Date:   Tue, 21 Dec 2021 10:06:48 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Logan Gunthorpe <logang@deltatee.com>
Cc:     linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org,
        Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Don Dutile <ddutile@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Jakowski Andrzej <andrzej.jakowski@intel.com>,
        Minturn Dave B <dave.b.minturn@intel.com>,
        Jason Ekstrand <jason@jlekstrand.net>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Xiong Jianxin <jianxin.xiong@intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Martin Oliveira <martin.oliveira@eideticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Subject: Re: [PATCH v4 21/23] mm: use custom page_free for P2PDMA pages
Message-ID: <20211221090648.GE7949@lst.de>
References: <20211117215410.3695-1-logang@deltatee.com> <20211117215410.3695-22-logang@deltatee.com>
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On Wed, Nov 17, 2021 at 02:54:08PM -0700, Logan Gunthorpe wrote:
> When P2PDMA pages are passed to userspace, they will need to be
> reference counted properly and returned to their genalloc after their
> reference count returns to 1. This is accomplished with the existing
> DEV_PAGEMAP_OPS and the .page_free() operation.
> 
> Change CONFIG_P2PDMA to select CONFIG_DEV_PAGEMAP_OPS and add
> MEMORY_DEVICE_PCI_P2PDMA to page_is_devmap_managed(),
> devmap_managed_enable_[put|get]() and free_devmap_managed_page().

Uuuh.  We are trying hard to kill off this magic free at refcount 1
behavior in the amdgpu device coherent series.  We really should not
add more of this.