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Date:   Wed, 30 Jan 2019 09:00:06 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Jason Gunthorpe <jgg@mellanox.com>
Cc:     Logan Gunthorpe <logang@deltatee.com>,
        Jerome Glisse <jglisse@redhat.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
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        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
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        Joerg Roedel <jroedel@suse.de>,
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Subject: Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device
 vma
Message-ID: <20190130080006.GB29665@lst.de>
References: <20190129191120.GE3176@redhat.com> <20190129193250.GK10108@mellanox.com> <99c228c6-ef96-7594-cb43-78931966c75d@deltatee.com> <20190129205749.GN3176@redhat.com> <2b704e96-9c7c-3024-b87f-364b9ba22208@deltatee.com> <20190129215028.GQ3176@redhat.com> <deb7ba21-77f8-0513-2524-ee40a8ee35d5@deltatee.com> <20190129234752.GR3176@redhat.com> <655a335c-ab91-d1fc-1ed3-b5f0d37c6226@deltatee.com> <20190130041841.GB30598@mellanox.com>
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On Wed, Jan 30, 2019 at 04:18:48AM +0000, Jason Gunthorpe wrote:
> Every attempt to give BAR memory to struct page has run into major
> trouble, IMHO, so I like that this approach avoids that.

Way less problems than not having struct page for doing anything
non-trivial.  If you map the BAR to userspace with remap_pfn_range
and friends the mapping is indeed very simple.  But any operation
that expects a page structure, which is at least everything using
get_user_pages won't work.

So you can't do direct I/O to your remapped BAR, you can't create MRs
on it, etc, etc.