atomic_add(plen, &iop->read_count);

  	if (ctx->bio && bio_end_sector(ctx->bio) == sector)
		if (__bio_try_merge_page(ctx->bio, page, plen, poff,
				&same_page))
			goto done;
		is_contig = true;
	}

	if (!is_contig || bio_full(ctx->bio, plen)) {
		// the existing case to allocate a new bio
	}

Also I think read_count should be renamed to be more descriptive,
something like read_bytes_pending?  Same for the write side.
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