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.
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,
then the function __sg_alloc_table_from_pages will exit. So the
function __sg_alloc_table_from_pages has few chances to call the
function sg_set_page.
This behavior is not like the function ib_umem_add_sg_table. In
ib_umem_add_sg_table, the "max_seg_sz - sg->length" has more chances
to be greater than (len << PAGE_SHIFT).
So the function sg_set_page is called more times than
__sg_alloc_table_from_pages.
The above causes the different sg lists from the 2 functions
__sg_alloc_table_from_pages and ib_umem_add_sg_table.
The most important thing is "if (prv->length + PAGE_SIZE >
max_segment)" in __sg_alloc_table_from_pages and "if ((max_seg_sz -
sg->length) >= (len << PAGE_SHIFT))" in the function
ib_umem_add_sg_table.
When max_segment is UINT_MAX, the different test in the 2 functions
causes the different sg lists.
Zhu Yanjun
>
> Jason
