linux-kernel - Re: [PATCH v4 21/23] mm: use custom page

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Date:   Tue, 21 Dec 2021 10:06:48 +0100
From:   Christoph Hellwig <hch@....de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>
Subject: Re: [PATCH v4 21/23] mm: use custom page_free for P2PDMA pages

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.