Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 29 Sep 2021 20:05:43 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
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>,
        Christian =?utf-8?B?S8O2bmln?= <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 v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()
Message-ID: <20210929230543.GB3544071@ziepe.ca>
References: <20210916234100.122368-1-logang@deltatee.com>
 <20210916234100.122368-20-logang@deltatee.com>
 <20210928195518.GV3544071@ziepe.ca>
 <8d386273-c721-c919-9749-fc0a7dc1ed8b@deltatee.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <8d386273-c721-c919-9749-fc0a7dc1ed8b@deltatee.com>
Precedence: bulk

On Wed, Sep 29, 2021 at 03:42:00PM -0600, Logan Gunthorpe wrote:

> The main reason is probably this: if we don't use VM_MIXEDMAP, then we
> can't set pte_devmap(). 

I think that is an API limitation in the fault routines..

finish_fault() should set the pte_devmap - eg by passing the
PFN_DEV|PFN_MAP somehow through the vma->vm_page_prot to mk_pte() or
otherwise signaling do_set_pte() that it should set those PTE bits
when it creates the entry.

(or there should be a vmf_* helper for this special case, but using
the vmf->page seems righter to me)

> If we don't set pte_devmap(), then every single page that GUP
> processes needs to check if it's a ZONE_DEVICE page and also if it's
> a P2PDMA page (thus dereferencing pgmap) in order to satisfy the
> requirements of FOLL_PCI_P2PDMA.

Definately not suggesting not to set pte_devmap(), only that
VM_MIXEDMAP should not be set on VMAs that only contain struct
pages. That is an abuse of what it is intended for.

At the very least there should be a big comment above the usage
explaining that this is just working around a limitation in
finish_fault() where it cannot set the PFN_DEV|PFN_MAP bits today.

Jason