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From: Christoph Hellwig <hch@lst.de>
To: Alexander Potapenko <glider@google.com>
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Subject: Re: [PATCH v5 34/38] kmsan: dma: unpoison memory mapped by
 dma_direct_map_page()
Message-ID: <20200328085233.GA26550@lst.de>
References: <20200325161249.55095-1-glider@google.com> <20200325161249.55095-35-glider@google.com> <20200325161905.GA19868@lst.de> <CAG_fn=WjuC+uRyc4ocQ9YLOkZtLY8a+EnNZiVG5SsvczJBunLw@mail.gmail.com> <20200327170647.GA22758@lst.de> <CAG_fn=VsTix2ff-3vB2kfv+tTgGPBHvMsKQijOuygeCrbXkg+g@mail.gmail.com>
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On Fri, Mar 27, 2020 at 07:46:08PM +0100, Alexander Potapenko wrote:
> > > Do you mean comparing the address to TASK_SIZE, or is there a more
> > > portable way to check that?
> >
> > !PageHighMem(page) implies the page has a kernel direct mapping.
>=20
> I tried adding this check and started seeing false positives because
> the virtio_ring driver actually uses highmem pages for DMA, and data
> from those pages is later copied to the kernel.
> Guess it's easier to just allow handling highmem pages? What problems
> do you anticipate?

For PageHighMem(page), page_address(page) is not actually valid, so =D0=86=
'm
not sure how your code in this patch even worked at all.  Note that
all drivers (well, except for a few buggy legacy ones with workarounds)
can DMA from/to highmem.