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

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Mar 12, 2021 at 04:04:35PM +0800, Zhu Yanjun wrote:
> On Fri, Mar 12, 2021 at 8:25 AM Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> > 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@nvidia.com> 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.
> 
> Sure.
> 
> When max_segment is UINT_MAX, prv->length is UINT_MAX - PAGE_SIZE.
> It is big. That is, segment size is UINT_MAX - PAGE_SIZE.
> 
> But from the function ib_umem_add_sg_table, the prv->length is SZ_2M.
> That is, segment size if SZ_2M.
> 
> It is the difference between the 2 functions.

I still have no idea what you are trying to say. Why would prv->length
be 'UINT - PAGE_SIZE'? That sounds like max_segment

Jason