user space vs kernel space address space
Lets consider 32 bit Linux system with 512 physical RAM. Suppose I have standard 3:1 address space split. Now what I understand is (In general) 1. In the fourth gigabyte I have kernel space 2. Out of 1GB for kernel address space only 896MB is used as direct mapping and other 128 MB is used for Noncontiguous Memory Area Management, Fixed Mapping and Permanent Mapping. PCMIIW Following are my doubts : 1. Since my system has only 512MB RAM, will there be only direct mappings since 896 is enough to hold 512 RAM? 2. When user space program do malloc, we get some virtual address from userspace region (from first 3GB) of process. So will it be like, when I access some memory from that region, there is some physical frame associated with it AND same physical frame will also be mapped in kernel space ? So what I want to know, for every physical frame is there a mapping in userspace as well as kernel space ? (given that some address in user space of process map to same physical frame) 3. If we consider example of Linux system with 4GB, now if I do malloc from user space from which memory region kernel will give memory ? Direct mapped region or fixed map or permanent map ? Thanks and regards, Pritam Bankar
Hi Pritam, 1) Yes, all 512RAM will be direct mapped to kernel address space IF the kernel have a 896MB direct mapping area. Actually you can change the range of kernel direct mapping by the vmalloc=<size> in boot command line. In 32-bit and 3:1 split configuration, kernel direct mapping area + vmalloc area is roughly 1G. 2) User space and kernel space can have different virtual addresses mapping to the same physical frame at the same time. 3) Direct mapped or fixed map or permanent map are describing kernel VIRTUAL address space, you can see the virtual memory layout by 'dmesg'. When user space needs memory, the kernel allocates a free memory frame, and remaps the frame to user space. The frame does not have to have a valid kernel virtual address. Regards, MH On Sun, Dec 29, 2013 at 5:42 PM, Pritam Bankar <pritambankar1988@gmail.com>wrote:
Lets consider 32 bit Linux system with 512 physical RAM. Suppose I have standard 3:1 address space split. Now what I understand is
(In general) 1. In the fourth gigabyte I have kernel space 2. Out of 1GB for kernel address space only 896MB is used as direct mapping and other 128 MB is used for Noncontiguous Memory Area Management, Fixed Mapping and Permanent Mapping.
PCMIIW
Following are my doubts : 1. Since my system has only 512MB RAM, will there be only direct mappings since 896 is enough to hold 512 RAM? 2. When user space program do malloc, we get some virtual address from userspace region (from first 3GB) of process. So will it be like, when I access some memory from that region, there is some physical frame associated with it AND same physical frame will also be mapped in kernel space ? So what I want to know, for every physical frame is there a mapping in userspace as well as kernel space ? (given that some address in user space of process map to same physical frame) 3. If we consider example of Linux system with 4GB, now if I do malloc from user space from which memory region kernel will give memory ? Direct mapped region or fixed map or permanent map ?
Thanks and regards, Pritam Bankar
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Miles thanks a lot for response but I still have some doubts please see my comments inline. On Sun, Dec 29, 2013 at 7:04 PM, Miles MH Chen <orca.chen@gmail.com> wrote:
Hi Pritam,
1) Yes, all 512RAM will be direct mapped to kernel address space IF the kernel have a 896MB direct mapping area. Actually you can change the range of kernel direct mapping by the vmalloc=<size> in boot command line. In 32-bit and 3:1 split configuration, kernel direct mapping area + vmalloc area is roughly 1G.
2) User space and kernel space can have different virtual addresses mapping to the same physical frame at the same time.
3) Direct mapped or fixed map or permanent map are describing kernel VIRTUAL address space, you can see the virtual memory layout by 'dmesg'. When user space needs memory, the kernel allocates a free memory frame, and remaps the frame to user space. The frame does not have to have a valid kernel virtual address.
But all physical memory/frames are eventually controlled by kernel , then how can there ever exist a frame which doesn't have mapping with kernel or kernel virtual address ?
Regards, MH
On Sun, Dec 29, 2013 at 5:42 PM, Pritam Bankar <pritambankar1988@gmail.com
wrote:
Lets consider 32 bit Linux system with 512 physical RAM. Suppose I have standard 3:1 address space split. Now what I understand is
(In general) 1. In the fourth gigabyte I have kernel space 2. Out of 1GB for kernel address space only 896MB is used as direct mapping and other 128 MB is used for Noncontiguous Memory Area Management, Fixed Mapping and Permanent Mapping.
PCMIIW
Following are my doubts : 1. Since my system has only 512MB RAM, will there be only direct mappings since 896 is enough to hold 512 RAM? 2. When user space program do malloc, we get some virtual address from userspace region (from first 3GB) of process. So will it be like, when I access some memory from that region, there is some physical frame associated with it AND same physical frame will also be mapped in kernel space ? So what I want to know, for every physical frame is there a mapping in userspace as well as kernel space ? (given that some address in user space of process map to same physical frame) 3. If we consider example of Linux system with 4GB, now if I do malloc from user space from which memory region kernel will give memory ? Direct mapped region or fixed map or permanent map ?
Thanks and regards, Pritam Bankar
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Thanks and regards, Pritam Bankar
Hi Pritam, Every page frame is controlled by kernel, but kernel does not have to map all pages in all time. On a 32-bit bit, 3:1 split Linux kernel, the kernel has at most 1GB VIRTUAL address space. (user space has 3GB). If there are 2GB of physical DRAM on the system, our kernel obviously cannot map 2GB memory into 1GB virtual address space. To solve this problem, the kernel map part of DRAM to normal area (1-to-1 mapping). A page from normal zone can be accessed by kernel by using the existing mapping. Other memory is called high memory. When kernel wants to access the high memory pages, it maps them to kernel address space (by vmalloc, fixed map or permanent map). (A 64-bit CPU can make life much easier. We can map a lot of DRAM directly.) Regards, MH On Mon, Dec 30, 2013 at 10:28 PM, Pritam Bankar <pritambankar1988@gmail.com>wrote:
Miles thanks a lot for response but I still have some doubts please see my comments inline.
On Sun, Dec 29, 2013 at 7:04 PM, Miles MH Chen <orca.chen@gmail.com>wrote:
Hi Pritam,
1) Yes, all 512RAM will be direct mapped to kernel address space IF the kernel have a 896MB direct mapping area. Actually you can change the range of kernel direct mapping by the vmalloc=<size> in boot command line. In 32-bit and 3:1 split configuration, kernel direct mapping area + vmalloc area is roughly 1G.
2) User space and kernel space can have different virtual addresses mapping to the same physical frame at the same time.
3) Direct mapped or fixed map or permanent map are describing kernel VIRTUAL address space, you can see the virtual memory layout by 'dmesg'. When user space needs memory, the kernel allocates a free memory frame, and remaps the frame to user space. The frame does not have to have a valid kernel virtual address.
But all physical memory/frames are eventually controlled by kernel , then how can there ever exist a frame which doesn't have mapping with kernel or kernel virtual address ?
Regards, MH
On Sun, Dec 29, 2013 at 5:42 PM, Pritam Bankar < pritambankar1988@gmail.com> wrote:
Lets consider 32 bit Linux system with 512 physical RAM. Suppose I have standard 3:1 address space split. Now what I understand is
(In general) 1. In the fourth gigabyte I have kernel space 2. Out of 1GB for kernel address space only 896MB is used as direct mapping and other 128 MB is used for Noncontiguous Memory Area Management, Fixed Mapping and Permanent Mapping.
PCMIIW
Following are my doubts : 1. Since my system has only 512MB RAM, will there be only direct mappings since 896 is enough to hold 512 RAM? 2. When user space program do malloc, we get some virtual address from userspace region (from first 3GB) of process. So will it be like, when I access some memory from that region, there is some physical frame associated with it AND same physical frame will also be mapped in kernel space ? So what I want to know, for every physical frame is there a mapping in userspace as well as kernel space ? (given that some address in user space of process map to same physical frame) 3. If we consider example of Linux system with 4GB, now if I do malloc from user space from which memory region kernel will give memory ? Direct mapped region or fixed map or permanent map ?
Thanks and regards, Pritam Bankar
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Thanks and regards, Pritam Bankar
Thanks Miles, I was in assumption that every physical HAS to be mapped in some kernel virtual address. I have come across one more doubt please see my comments inline.
On Sun, Dec 29, 2013 at 7:04 PM, Miles MH Chen <orca.chen@gmail.com>wrote:
Hi Pritam,
1) Yes, all 512RAM will be direct mapped to kernel address space IF the kernel have a 896MB direct mapping area. Actually you can change the range of kernel direct mapping by the vmalloc=<size> in boot command line. In 32-bit and 3:1 split configuration, kernel direct mapping area + vmalloc area is roughly 1G.
2) User space and kernel space can have different virtual addresses mapping to the same physical frame at the same time.
How above scenario of user space and kernel space having mapping to the same physical frame is handled. This would be like sharing kernel data with user which is not allowed.
3) Direct mapped or fixed map or permanent map are describing kernel
VIRTUAL address space, you can see the virtual memory layout by 'dmesg'. When user space needs memory, the kernel allocates a free memory frame, and remaps the frame to user space. The frame does not have to have a valid kernel virtual address.
But all physical memory/frames are eventually controlled by kernel , then how can there ever exist a frame which doesn't have mapping with kernel or kernel virtual address ?
Regards, MH
On Sun, Dec 29, 2013 at 5:42 PM, Pritam Bankar < pritambankar1988@gmail.com> wrote:
Lets consider 32 bit Linux system with 512 physical RAM. Suppose I have standard 3:1 address space split. Now what I understand is
(In general) 1. In the fourth gigabyte I have kernel space 2. Out of 1GB for kernel address space only 896MB is used as direct mapping and other 128 MB is used for Noncontiguous Memory Area Management, Fixed Mapping and Permanent Mapping.
PCMIIW
Following are my doubts : 1. Since my system has only 512MB RAM, will there be only direct mappings since 896 is enough to hold 512 RAM? 2. When user space program do malloc, we get some virtual address from userspace region (from first 3GB) of process. So will it be like, when I access some memory from that region, there is some physical frame associated with it AND same physical frame will also be mapped in kernel space ? So what I want to know, for every physical frame is there a mapping in userspace as well as kernel space ? (given that some address in user space of process map to same physical frame) 3. If we consider example of Linux system with 4GB, now if I do malloc from user space from which memory region kernel will give memory ? Direct mapped region or fixed map or permanent map ?
Thanks and regards, Pritam Bankar
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Thanks and regards, Pritam Bankar
On 31 Dec 2013 13:39, "Pritam Bankar" <pritambankar1988@gmail.com> wrote:
On Sun, Dec 29, 2013 at 7:04 PM, Miles MH Chen <orca.chen@gmail.com> wrote:
Hi Pritam,
1) Yes, all 512RAM will be direct mapped to kernel address space IF
the kernel have a 896MB direct mapping area.
Actually you can change the range of kernel direct mapping by the vmalloc=<size> in boot command line. In 32-bit and 3:1 split configuration, kernel direct mapping area + vmalloc area is roughly 1G.
2) User space and kernel space can have different virtual addresses mapping to the same physical frame at the same time.
How above scenario of user space and kernel space having mapping to the same physical frame is handled. This would be like sharing kernel data with user which is not allowed.
One trivial example would be a driver that meets the userspace application to mmap a kernel buffer. In that case, there will exist both a kernelspace and a userspace mapping to the same physical frame(s). It'll let you share data between the two in a rather efficient way. Of course, you need to handle any necessary syncing yourself, so that you're not reading inconsistent data. Cheers, Anders
But all physical memory/frames are eventually controlled by kernel , then how can there ever exist a frame which doesn't have mapping with kernel or kernel virtual address ?
The key point is that frames are in physical memory while pages are in virtual memory. For a 4G Linux system, low 896M physical memory (low memory) is mapped directly into kernel space(virtual), and the rest is high memory at all (initally not mapped into any virtual space). If kernel needs more frames other than those directly mapped, it can map frames from high memory into its 128M virtaul space. If a user task needs frames, these frames might be from low memory or from high memory. In the former case, kernel shares these frames with user task. While in the latter case, these frames are only used by the user task. Therefore, some frames in high memory might not be mapped into kernel space at all. In addition, 896M low memory(physical) can be used directly by kernel, while all physical memory frames are allocated and mapped by kernel.
participants (4)
-
Anders Darander -
Miles MH Chen -
parmenides -
Pritam Bankar