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Date:   Fri, 16 Aug 2019 14:28:46 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Jerome Glisse <jglisse@redhat.com>, Christoph Hellwig <hch@lst.de>,
        Ben Skeggs <bskeggs@redhat.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "nouveau@lists.freedesktop.org" <nouveau@lists.freedesktop.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk
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On Fri, Aug 16, 2019 at 10:21:41AM -0700, Dan Williams wrote:

> > We can do a get_dev_pagemap inside the page_walk and touch the pgmap,
> > or we can do the 'device mutex && retry' pattern and touch the pgmap
> > in the driver, under that lock.
> >
> > However in all cases the current get_dev_pagemap()'s in the page walk
> > are not necessary, and we can delete them.
> 
> Yes, as long as 'struct page' instances resulting from that lookup are
> not passed outside of that lock.

Indeed.

Also, I was reflecting over lunch that the hmm_range_fault should only
return DEVICE_PRIVATE pages for the caller's device (see other thread
with HCH), and in this case, the caller should also be responsible to
ensure that the driver is not calling hmm_range_fault at the same time
it is deleting it's own DEVICE_PRIVATE mapping - ie by fencing its
page fault handler.

This does not apply to PCI_P2PDMA, but, lets see how that looks when
we get there.

So the whole thing seems pretty safe.

Jason