> -----Original Message----- > From: pramod gurav [mailto:pramod.gurav.etc@xxxxxxxxx] > 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@xxxxxxxxxx> wrote: > > > > > > > > > > From: kernelnewbies-bounces@xxxxxxxxxxxxxxxxx > > [mailto:kernelnewbies-bounces@xxxxxxxxxxxxxxxxx] 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@xxxxxxxxx> > > 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 _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
