Re: Fwd: [PATCH 1/1] RDMA/umem: add back hugepage sg list

[Zhu Yanjun <zyjzyj2000@gmail.com>]

On Fri, Mar 19, 2021 at 9:01 PM Jason Gunthorpe <jgg@nvidia.com> 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@nvidia.com> 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.
 461                                 break;
 462                         prv->length += PAGE_SIZE;
 463                         paddr++;
 464                         pages++;
 465                         n_pages--;
 466                 }
 467                 if (!n_pages)
 468                         goto out;
...
"

>
> Jason