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Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI
 Memory
From:   Benjamin Herrenschmidt <benh@au1.ibm.com>
Reply-To: benh@au1.ibm.com
To:     Logan Gunthorpe <logang@deltatee.com>,
        Dan Williams <dan.j.williams@intel.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Keith Busch <keith.busch@intel.com>,
        Oliver OHalloran <oliveroh@au1.ibm.com>,
        Alex Williamson <alex.williamson@redhat.com>,
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        =?ISO-8859-1?Q?J=E9r=F4me?= Glisse <jglisse@redhat.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Christoph Hellwig <hch@lst.de>
Date:   Fri, 02 Mar 2018 10:00:04 +1100
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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