linux-kernel - Re: [PATCH V2 1/6] Drivers: net: hyperv: Enable scatter gather I/O

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Date:	Fri, 07 Mar 2014 16:47:56 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	kys@...rosoft.com
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	devel@...uxdriverproject.org, olaf@...fle.de, apw@...onical.com,
	jasowang@...hat.com
Subject: Re: [PATCH V2 1/6] Drivers: net: hyperv: Enable scatter gather I/O

From: "K. Y. Srinivasan" <kys@...rosoft.com>
Date: Thu,  6 Mar 2014 21:32:36 -0800

> +static u32 fill_pg_buf(struct page *page, u32 offset, u32 len,
> +			struct hv_page_buffer *pb)
> +{
> +	int j = 0;
> +
> +	/* Deal with compund pages by ignoring unused part
> +	 * of the page.
> +	 */
> +	page += offset  >> PAGE_SHIFT;

Please only one space between "offset" and ">>"

> +	offset &= ~PAGE_MASK;
> +
> +	while (len > 0) {
> +		unsigned long bytes;
> +
> +		bytes = PAGE_SIZE - offset;
> +		if (bytes > len)
> +			bytes = len;
> +		pb[j].pfn = page_to_pfn(page);
> +		pb[j].offset = offset;
> +		pb[j].len = bytes;
> +
> +		offset += bytes;
> +		len -= bytes;
> +
> +		if (offset == PAGE_SIZE && len) {
> +			page++;
> +			offset = 0;
> +			j++;
> +		}
> +	}
> +
> +	return j + 1;
> +}

I think this function has some edge case errors.

As I understand it, this function returns how many page buffer entries
were filled in.

But if we fill exactly the end of a page, we will report one too many
in the return value.

Consider an "offset" of X, where X is smaller than PAGE_SIZE.  And a
"len" which is exactly "PAGE_SIZE - offset".

We will fill in exactly one page buffer entry, however we will
erroneously return 2 from this function.

I believe the fix is to first replace the test at the end of the loop
with:

	page++;
	offset = 0;
	j++;

And subsequently change the function to just return plain "j".
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