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Question

Virtual Memory is a quite complex topic for me. I am trying to understand it. Here is my understanding for a 32-bit system. Example RAM is just 2GB. I have tried reading many links, and I am not confident at the moment. I would like you people to help me in clearing up my concepts. Please acknowledge my points, and also please answer for what you feel is wrong. I have also a confused section in my points. So, here starts the summary.

1. Every process thinks it is only running. It can access the 4GB of memory - virtual address space.

2. When a process access a virtual address it is translated to physical address via MMU. This MMU is a part of a CPU - a hardware.

3. When the MMU cannot translate the address to a physical one, it raises a page fault.

4. On page fault, the kernel is notified. The kernel check the VM area struct. If it can find it - may be on disk. It will do some page-in /page-out. And get this memory on the RAM.

5. Now MMU will again try and will succeed this time.

6. In case the kernel cannot find the address, it will raise a signal. For example, invalid access will raise a SIGSEGV.

Confused points.

1. Does Page table is maintained in Kernel? This VM area struct has a page table ?

2. How MMU cannot find the address in physical RAM. Let's say it translates to some wrong address in RAM. Still the code will execute, but it will be a bad address. How MMU ensures that it is reading a right data? Does it consult Kernel VM area everytime?

3. Is the Mapping table - virtual to physical is inside a MMU. I have read it that is maintained by an individual process. If it is inside a process, why I can't see it. Or if it is MMU, how MMU generates the address - is it that Segment + 12-bit shift -> Page frame number, and then the addition of offset (bits -1 to 10) -> gives a physical address. Does it mean that for a 32-bit architecture, with this calculation in my mind. I can determine the physical address from a virtual address.

4. cat /proc/pid_value/maps. This shows me the current mapping of the vmarea. Basically, it reads the Vmarea struct and prints it. That means that this is important. I am not able to fit this piece in the complete picture. When the program is executed does the vmarea struct is generated. Is VMAREA comes only into the picture when the MMU cannnot translate the address ie Page fault? When I print the vmarea it displays the address range , permission and mapped to file descriptor, and offset. I am sure this file descriptor is the one in the hard-disk and the offset is for that file.

5. The high-mem concept is that kernel cannot directly access the Memory region greater than 1 GB(approx). Thus, it needs a page table to indirectly map it. Thus, it will temporarily load some page table to map the address. Does HIGH MEM will come into the picture everytime. Because Userspace can directly translate the address via MMU. On what scenario, does kernel really want to access the High MEM. I believe the kernel drivers will mostly be using kmalloc. This is a direct memory + offset address. In this case no mapping is really required. So, the question is on what scenario a kernel needs to access the High Mem.

6. Does the processor specifically comes with the MMU support. Those who doesn't have MMU support cannot run LInux?

Answer 1

• Does Page table is maintained in Kernel? This VM area struct has a page table ?

Yes. Not exactly: each process has a mm_struct, which contains a list of vm_area_struct 's (which represent abstract, processor-independent memory regions, aka mappings), and a field called pgd , which is a pointer to the processor-specific page table (which contains the current state of each page: valid, readable, writable, dirty, ...).

The page table doesn't need to be complete, the OS can generate each part of it from the VMAs.

• How MMU cannot find the address in physical RAM. Let's say it translates to some wrong address in RAM. Still the code will execute, but it will be a bad address. How MMU ensures that it is reading a right data? Does it consult Kernel VM area everytime?

The translation fails, eg because the page was marked as invalid, or a write access was attempted against a readonly page.

• Is the Mapping table - virtual to physical is inside a MMU. I have read it that is maintained by an individual process. If it is inside a process, why I can't see it. Or if it is MMU, how MMU generates the address - is it that Segment + 12-bit shift -> Page frame number, and then the addition of offset (bits -1 to 10) -> gives a physical address. Does it mean that for a 32-bit architecture, with this calculation in my mind. I can determine the physical address from a virtual address.

There are two kinds of MMUs in common use. One of them only has a TLB (Translation Lookaside Buffer), which is a cache of the page table. When the TLB doesn't have a translation for an attempted access, a TLB miss is generated, the OS does a page table walk, and puts the translation in the TLB.

The other kind of MMU does the page table walk in hardware.

In any case, the OS maintains a page table per process, this maps Virtual Page Numbers to Physical Frame Numbers. This mapping can change at any moment, when a page is paged-in, the physical frame it is mapped to depends on the availability of free memory.

• cat /proc/pid_value/maps. This shows me the current mapping of the vmarea. Basically, it reads the Vmarea struct and prints it. That means that this is important. I am not able to fit this piece in the complete picture. When the program is executed does the vmarea struct is generated. Is VMAREA comes only into the picture when the MMU cannnot translate the address ie Page fault? When I print the vmarea it displays the address range , permission and mapped to file descriptor, and offset. I am sure this file descriptor is the one in the hard-disk and the offset is for that file.

To a first approximation, yes. Beyond that, there are many reasons why the kernel may decide to fiddle with a process' memory, eg: if there is memory pressure it may decide to page out some rarely used pages from some random process. User space can also manipulate the mappings via mmap() , execve() and other system calls.

• The high-mem concept is that kernel cannot directly access the Memory region greater than 1 GB(approx). Thus, it needs a page table to indirectly map it. Thus, it will temporarily load some page table to map the address. Does HIGH MEM will come into the picture everytime. Because Userspace can directly translate the address via MMU. On what scenario, does kernel really want to access the High MEM. I believe the kernel drivers will mostly be using kmalloc. This is a direct memory + offset address. In this case no mapping is really required. So, the question is on what scenario a kernel needs to access the High Mem.

Totally unrelated to the other questions. In summary, high memory is a hack to be able to access lots of memory in a limited address space computer.

Basically, the kernel has a limited address space reserved to it (on x86, a typical user/kernel split is 3Gb/1Gb [processes can run in user space or kernel space. A process runs in kernel space when a syscall is invoked. To avoid having to switch the page table on every context-switch, on x86 typically the address space is split between user-space and kernel-space]). So the kernel can directly access up to ~1Gb of memory. To access more physical memory, there is some indirection involved, which is what high memory is all about.

• Does the processor specifically comes with the MMU support. Those who doesn't have MMU support cannot run Linux?

Laptop/desktop processors come with an MMU. x86 supports paging since the 386.

Linux, specially the variant called µCLinux, supports processors without MMUs (!MMU). Many embedded systems (ADSL routers, ...) use processors without an MMU. There are some important restrictions, among them:

• Some syscalls don't work at all: eg fork() .
• Some syscalls work with restrictions and non-POSIX conforming behavior: eg mmap()
• The executable file format is different: eg bFLT or ELF-FDPIC instead of ELF.
• The stack cannot grow, and its size has to be set at link-time.

When a program is loaded first the kernel will setup a kernel VM-Area for that process is it? This Kernel VM Area actually holds where the program sections are there in the memory/HDD. Then the entire story of updating CR3 register, and page walkthrough or TLB comes into the picture right? So, whenever there is a pagefault - Kernel will update the page table by looking at Kernel virtual memory area is it? But they say Kernel VM area keeps updating. How this is possible, since cat /proc/pid_value/map will keep updating.The map won't be constant from start to end. SO, the real information is available in the Kernel VM area struct is it? This is the acutal information where the section of program lies, it could be HDD or physical memory -- RAM? So, this is filled during process loading is it, the first job? Kernel does the page in page out on page fault, and will update the Kernel VM area is it? So, it should also know the entire program location on the HDD for page-in / page out right? Please correct me here. This is in continuation to my first question of the previous comment.

When the kernel loads a program, it will setup several VMAs (mappings), according to the segments in the executable file (which on ELF files you can see with readelf --segments ), which will be text/code segment, data segment, etc... During the lifetime of the program, additional mappings may be created by the dynamic/runtime linkers, by the memory allocator ( malloc() , which may also extend the data segment via brk() ), or directly by the program via mmap() , shm_open() , etc..

The VMAs contain the necessary information to generate the page table, eg they tell whether that memory is backed by a file or by swap (anonymous memory). So, yes, the kernel will update the page table by looking at the VMAs. The kernel will page in memory in response to page faults, and will page out memory in response to memory pressure.

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Using x86 no PAE as an example:

On x86 with no PAE, a linear address can be split into 3 parts: the top 10 bits point to an entry in the page directory, the middle 10 bits point to an entry in the page table pointed to by the aforementioned page directory entry. The page table entry may contain a valid physical frame number: the top 22 bits of a physical address. The bottom 12 bits of the virtual address is an offset into the page that goes untranslated into the physical address.

Each time the kernel schedules a different process, the CR3 register is written to with a pointer to the page directory for the current process. Then, each time a memory access is made, the MMU tries to look for a translation cached in the TLB, if it doesn't find one, it looks for one doing a page table walk starting from CR3. If it still doesn't find one, a GPF fault is raised, the CPU switches to Ring 0 (kernel mode), and the kernel tries to find one in the VMAs.

Also, I believe this reading from CR, page directory->page-table->Page frame number-memory address this all done by MMU. Am I correct?

On x86, yes, the MMU does the page table walk. On other systems (eg: MIPS), the MMU is little more than the TLB, and on TLB miss exceptions the kernel does the page table walk by software.

Answer 2

Though this is not going to be the best answer, iw ould like to share my thoughts on confused points.

1. Does Page table is maintained...

Yes. kernel maintains the page tables. In fact it maintains nested page tables. And top of the page tables is stored in top_pmd. pmd i suppose it is page mapping directory. You can traverse through all the page tables using this structure.

2. How MMU cannot find the address in physical RAM.....

I am not sure i understood the question. But in case because of some problem, the instruction is faulted or out of its instruction area is being accessed, you generally get undefined instruction exception resulting in undefined exception abort. If you look at the crash dumps, you can see it in the kernel log.

3. Is the Mapping table - virtual to physical is inside a MMU...

Yes. MMU is SW+HW. HW is like TLB and all. The mapping tables are stored here. For instructions, that is for code section i always converted the physical-virtual address and always they matched. And almost all the times it matches for Data sections as well.

4. cat /proc/pid_value/maps. This shows me the current mapping of the vmarea....

This is more used for analyzing the virtual addresses of user space stacks. As you know virtually all the user space programs can have 4 GB of virtual address. So unlike kernel if i say 0xc0100234. You cannot directly go and point to the istruction. So you need this mapping and the virtual address to point the instruction based on the data you have.

5. The high-mem concept is that kernel cannot directly access the Memory...

High-mem corresponds to user space memory(some one correct me if i am wrong). When kernel wants to read some data from a address at user space you will be accessing the HIGHMEM.

6. Does the processor specifically comes with the MMU support. Those who doesn't have MMU support cannot run LInux?

MMU as i mentioned is HW + SW. So mostly it would be coming with the chipset. and the SW would be generally architecture dependent. You can disable MMU from kernel config and build. I have never tried it though. Mostly these days allthe chipsets have it. But small boards i think they disable MMU. I am not entirely sure though.

As all these are conceptual questions, i may be lacking some knowledge and be wrong at places. If so others please correct me.