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Date:   Sun, 6 Oct 2019 23:12:12 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Alan Mikhak <alan.mikhak@sifive.com>, 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@sifive.com,
        paul.walmsley@sifive.com
Subject: Re: [PATCH] scatterlist: Validate page before calling PageSlab()
Message-ID: <20191007061212.GA17978@infradead.org>
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 <20191001121623.GA22532@ziepe.ca>
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On Tue, Oct 01, 2019 at 09:16:23AM -0300, Jason Gunthorpe wrote:
> > If the virtual address obtained from ioremap() is not
> > backed by a page struct, virt_to_page() returns an
> > invalid page pointer. However, sg_copy_buffer() can
> > correctly recover the original virtual address. Such
> > addresses can successfully be assigned to scatterlist
> > segments to transfer data across the PCI bus with
> > sg_copy_buffer() if it were not for the crash in
> > PageSlab() when called by sg_miter_stop().
> 
> I thought we already agreed in general that putting things that don't
> have struct page into the scatter list was not allowed?

Yes, that absolutely is the case.