On Sat, Mar 20, 2021 at 11:38 AM Zhu Yanjun <zyjzyj2000@xxxxxxxxx> wrote:
>
> On Fri, Mar 19, 2021 at 9:48 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> >
> > On Fri, Mar 19, 2021 at 09:33:13PM +0800, Zhu Yanjun wrote:
> > > On Fri, Mar 19, 2021 at 9:01 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> > > >
> > > > On Sat, Mar 13, 2021 at 11:02:41AM +0800, Zhu Yanjun wrote:
> > > > > On Fri, Mar 12, 2021 at 10:01 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Fri, Mar 12, 2021 at 09:49:52PM +0800, Zhu Yanjun wrote:
> > > > > > > In short, the sg list from __sg_alloc_table_from_pages is different
> > > > > > > from the sg list from ib_umem_add_sg_table.
> > > > > >
> > > > > > I don't care about different. Tell me what is wrong with what we have
> > > > > > today.
> > > > > >
> > > > > > I thought your first message said the sgl's were too small, but now
> > > > > > you seem to say they are too big?
> > > > >
> > > > > Sure.
> > > > >
> > > > > The sg list from __sg_alloc_table_from_pages, length of sg is too big.
> > > > > And the dma address is like the followings:
> > > > >
> > > > > "
> > > > > sg_dma_address(sg):0x4b3c1ce000
> > > > > sg_dma_address(sg):0x4c3c1cd000
> > > > > sg_dma_address(sg):0x4d3c1cc000
> > > > > sg_dma_address(sg):0x4e3c1cb000
> > > > > "
> > > >
> > > > Ok, so how does too big a dma segment side cause
> > > > __sg_alloc_table_from_pages() to return sg elements that are too
> > > > small?
> > > >
> > > > I assume there is some kind of maths overflow here?
> > > Please check this function __sg_alloc_table_from_pages
> > > "
> > > ...
> > > 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) <--max_segment is too big. So n_pages will be 0. Then
> > > the function will goto out to exit.
> >
> > You already said this.
> >
> > You are reporting 4k pages, if max_segment is larger than 4k there is
> > no such thing as "too big"
> >
> > I assume it is "too small" because of some maths overflow.
>
> 459 while (n_pages && page_to_pfn(pages[0]) == paddr) {
> 460 if (prv->length + PAGE_SIZE >
> max_segment) <--it max_segment is big, n_pages is zero.
> 461 break;
> 462 prv->length += PAGE_SIZE;
> 463 paddr++;
> 464 pages++;
> 465 n_pages--;
> 466 }
> 467 if (!n_pages) <---here, this function will goto out.
> 468 goto out;
> ...
> 509 chunk_size = ((j - cur_page) << PAGE_SHIFT) - offset;
> 510 sg_set_page(s, pages[cur_page],
> 511 min_t(unsigned long, size,
> chunk_size), offset); <----this function will not have many chance to
> be called if max_segment is big.
> 512 added_nents++;
> 513 size -= chunk_size;
>
> If the max_segment is not big enough, for example it is SZ-2M,
> sg_set_page will be called every SZ_2M.
> To now, I do not find any math overflow.
147 struct ib_umem *ib_umem_get(struct ib_device *device, unsigned long addr,
148 size_t size, int access)
149 {
...
244 umem->nmap =
245 ib_dma_map_sg_attrs(device, umem->sg_head.sgl,
umem->sg_nents,
246 DMA_BIDIRECTIONAL, dma_attr);
...
And after the function ib_dma_map_sg_attrs, dma address is set.
To now, I can not find maths overflow.
Zhu Yanjun
>
> Zhu Yanjun
> >
> > You should add some prints and find out what is going on.
> >
> > Jason
