Re: [PATCH v6 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()

From: Logan Gunthorpe <logang@deltatee.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
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 König" <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>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"Bjorn Helgaas" <bhelgaas@google.com>
Subject: Re: [PATCH v6 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()
Date: Fri, 27 May 2022 16:41:08 -0600	[thread overview]
Message-ID: <d336cfe8-2451-04c3-a2ce-0e8e47afd1e3@deltatee.com> (raw)
In-Reply-To: <20220527190307.GG2960187@ziepe.ca>

On 2022-05-27 13:03, Jason Gunthorpe wrote:
> On Fri, May 27, 2022 at 09:35:07AM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2022-05-27 06:55, Jason Gunthorpe wrote:
>>> On Thu, Apr 07, 2022 at 09:47:16AM -0600, Logan Gunthorpe wrote:
>>>> +static void pci_p2pdma_unmap_mappings(void *data)
>>>> +{
>>>> +	struct pci_dev *pdev = data;
>>>> +	struct pci_p2pdma *p2pdma = rcu_dereference_protected(pdev->p2pdma, 1);
>>>> +
>>>> +	/* Ensure no new pages can be allocated in mappings */
>>>> +	p2pdma->active = false;
>>>> +	synchronize_rcu();
>>>> +
>>>> +	unmap_mapping_range(p2pdma->inode->i_mapping, 0, 0, 1);
>>>> +
>>>> +	/*
>>>> +	 * On some architectures, TLB flushes are done with call_rcu()
>>>> +	 * so to ensure GUP fast is done with the pages, call synchronize_rcu()
>>>> +	 * before freeing them.
>>>> +	 */
>>>> +	synchronize_rcu();
>>>> +	pci_p2pdma_free_mappings(p2pdma->inode->i_mapping);
>>>
>>> With the series from Felix getting close this should get updated to
>>> not set pte_devmap and use proper natural refcounting without any of
>>> this stuff.
>>
>> Can you send a link? I'm not sure what you are referring to.
> 
> IIRC this is the last part:
> 
> https://lore.kernel.org/linux-mm/20220524190632.3304-1-alex.sierra@amd.com/
> 
> And the earlier bit with Christoph's pieces looks like it might get
> merged to v5.19..
> 
> The general idea is once pte_devmap is not set then all the
> refcounting works the way it should. This is what all new ZONE_DEVICE
> users should do..

Ok, I don't actually follow how those patches relate to this.

Based on your description I guess I don't need to set PFN_DEV and
perhaps not use vmf_insert_mixed()? And then just use vm_normal_page()?

But the refcounting of the pages seemed like it was already sane to me,
unless you mean that the code no longer has to synchronize_rcu() before
returning the pages... that would be spectacular and clean things up a
lot (plus fix an annoying issue where if you use then free all the
memory you can't allocate new memory for an indeterminate amount of time).

Logan