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

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Date:   Thu, 2 Jun 2022 10:45:55 -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.

This is not my experience. All the drivers I've worked with do not block
unbind with open file descriptors (at least for char devices). I know,
for example, that having a file descriptor open of /dev/nvmeX does not
cause unbinding to block. I figured this was the expectation as the
userspace process doing the unbind won't be able to be interrupted
seeing there's no way to fail on that path. Though, it certainly would
make things a lot easier if the unbind can block indefinitely as it
usually requires some complicated locking.

Do you have an example of this? What mechanisms are developers using to
block unbind with open file descriptors?

Logan