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

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Message-ID: <51cc6645-2504-6d01-5626-238ed6ffff04@deltatee.com>
Date:   Thu, 2 Jun 2022 10:49:15 -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 2022-06-02 10:30, Jason Gunthorpe wrote:
> On Thu, Jun 02, 2022 at 10:16:10AM -0600, Logan Gunthorpe wrote:
> 
>>> Just stuff the pages into the mmap, and your driver unprobe will
>>> automatically block until all the mmaps are closed - no different than
>>> having an open file descriptor or something.
>>
>> Oh is that what we want?
> 
> Yes, it is the typical case - eg if you have a sysfs file open unbind
> hangs indefinitely. Many drivers can't unbind while they have open file
> descriptors/etc.
> 
> A couple drivers go out of their way to allow unbinding while a live
> userspace exists but this can get complicated. Usually there should be
> a good reason.
> 
> The module will already be refcounted anyhow because the mmap points
> to a char file which holds a module reference - meaning a simple rmmod
> of the driver shouldn't work already..

Also, I just tried it... If I open a sysfs file for an nvme device (ie.
/sys/class/nvme/nvme4/cntlid) and unbind the device, it does not block.
A subsequent read on that file descriptor returns ENODEV. Which is what
I would have expected.

Logan