Re: direct-to-BIO for O_DIRECT

[Ingo Oeser]

On Mon, Jul 08, 2002 at 09:26:48PM -0700, Andrew Morton wrote:
> > > It would be nice if we could just map a set of user pages
> > > to a scatterlist.
> > 
> > After disabling kiobufs in sg I would like such a drop
> > in replacement.
> 
> Ben had lightweight sg structures called `kvecs' and `kveclets'. And
> library functions to map pages into them.  And code to attach them
> to BIOs.  So we'll be looking at getting that happening.

BIOs are for BLOCK devices we want sth. like this for CHARACTER
devices.

I just want sth. along the lines of this:

/* Pin down (COMPLETE!) user pages and put them into a scatter gather list */
int sg_map_user_pages(struct scatterlist *sgl, const unsigned int nr_pages, 
		unsigned long uaddr, int rw) {
	int res, i;
	struct page *pages[nr_pages];

	down_read(&current->mm->mmap_sem);
	res = get_user_pages(
			current,
			current->mm,
			uaddr,
			nr_pages,
			rw == READ, /* logic is perversed^Wreversed here :-( */
			0, /* don't force */
			&pages[0],
			NULL);
	up_read(&current->mm->mmap_sem);

	/* Errors and no page mapped should return here */
	if (res <= 0) return res;

	for (i=1; i < res; i++) {
		sgl[i].page = pages[i];	
	}
	return res;
}

/* And unmap them... */
int sg_unmap_user_pages(struct scatterlist *sgl, const unsigned int nr_pages) {
	int i;

	for (i=0; i < nr_pages; i++)
		page_cache_release(sgl[i].page);

	return 0;
}

Possibly more complicated and less error prone, but you get the
idea ;-)

Regards

Ingo Oeser
-- 
Science is what we can tell a computer. Art is everything else. --- D.E.Knuth