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Message-Id: <1519945204.4592.45.camel@au1.ibm.com>
Date: Fri, 02 Mar 2018 10:00:04 +1100
From: Benjamin Herrenschmidt <benh@....ibm.com>
To: Logan Gunthorpe <logang@...tatee.com>,
Dan Williams <dan.j.williams@...el.com>
Cc: Jens Axboe <axboe@...nel.dk>, Keith Busch <keith.busch@...el.com>,
Oliver OHalloran <oliveroh@....ibm.com>,
Alex Williamson <alex.williamson@...hat.com>,
linux-nvdimm <linux-nvdimm@...ts.01.org>,
linux-rdma <linux-rdma@...r.kernel.org>,
linux-pci@...r.kernel.org,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
Jérôme Glisse <jglisse@...hat.com>,
Jason Gunthorpe <jgg@...lanox.com>,
Bjorn Helgaas <bhelgaas@...gle.com>,
Max Gurtovoy <maxg@...lanox.com>,
Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI
Memory
On Thu, 2018-03-01 at 14:57 -0700, Logan Gunthorpe wrote:
>
> On 01/03/18 02:45 PM, Logan Gunthorpe wrote:
> > It handles it fine for many situations. But when you try to map
> > something that is at the end of the physical address space then the
> > spares-vmemmap needs virtual address space that's the size of the
> > physical address space divided by PAGE_SIZE which may be a little bit
> > too large...
>
> Though, considering this more, maybe this shouldn't be a problem...
>
> Lets say you have 56bits of address space.
We use only 52 in practice but yes.
> That's 64PB. If you use need
> a sparse vmemmap for the entire space it will take 16TB which leaves you
> with 63.98PB of address space left. (Similar calculations for other
> numbers of address bits.)
We only have 52 bits of virtual space for the kernel with the radix
MMU.
> So I'm not sure what the problem with this is.
>
> We still have to ensure all the arches map the memory with the right
> cache bits but that should be relatively easy to solve.
>
> Logan
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