Re: [for-next PATCH 4/5] RDMA/rxe: refactor iova_to_vaddr

Fabio M. De Francesco
2022-11-22T23:24:36Z

On lunedì 21 novembre 2022 19:53:54 CET Jason Gunthorpe wrote:
> On Wed, Nov 16, 2022 at 01:37:14PM +0100, Fabio M. De Francesco wrote:
> > > - return (void *)(uintptr_t)mr->map[m]->buf[n].addr + offset;
> > > + if (mr->ibmr.type == IB_MR_TYPE_USER) {
> > > + char *paddr;
> > > + struct page *pg = mr->map[m]->buf[n].page;
> > > +
> > > + paddr = kmap_local_page(pg);
> > > + if (paddr == NULL) {
> > > + pr_warn("Failed to map page");
> > > + return NULL;
> > > + }
> >
> > I know nothing about this code but I am here as a result of regular checks
> > for changes to HIGHMEM mappings across the entire kernel. So please
forgive
> > me if I'm objecting to the correct changes.
> >
> > 1) It looks like this code had a call to page_address() and you converted
it
> > to mapping with kmap_local_page().
>
> It only worked properly because the kconfig is blocking CONFIG_HIGHMEM
> so ZONE_HIGHMEM doesn't exist. These pages are obtained from GUP and
> could be anything.
>
> This is done indirectly through
>
> config INFINIBAND_VIRT_DMA
> def_bool !HIGHMEM
> But this patch is undoing parts of what are causing that restriction
> by using the proper APIs. Though I'm not sure if we can eventually get
> to remove it for RXE completely.
> Jason

Ah, OK. This code was for !HIGHMEM kernels...

I understand but, FWIW I doubt that the use of page_address(), instead of
kmapping, should ever be made on the bases that kconfig is blocking HIGHMEM.

However, if I understand correctly, that restriction (!HIGHMEM) is going to be
removed. Therefore, page_address() will break on HIGHMEM and Jason is
correctly converting to kmap_local_page().

There is only one thing left... I think that he missed another mail from me.
The one you responded to was missing the final paragraph, so I sent another
message few minutes later.

kmap_local_page() returns always valid pointers to kernel virtual addresses. I
can't see any ways for kmap_local_page() to return NULL. Therefore, I was
asking for the reason to check for NULL.

Thanks,

Fabio