On Thu, Mar 11, 2021 at 06:41:43PM +0800, Zhu Yanjun wrote:
> On Mon, Mar 8, 2021 at 8:16 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> >
> > On Mon, Mar 08, 2021 at 06:13:52PM +0800, Zhu Yanjun wrote:
> >
> > > And I delved into the source code of __sg_alloc_table_from_pages. I
> > > found that this function is related with ib_dma_max_seg_size. So
> > > when ib_dma_max_seg_size is set to UINT_MAX, the sg dma address is
> > > 4K (one page). When ib_dma_max_seg_size is set to SZ_2M, the sg dma
> > > address is 2M now.
> >
> > That seems like a bug, you should fix it
>
> Hi, Jason && Leon
>
> I compared the function __sg_alloc_table_from_pages with ib_umem_add_sg_table.
> In __sg_alloc_table_from_pages:
>
> "
> 449 if (prv) {
> 450 unsigned long paddr = (page_to_pfn(sg_page(prv))
> * PAGE_SIZE +
> 451 prv->offset + prv->length) /
> 452 PAGE_SIZE;
> 453
> 454 if (WARN_ON(offset))
> 455 return ERR_PTR(-EINVAL);
> 456
> 457 /* Merge contiguous pages into the last SG */
> 458 prv_len = prv->length;
> 459 while (n_pages && page_to_pfn(pages[0]) == paddr) {
> 460 if (prv->length + PAGE_SIZE > max_segment)
> 461 break;
> 462 prv->length += PAGE_SIZE;
> 463 paddr++;
> 464 pages++;
> 465 n_pages--;
> 466 }
> 467 if (!n_pages)
> 468 goto out;
> 469 }
>
> "
> if prv->length + PAGE_SIZE > max_segment, then set another sg.
> In the commit "RDMA/umem: Move to allocate SG table from pages",
> max_segment is dma_get_max_seg_size.
> Normally it is UINT_MAX. So in my host, prv->length + PAGE_SIZE is
> usually less than max_segment
> since length is unsigned int.
I don't understand what you are trying to say
460 if (prv->length + PAGE_SIZE > max_segment)
max_segment should be a very big number and "prv->length + PAGE_SIZE" should
always be < max_segment so it should always be increasing the size of
prv->length and 'rpv' here is the sgl.
The other loops are the same.
Jason
