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

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Date:	Sat, 08 Mar 2014 01:36:58 -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: KY Srinivasan <kys@...rosoft.com>
Date: Sat, 8 Mar 2014 04:12:01 +0000

> 
> 
>> -----Original Message-----
>> From: David Miller [mailto:davem@...emloft.net]
>> Sent: Saturday, March 8, 2014 3:18 AM
>> To: KY Srinivasan
>> 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.
> 
> I may be missing something here; but in the case you are describing we would return a value
> of one as we should. When offset + len is exactly equal to the page size, we would not increment
> the value of j, since at that point len == 0. Since the initial value of j was 0 and we return (j + 1),
> we will return 1.

That's not what happens, you execute the loop code at least once, and
hit:

>> > +		if (offset == PAGE_SIZE && len) {
>> > +			page++;
>> > +			offset = 0;
>> > +			j++;
>> > +		}

thus incrementing j before the return statement.
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