Linux MM : virtual memory address space

[Jeff Haran]

> -----Original Message-----
> From: pramod gurav [mailto:pramod.gurav.etc at gmail.com]
> Sent: Thursday, March 06, 2014 12:56 AM
> To: Jeff Haran
> Cc: priyaranjan; kernelnewbies
> Subject: Re: Linux MM : virtual memory address space
> 
> On Wed, Mar 5, 2014 at 12:39 AM, Jeff Haran <Jeff.Haran at citrix.com> wrote:
> >
> >
> >
> >
> > From: kernelnewbies-bounces at kernelnewbies.org
> > [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of pramod gurav
> > Sent: Monday, March 03, 2014 11:33 PM
> > To: priyaranjan
> > Cc: kernelnewbies
> > Subject: Re: Linux MM : virtual memory address space
> >
> >
> >
> >
> >
> >
> >
> > On Mon, Mar 3, 2014 at 11:52 PM, priyaranjan <priyaranjan45678 at gmail.com>
> > wrote:
> >
> > I was going through http://linux-mm.org/HighMemory
> >
> >
> > "Currently the 32 bit x86 architecture is the most popular type of computer.
> > In this architecture, traditionally the Linux kernel has split the 4GB of
> > virtual memory address space into 3GB for user programs and 1GB for the
> > kernel"
> >
> >
> >
> > What about 64-bit system? Where does the code lie in linux kernel for the
> > check?
> >
> > Is there any latest and updated memory management documentation for Linux
> > kernel?
> >
> >
> >
> > Regards,
> >
> > Priyaranjan
> >
> >
> >
> >
> >
> > Priyaranjan,
> >
> >
> >
> > As below link suggests:
> >
> > http://users.nccs.gov/~fwang2/linux/lk_addressing.txt
> >
> >
> >
> > Also read this blog written in chinese:
> >
> >
> >
> > http://adam8157.info/blog/2012/07/linux-x86-64-vm/
> >
> >
> >
> > on 64 bit arch the virtual address space is huge (2 to thr power of 64). So
> > the overhead of translating the virtual addresses will be high. TO avoid
> > this only lower 48 bits are used to form virtual addresses.
> >
> >
> >
> > I believe this statement about only the lower 48 bits being used it not
> > correct. That would imply that the upper 16 bits of all virtual addresses on
> > x86_64 would be the same, which is clearly not the case since the upper 16
> > bits of user space vas are all 0s yet the upper 16 bits of kernel space vas
> > are all 1s.
> >
> >
> >
> > Jeff Haran
> >
> >
> Hi Jeff,
> Just noticed a line in /proc/cpuinfo on my 64 bit linux Machine which is:
> 
> --> address sizes : 36 bits physical, 48 bits virtual
> 
> This should confirm the statement that, only lower 48 bits are used
> for virtual address space on a 64 it arch.
> And about upper 16 bits in kernel being 1s in the address range shown
> in the link, I think they are not correct.

All I can suggest is that you take a kernel and modify it to prink() the virtual address of some kernel data structure (or write a module to do the same) and see for yourself. At least on the x86_64 systems I use, the address of for example a struct sk_buff, which is allocated from a kmem cache, is in the upper end of the 64 bit address range. The top 16 bits are all 1s. Always. This is running recent Redhat EL6, but I believe the same will be true for kernels from kernel.org.

Jeff Haran