From: Jason Gunthorpe <jgg@ziepe.ca>
To: Logan Gunthorpe <logang@deltatee.com>
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: Thu, 2 Jun 2022 14:18:07 -0300 [thread overview]
Message-ID: <20220602171807.GM2960187@ziepe.ca> (raw)
In-Reply-To: <28824558-4fd5-e054-6c8d-5e045d52f795@deltatee.com>
On Thu, Jun 02, 2022 at 10:45:55AM -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.
>
> 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.
So there are lots of bugs in the kernel, and I've seen many drivers
that think calling cdev_device_del() is all they need to do - and then
happily allow cdev ioctl's/etc on a de-initialized driver struct.
Drivers that do take care of this usually have to put a lock around
all their fops to serialize against unbind. RDMA uses SRCU, iirc TPM
used a rwlock. But this is tricky and hurts fops performance.
I don't know what nvme did to protect against this, I didn't notice
an obvious lock.
> 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.
As I said, this is what sysfs does today and I don't see that ever
changing. If you userspace has a sysfs file open then the driver
unbind hangs until the file is closed.
So, doing as bad as sysfs seems like a reasonable baseline to me.
> Do you have an example of this? What mechanisms are developers using to
> block unbind with open file descriptors?
Sysfs maintains a refcount with a bias that is basically a fancied
rwlock. Most places use some kind of refcount triggering a
completion. Sleep on the completion until refcount is 0 on unbind kind
of thing.
Jason
next prev parent reply other threads:[~2022-06-02 17:19 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-07 15:46 [PATCH v6 00/21] Userspace P2PDMA with O_DIRECT NVMe devices Logan Gunthorpe
2022-04-07 15:46 ` [PATCH v6 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL Logan Gunthorpe
2022-04-07 15:46 ` [PATCH v6 02/21] PCI/P2PDMA: Attempt to set map_type if it has not been set Logan Gunthorpe
2022-04-07 15:46 ` [PATCH v6 03/21] PCI/P2PDMA: Expose pci_p2pdma_map_type() Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 04/21] PCI/P2PDMA: Introduce helpers for dma_map_sg implementations Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 05/21] dma-mapping: allow EREMOTEIO return code for P2PDMA transfers Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 06/21] dma-direct: support PCI P2PDMA pages in dma-direct map_sg Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 07/21] dma-mapping: add flags to dma_map_ops to indicate PCI P2PDMA support Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 08/21] iommu/dma: support PCI P2PDMA pages in dma-iommu map_sg Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 09/21] nvme-pci: check DMA ops when indicating support for PCI P2PDMA Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 10/21] nvme-pci: convert to using dma_map_sgtable() Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 11/21] RDMA/core: introduce ib_dma_pci_p2p_dma_supported() Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 12/21] RDMA/rw: drop pci_p2pdma_[un]map_sg() Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 13/21] PCI/P2PDMA: Remove pci_p2pdma_[un]map_sg() Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 14/21] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 15/21] iov_iter: introduce iov_iter_get_pages_[alloc_]flags() Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 16/21] block: add check when merging zone device pages Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 17/21] lib/scatterlist: " Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 18/21] block: set FOLL_PCI_P2PDMA in __bio_iov_iter_get_pages() Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 19/21] block: set FOLL_PCI_P2PDMA in bio_map_user_iov() Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem() Logan Gunthorpe
2022-05-27 12:55 ` Jason Gunthorpe
2022-05-27 15:35 ` Logan Gunthorpe
2022-05-27 19:03 ` Jason Gunthorpe
2022-05-27 22:41 ` Logan Gunthorpe
2022-06-02 0:00 ` Jason Gunthorpe
2022-06-02 16:16 ` Logan Gunthorpe
2022-06-02 16:30 ` Jason Gunthorpe
2022-06-02 16:45 ` Logan Gunthorpe
2022-06-02 17:18 ` Jason Gunthorpe [this message]
2022-06-02 16:49 ` Logan Gunthorpe
2022-06-02 17:28 ` Jason Gunthorpe
2022-06-02 17:49 ` Logan Gunthorpe
2022-04-07 15:47 ` [PATCH v6 21/21] nvme-pci: allow mmaping the CMB in userspace Logan Gunthorpe
2022-05-16 22:31 ` [PATCH v6 00/21] Userspace P2PDMA with O_DIRECT NVMe devices Chaitanya Kulkarni
2022-05-16 22:35 ` Logan Gunthorpe
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