Re: [PATCH] scsi_debug: disable clustering

Matthew Wilcox
2008-02-17T14:28:49Z

On Sun, Feb 17, 2008 at 08:18:11AM -0600, James Bottomley wrote:
> No, he means that kmap_atomic can only map a page of data. This makes
> single page only sg list entries and input assumption into this loop.
> with ENABLE_CLUSTERING, that's potentially not true. Of course, this
> accidentally works most of the time because of the way kmap functions.

Ah, right. I'm on the verge of releasing a ram-based scsi driver I've
been working on ... this loop should work fine with clustering as it
takes account of the sg potentially having multiple pages:

scsi_for_each_sg(cmnd, sg, scsi_sg_count(cmnd), i) {
struct page *sgpage = sg_page(sg);
unsigned int to_off = sg->offset;
unsigned int sg_copy = sg->length;
if (sg_copy > len)
sg_copy = len;
len -= sg_copy;

while (sg_copy > 0) {
char *vto, *vfrom;
unsigned int page_copy;

if (from_off > to_off)
page_copy = PAGE_SIZE - from_off;
else
page_copy = PAGE_SIZE - to_off;
if (page_copy > sg_copy)
page_copy = sg_copy;

vfrom = get_data_page(data_pfn);
vto = get_sg_page(sgpage);
memcpy(vto + to_off, vfrom + from_off, page_copy);
put_sg_page(vto);
put_data_page(vfrom);

from_off += page_copy;
if (from_off == PAGE_SIZE) {
from_off = 0;
data_pfn++;
}
to_off += page_copy;
if (to_off == PAGE_SIZE) {
to_off = 0;
sgpage++;
}
sg_copy -= page_copy;
}
if (!len)
break;
}

get_data_page() and get_sg_page() each do a kmap_atomic, so I was
concerned that I might be out of KM_USER* pages.

--
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