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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Logan Gunthorpe <logang@deltatee.com>
Cc: "Sagi Grimberg" <sagi@grimberg.me>,
	linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-nvme@lists.infradead.org, linux-rdma@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-block@vger.kernel.org,
	"Stephen Bates" <sbates@raithlin.com>,
	"Christoph Hellwig" <hch@lst.de>, "Jens Axboe" <axboe@kernel.dk>,
	"Keith Busch" <keith.busch@intel.com>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Max Gurtovoy" <maxg@mellanox.com>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Steve Wise" <swise@opengridcomputing.com>
Subject: Re: [PATCH v2 10/10] nvmet: Optionally use PCI P2P memory
Date: Thu, 1 Mar 2018 15:45:40 -0700	[thread overview]
Message-ID: <20180301224540.GL19007@ziepe.ca> (raw)
In-Reply-To: <edaf476a-cfe9-0598-1aa5-14802a9a1477@deltatee.com>

On Thu, Mar 01, 2018 at 12:27:03PM -0700, Logan Gunthorpe wrote:
> 
> 
> On 01/03/18 11:42 AM, Jason Gunthorpe wrote:
> >On Thu, Mar 01, 2018 at 08:35:55PM +0200, Sagi Grimberg wrote:
> >This is also why I don't entirely understand why this series has a
> >generic allocator for p2p mem, it makes little sense to me.
> 
> >Why wouldn't the nmve driver just claim the entire CMB of its local
> >device for its own use? Why involve p2p core code in this?
> 
> We'd prefer to have a generic way to get p2pmem instead of restricting
> ourselves to only using CMBs. We did work in the past where the P2P memory
> was part of an IB adapter and not the NVMe card. So this won't work if it's
> an NVMe only interface.

It just seems like it it making it too complicated.

In 99% of cases locality is important and you don't want to accidently
use a buffer in a 3rd device that has no relation to the two doing the
transfer just because it happened to be returned by the generic
allocator.

I can appreciate you might have some special use case for that, but it
absolutely should require special configuration and not just magically
happen.

You bring up IB adaptor memory - if we put that into the allocator
then what is to stop the NMVe driver just using it instead of the CMB
buffer? That would be totally wrong in almost all cases.

Seems like a very subtle and hard to debug performance trap to leave
for the users, and pretty much the only reason to use P2P is
performance... So why have such a dangerous interface?

Jason

  reply	other threads:[~2018-03-01 22:45 UTC|newest]

Thread overview: 124+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-28 23:39 [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory Logan Gunthorpe
2018-02-28 23:39 ` [PATCH v2 01/10] PCI/P2PDMA: Support peer to peer memory Logan Gunthorpe
2018-03-01 17:37   ` Bjorn Helgaas
2018-03-01 18:55     ` Logan Gunthorpe
2018-03-01 23:00       ` Bjorn Helgaas
2018-03-01 23:06         ` Logan Gunthorpe
2018-03-01 23:14           ` Stephen  Bates
2018-03-01 23:45             ` Bjorn Helgaas
2018-02-28 23:39 ` [PATCH v2 02/10] PCI/P2PDMA: Add sysfs group to display p2pmem stats Logan Gunthorpe
2018-03-01 17:44   ` Bjorn Helgaas
2018-03-02  0:15     ` Logan Gunthorpe
2018-03-02  0:36       ` Dan Williams
2018-03-02  0:37         ` Logan Gunthorpe
2018-02-28 23:39 ` [PATCH v2 03/10] PCI/P2PDMA: Add PCI p2pmem dma mappings to adjust the bus offset Logan Gunthorpe
2018-03-01 17:49   ` Bjorn Helgaas
2018-03-01 19:36     ` Logan Gunthorpe
2018-02-28 23:40 ` [PATCH v2 04/10] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches Logan Gunthorpe
2018-03-01 18:02   ` Bjorn Helgaas
2018-03-01 18:54     ` Stephen  Bates
2018-03-01 21:21       ` Alex Williamson
2018-03-01 21:26         ` Logan Gunthorpe
2018-03-01 21:32         ` Stephen  Bates
2018-03-01 21:35           ` Jerome Glisse
2018-03-01 21:37             ` Logan Gunthorpe
2018-03-01 23:15       ` Bjorn Helgaas
2018-03-01 23:59         ` Logan Gunthorpe
2018-03-01 19:13     ` Logan Gunthorpe
2018-03-05 22:28       ` Bjorn Helgaas
2018-03-05 23:01         ` Logan Gunthorpe
2018-02-28 23:40 ` [PATCH v2 05/10] block: Introduce PCI P2P flags for request and request queue Logan Gunthorpe
2018-03-01 11:08   ` Sagi Grimberg
2018-02-28 23:40 ` [PATCH v2 06/10] IB/core: Add optional PCI P2P flag to rdma_rw_ctx_[init|destroy]() Logan Gunthorpe
2018-03-01 10:32   ` Sagi Grimberg
2018-03-01 17:16     ` Logan Gunthorpe
2018-02-28 23:40 ` [PATCH v2 07/10] nvme-pci: Use PCI p2pmem subsystem to manage the CMB Logan Gunthorpe
2018-03-05  1:33   ` Oliver
2018-03-05 16:00     ` Keith Busch
2018-03-05 17:10       ` Logan Gunthorpe
2018-03-05 18:02         ` Sinan Kaya
2018-03-05 18:09           ` Logan Gunthorpe
2018-03-06  0:49         ` Oliver
2018-03-06  1:14           ` Logan Gunthorpe
2018-03-06 10:40             ` Oliver
2018-03-05 19:57       ` Sagi Grimberg
2018-03-05 20:10         ` Jason Gunthorpe
2018-03-05 20:16           ` Logan Gunthorpe
2018-03-05 20:42           ` Keith Busch
2018-03-05 20:50             ` Jason Gunthorpe
2018-03-05 20:13         ` Logan Gunthorpe
2018-02-28 23:40 ` [PATCH v2 08/10] nvme-pci: Add support for P2P memory in requests Logan Gunthorpe
2018-03-01 11:07   ` Sagi Grimberg
2018-03-01 15:58     ` Stephen  Bates
2018-03-09  5:08       ` Bart Van Assche
2018-02-28 23:40 ` [PATCH v2 09/10] nvme-pci: Add a quirk for a pseudo CMB Logan Gunthorpe
2018-03-01 11:03   ` Sagi Grimberg
2018-02-28 23:40 ` [PATCH v2 10/10] nvmet: Optionally use PCI P2P memory Logan Gunthorpe
2018-03-01 11:03   ` Sagi Grimberg
2018-03-01 16:15     ` Stephen  Bates
2018-03-01 17:40     ` Logan Gunthorpe
2018-03-01 18:35       ` Sagi Grimberg
2018-03-01 18:42         ` Jason Gunthorpe
2018-03-01 19:01           ` Stephen  Bates
2018-03-01 19:27           ` Logan Gunthorpe
2018-03-01 22:45             ` Jason Gunthorpe [this message]
2018-03-01 22:56               ` Logan Gunthorpe
2018-03-01 23:00               ` Stephen  Bates
2018-03-01 23:20                 ` Jason Gunthorpe
2018-03-01 23:29                   ` Logan Gunthorpe
2018-03-01 23:32                   ` Stephen  Bates
2018-03-01 23:49                 ` Keith Busch
2018-03-01 23:52                   ` Logan Gunthorpe
2018-03-01 23:53                   ` Stephen  Bates
2018-03-02 15:53                     ` Christoph Hellwig
2018-03-02 20:51                       ` Stephen  Bates
2018-03-01 23:57                   ` Stephen  Bates
2018-03-02  0:03                     ` Logan Gunthorpe
2018-03-02 16:18                     ` Jason Gunthorpe
2018-03-02 17:10                       ` Logan Gunthorpe
2018-03-01 19:10         ` Logan Gunthorpe
2018-03-01  3:54 ` [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory Benjamin Herrenschmidt
2018-03-01  3:56   ` Benjamin Herrenschmidt
2018-03-01 18:04     ` Logan Gunthorpe
2018-03-01 20:29       ` Benjamin Herrenschmidt
2018-03-01 20:55         ` Jerome Glisse
2018-03-01 21:03           ` Logan Gunthorpe
2018-03-01 21:10             ` Jerome Glisse
2018-03-01 21:15               ` Logan Gunthorpe
2018-03-01 21:25                 ` Jerome Glisse
2018-03-01 21:37               ` Stephen  Bates
2018-03-02 21:38               ` Stephen  Bates
2018-03-02 22:09                 ` Jerome Glisse
2018-03-05 20:36                   ` Stephen  Bates
2018-03-01 20:55         ` Logan Gunthorpe
2018-03-01 18:09     ` Stephen  Bates
2018-03-01 20:32       ` Benjamin Herrenschmidt
2018-03-01 19:21     ` Dan Williams
2018-03-01 19:30       ` Logan Gunthorpe
2018-03-01 20:34       ` Benjamin Herrenschmidt
2018-03-01 20:40         ` Benjamin Herrenschmidt
2018-03-01 20:53           ` Jason Gunthorpe
2018-03-01 20:57             ` Logan Gunthorpe
2018-03-01 22:06             ` Benjamin Herrenschmidt
2018-03-01 22:31               ` Linus Torvalds
2018-03-01 22:34                 ` Benjamin Herrenschmidt
2018-03-02 16:22                   ` Kani, Toshi
2018-03-02 16:57                     ` Linus Torvalds
2018-03-02 17:34                       ` Linus Torvalds
2018-03-02 17:38                       ` Kani, Toshi
2018-03-01 21:37         ` Dan Williams
2018-03-01 21:45           ` Logan Gunthorpe
2018-03-01 21:57             ` Logan Gunthorpe
2018-03-01 23:00               ` Benjamin Herrenschmidt
2018-03-01 23:19                 ` Logan Gunthorpe
2018-03-01 23:25                   ` Benjamin Herrenschmidt
2018-03-02 21:44                     ` Benjamin Herrenschmidt
2018-03-02 22:24                       ` Logan Gunthorpe
2018-03-01 23:26                   ` Benjamin Herrenschmidt
2018-03-01 23:54                     ` Logan Gunthorpe
2018-03-01 21:03       ` Benjamin Herrenschmidt
2018-03-01 21:11         ` Logan Gunthorpe
2018-03-01 21:18           ` Jerome Glisse
2018-03-01 21:22             ` Logan Gunthorpe
2018-03-01 10:31 ` Sagi Grimberg
2018-03-01 19:33   ` Logan Gunthorpe

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