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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Li Zhijian <lizhijian@fujitsu.com>,
        Zhu Yanjun <zyjzyj2000@gmail.com>,
        Leon Romanovsky <leon@kernel.org>, linux-rdma@vger.kernel.org,
        linux-kernel@vger.kernel.org, Ira Weiny <ira.weiny@intel.com>
Subject: Re: [for-next PATCH 4/5] RDMA/rxe: refactor iova_to_vaddr
Date:   Wed, 23 Nov 2022 00:24:26 +0100
Message-ID: <2158152.NgBsaNRSFp@suse>
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On luned=EC 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 =3D=3D IB_MR_TYPE_USER) {
> > > +             char *paddr;
> > > +             struct page *pg =3D mr->map[m]->buf[n].page;
> > > +
> > > +             paddr =3D kmap_local_page(pg);
> > > +             if (paddr =3D=3D NULL) {
> > > +                     pr_warn("Failed to map page");
> > > +                     return NULL;
> > > +             }
> >=20
> > I know nothing about this code but I am here as a result of regular che=
cks
> > for changes to HIGHMEM mappings across the entire kernel. So please=20
forgive
> > me if I'm objecting to the correct changes.
> >=20
> > 1) It looks like this code had a call to page_address() and you convert=
ed=20
it
> > to mapping with kmap_local_page().
>=20
> 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.
>=20
> This is done indirectly through
>=20
> 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=20
kmapping, should ever be made on the bases that kconfig is blocking HIGHMEM=
=2E =20

However, if I understand correctly, that restriction (!HIGHMEM) is going to=
 be=20
removed. Therefore, page_address() will break on HIGHMEM and Jason is=20
correctly converting to kmap_local_page().

There is only one thing left... I think that he missed another mail from me=
=2E=20
The one you responded to was missing the final paragraph, so I sent another=
=20
message few minutes later.

kmap_local_page() returns always valid pointers to kernel virtual addresses=
=2E I=20
can't see any ways for kmap_local_page() to return NULL. Therefore, I was=20
asking for the reason to check for NULL.

Thanks,

=46abio