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

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Message-ID: <20220602172811.GN2960187@ziepe.ca>
Date:   Thu, 2 Jun 2022 14:28:11 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
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 Thu, Jun 02, 2022 at 10:49:15AM -0600, Logan Gunthorpe wrote:
> 
> 
> 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.

Oh interesting, this has been changed since years ago when I last
looked, the kernfs_get_active() is now more narrowed than it once
was. So manybe sysfs isn't the same concern it used to be!

Thanks,
Jason