On Sun, Mar 21, 2021 at 9:03 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> On Sun, Mar 21, 2021 at 08:54:31PM +0800, Zhu Yanjun wrote:
> > On Sun, Mar 21, 2021 at 8:07 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> > >
> > > On Sun, Mar 21, 2021 at 04:06:07PM +0800, Zhu Yanjun wrote:
> > > > > > > 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?
> > > >
> > > > With the following snippet
> > > > "
> > > > struct ib_umem *region;
> > > > region = ib_umem_get(pd->device, start, len, access);
> > > >
> > > > page_size = ib_umem_find_best_pgsz(region,
> > > > SZ_4K | SZ_2M | SZ_1G,
> > > > virt);
> > > > "
> > >
> > > Can you please stop posting random bits of code that do nothing to
> > > answer the question?
> > >
> > > > IMHO, you can reproduce this problem in your local host.
> > >
> > > Uh, no, you need to communicate effectively or stop posting please.
> >
> > can you read the following link again
> >
> > https://patchwork.kernel.org/project/linux-rdma/patch/CAD=hENeqTTmpS5V+G646V0QvJFLVSd3Sq11ffQFcDXU-OSsQEg@xxxxxxxxxxxxxx/#24031643
> >
> > In this link, I explained it in details.
>
> No, this is all nonsense
>
> > Since the max_segment is very big, so normally the prv->length +
> > PAGE_SIZE is less than max_segment, it will loop over and over again,
> > until n_pages is zero,
>
> And each iteration increases
>
> prv->length += PAGE_SIZE;
> n_pages--;
>
> Where prv is the last sgl entry, so decreasing n_pages cannot also
> yield a 4k sgl entry like you are reporting.
No. You suppose n_pages is very big, then prv->length will increase to
reach max_segment.
If n_pages is very small, for example, we suppose n_pages is 1, then
prv->length will
not reach max_segment.
In fact, in my test case, n_pages is very small.
Zhu Yanjun
>
> You get 4k sgl entries if max_segment is *too small* not too big!
>
> Jason
