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Date:   Tue, 1 Oct 2019 14:43:58 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Alan Mikhak <alan.mikhak@sifive.com>
Cc:     linux-kernel@vger.kernel.org, martin.petersen@oracle.com,
        alexios.zavras@intel.com, ming.lei@redhat.com,
        gregkh@linuxfoundation.org, tglx@linutronix.de,
        christophe.leroy@c-s.fr, Palmer Dabbelt <palmer@sifive.com>,
        Paul Walmsley <paul.walmsley@sifive.com>
Subject: Re: [PATCH] scatterlist: Validate page before calling PageSlab()
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On Tue, Oct 01, 2019 at 10:26:38AM -0700, Alan Mikhak wrote:

> > > Cost of adding page structs to a large PCI I/O address range can be
> > > quite substantial. Allowing PCI I/O pages without page structs may be
> > > desirable. Perhaps it is worth considering this cost.
> >
> > This is generally agreed, but nobody has figured out a solution.
> >
> > > Scatterlist has no problem doing its memcpy() from pages without a
> > > page struct that were obtained from ioremap(). It is only at
> >
> > Calling memcpy on pointers from ioremap is very much not allowed. Code
> > has to use the iomem safe memcpy.
>
> Is it in the realm of possible to add support for such PCI I/O pages
> to scatterlist? Perhaps some solution is possible by adding a new
> function, say sg_set_iomem_page(), and a new SG_MITER_IOMEM flag that
> tells scatterlist to use iomem safe memcpy functions when the page is
> not backed by page struct because it was obtained from ioremap(). This
> flag can also be used at sg_miter_stop() to not call PageSlab() or
> flush_kernel_dcache_page().

People have tried many different things so far, it is more comple than
just the copy functions as there is also sg_page to worry about.

> If supporting PCI I/O pages is not possible, would it be possible to
> check for invalid page pointers in sg_set_page() and communicate the
> requirement for a page struct backing in its description?

Clarifying the comments seems reasonable to me.

Jason