linux-kernel - Re: [PATCH v6 20/21] PCI/P2PDMA: Introduce pci_mmap

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Message-ID: <d336cfe8-2451-04c3-a2ce-0e8e47afd1e3@deltatee.com>
Date:   Fri, 27 May 2022 16:41:08 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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>,
        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>,
        Ralph Campbell <rcampbell@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v6 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()



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