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SubjectRe: [PATCH V2 1/6] Drivers: net: hyperv: Enable scatter gather I/O
From
From: KY Srinivasan <kys@microsoft.com>
Date: Sat, 8 Mar 2014 04:12:01 +0000

>
>
>> -----Original Message-----
>> From: David Miller [mailto:davem@davemloft.net]
>> Sent: Saturday, March 8, 2014 3:18 AM
>> To: KY Srinivasan
>> Cc: netdev@vger.kernel.org; linux-kernel@vger.kernel.org;
>> devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
>> jasowang@redhat.com
>> Subject: Re: [PATCH V2 1/6] Drivers: net: hyperv: Enable scatter gather I/O
>>
>> From: "K. Y. Srinivasan" <kys@microsoft.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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