> -----Original Message----- > From: David Miller [mailto:davem@xxxxxxxxxxxxx] > Sent: Saturday, March 8, 2014 12:07 PM > To: KY Srinivasan > Cc: netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx; > jasowang@xxxxxxxxxx > Subject: Re: [PATCH V2 1/6] Drivers: net: hyperv: Enable scatter gather I/O > > From: KY Srinivasan <kys@xxxxxxxxxxxxx> > Date: Sat, 8 Mar 2014 04:12:01 +0000 > > > > > > >> -----Original Message----- > >> From: David Miller [mailto:davem@xxxxxxxxxxxxx] > >> Sent: Saturday, March 8, 2014 3:18 AM > >> To: KY Srinivasan > >> Cc: netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > >> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx; > >> jasowang@xxxxxxxxxx > >> Subject: Re: [PATCH V2 1/6] Drivers: net: hyperv: Enable scatter > >> gather I/O > >> > >> From: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx> > >> 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: > > >> > + offset += bytes; > >> > + len -= bytes; > >> > + if (offset == PAGE_SIZE && len) { > >> > + page++; > >> > + offset = 0; > >> > + j++; > >> > + } > > thus incrementing j before the return statement. I don't mean to be argumentative, but I still don't see the issue here. You are right, I will execute the loop code at least once, but j gets incremented only if len is non-zero as well as offset being == PAGE_SIZE. "len" would have gotten decremented prior to the check and in the case we are talking about, "len" would be zero and so j would not get incremented. Regards, K. Y _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
