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Date:   Wed, 24 Jun 2020 09:22:00 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ralph Campbell <rcampbell@nvidia.com>
Cc:     Christoph Hellwig <hch@lst.de>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Jerome Glisse <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RESEND PATCH] mm: fix migrate_vma_setup() src_owner and
 normal pages
Message-ID: <20200624072200.GA18609@lst.de>
References: <20200622222008.9971-1-rcampbell@nvidia.com> <20200623114018.GA17496@lst.de> <a0dbf913-2bd0-b032-596e-520e678d5b5b@nvidia.com>
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On Tue, Jun 23, 2020 at 10:05:19AM -0700, Ralph Campbell wrote:
>
> On 6/23/20 4:40 AM, Christoph Hellwig wrote:
>> On Mon, Jun 22, 2020 at 03:20:08PM -0700, Ralph Campbell wrote:
>>> The caller of migrate_vma_setup() does not know what type of page is
>>> stored in the CPU's page tables. Pages within the specified range are
>>> free to be swapped out, migrated, or freed until after migrate_vma_setup()
>>> returns. The caller needs to set struct migrate_vma.src_owner in case a
>>> page is a ZONE device private page that the device owns and might want to
>>> migrate. However, the current code skips normal anonymous pages if
>>> src_owner is set, thus preventing those pages from being migrated.
>>> Remove the src_owner check for normal pages since src_owner only applies
>>> to device private pages and allow a range of normal and device private
>>> pages to be migrated.
>>
>> src_owner being set means we want to migrate from device private
>> memory to normal host DRAM.  What kind of problem do you see of
>> not touching already present pages in that path?
>>
>
> The problem is that migrate_vma_setup() invalidates the address range so any
> previously migrated pages to device private memory have to be faulted in
> again. By having the PFN of those device private pages in the src array, the
> driver can reinstate the device MMU mappings and avoid the page faults.

Maybe add that to the changelog?