Re: [v2,4/7] scatterlist: add sg_alloc_table_from_buf() helper

[Boris Brezillon <boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

Hi Vignesh,

On Thu, 31 Mar 2016 10:26:59 +0530
Vignesh R <vigneshr-l0cyMroinI0@public.gmane.org> wrote:

> Hi,
> 
> On 03/30/2016 09:09 PM, Boris BREZILLON wrote:
> 
> [...]
> 
> > +int sg_alloc_table_from_buf(struct sg_table *sgt, const void *buf, size_t len,
> > +			    const struct sg_constraints *constraints,
> > +			    gfp_t gfp_mask)
> > +{
> > +	struct sg_constraints cons = { };
> > +	size_t remaining, chunk_len;
> > +	const void *sg_buf;
> > +	int i, ret;
> > +
> > +	if (constraints)
> > +		cons = *constraints;
> > +
> > +	ret = sg_check_constraints(&cons, buf, len);
> > +	if (ret)
> > +		return ret;
> > +
> > +	sg_buf = buf;
> > +	remaining = len;
> > +	i = 0;
> > +	sg_for_each_chunk_in_buf(sg_buf, remaining, chunk_len, &cons)
> > +		i++;
> > +
> > +	ret = sg_alloc_table(sgt, i, gfp_mask);
> > +	if (ret)
> > +		return ret;
> > +
> > +	sg_buf = buf;
> > +	remaining = len;
> > +	i = 0;
> > +	sg_for_each_chunk_in_buf(sg_buf, remaining, chunk_len, &cons) {
> > +		if (is_vmalloc_addr(sg_buf)) {
> > +			struct page *vm_page;
> > +
> > +			vm_page = vmalloc_to_page(sg_buf);
> > +			if (!vm_page) {
> > +				ret = -ENOMEM;
> > +				goto err_free_table;
> > +			}
> > +
> > +			sg_set_page(&sgt->sgl[i], vm_page, chunk_len,
> > +				    offset_in_page(sg_buf));
> > +		} else {
> > +			sg_set_buf(&sgt->sgl[i], sg_buf, chunk_len);
> > +		}
> > +
> 
> If the buf address is in PKMAP_BASE - PAGE_OFFSET-1 region (I have
> observed that JFFS2 FS provides buffers in above region to MTD layer),
> if CONFIG_DEBUG_SG is set then sg_set_buf() will throw a BUG_ON() as
> virt_addr_is_valid() will return false. Is there a sane way to handle
> buffers of PKMAP_BASE region with sg_*  APIs?
> Or, is the function sg_alloc_table_from_buf() not to be used with such
> buffers?

It should be usable with kmapped buffers too: I'll provide a new version
to support that.
That makes me realize I'm not checking the virtual address consistency
in sg_check_constraints().

Thanks,

Boris

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com