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

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Mar 20, 2021 at 11:38:26AM +0800, Zhu Yanjun wrote:
> On Fri, Mar 19, 2021 at 9:48 PM Jason Gunthorpe <jgg@nvidia.com> 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@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.
> >
> > 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.

What does n_pages have to do with max_segment?

Please try to explain clearly.

Jason