Hey Vijay I am a newbie too. Just sharing what I could go through. It is said that Kernel or atleast a part of kernel needs to be non paged for fast interrupt access etc as pinned memory Wiki says Pinned/Locked/Fixed pages Operating systems have memory areas that are pinned (never swapped to secondary storage). For example, interrupt<http://en.wikipedia.org/wiki/Interrupt>mechanisms rely on an array of pointers to their handlers, such as I/O <http://en.wikipedia.org/wiki/I/O> completion and page fault<http://en.wikipedia.org/wiki/Page_fault>. If the pages containing these pointers or the code that they invoke were pageable, interrupt-handling would become far more complex and time-consuming, particularly in the case of page fault interrupts. Hence, some part of the page table structures is not pageable. Some pages may be pinned for short periods of time, others may be pinned for long periods of time, and still others may need to be permanently pinned. For example: - The paging supervisor code and drivers for secondary storage devices on which pages reside must be permanently pinned, as otherwise paging wouldn't even work because the necessary code wouldn't be available. - Timing-dependent components may be pinned to avoid variable paging delays. - Data buffers <http://en.wikipedia.org/wiki/Data_buffer> that are accessed directly by peripheral devices that use direct memory access<http://en.wikipedia.org/wiki/Direct_memory_access>or I/O channels <http://en.wikipedia.org/wiki/I/O_channel> must reside in pinned pages while the I/O operation is in progress because such devices and the buses <http://en.wikipedia.org/wiki/Bus_%28computing%29> to which they are attached expect to find data buffers located at physical memory addresses; regardless of whether the bus has a memory management unit for I/O <http://en.wikipedia.org/wiki/IOMMU>, transfers cannot be stopped if a page fault occurs and then restarted when the page fault has been processed. There are other two discussion thread which say kernel is non-pageable and now due to growing kernel Data structures it is allowed http://kerneltrap.org/node/6404 http://kerneltrap.org/node/8206 Regards Kishore On Thu, Jun 21, 2012 at 5:57 PM, Vijay Chauhan <kernel.vijay@gmail.com>wrote:

Hello,

I am newbie. It has been said "kernel memory is not pageable" What does it mean? There is no concept of kernel virtual address?

Any simple explanation will help me to udnerstand.

Thanks, Vijay

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Hello Vijay, On Thu, Jun 21, 2012 at 5:57 PM, Vijay Chauhan <kernel.vijay@gmail.com> wrote:

Hello,

I am newbie. It has been said "kernel memory is not pageable" What does it mean? There is no concept of kernel virtual address? You might have heard about 3G/1G split. This 1GB is the virtual address of the kernel. And whenever kernel try to access any address in this range(for ARM architecture it is 0xC0000000 - 0xFFFFFFFF), there should not be any page fault. That means MMU should be able to convert your virtual address to physical. This 1GB contains your IO address, RAM address.

Paging is a mechanism OS uses to pull the data(in pagesizes) to and fro between system RAM and secondary memory. Kernel memory is not pageable. This means memory allocated for the kernel will not be pagged out. If you try to access any memory in kernel with out creating page tables(this can be done by ioremap) you will end up in OOPS. The main reason of kernel not being swapable or pageable is as follows. Think this way. What will happen if we have paged out that portion of the logic which decides what to do when a page fault occurs? Who will take care of the page fault then? But if a user program hit a page fault(ie accessed address is not in main memory), kernel will load the page from secondary memory if it is a valid address. And if the address accesses is illegal, kernel kill the user application(Segmentation fault). Thanks, Arun

Any simple explanation will help me to udnerstand.

Thanks, Vijay

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Hi! On 17:57 Thu 21 Jun , Vijay Chauhan wrote:

Hello,

I am newbie. It has been said "kernel memory is not pageable" What does it mean? There is no concept of kernel virtual address?

Any simple explanation will help me to udnerstand.

The right term is actually "kernel memory is not swappable". Swapping means writing inactive memory to disk and then using it for something else. Kernel memory not being swappable is a design decicion made in the early linux days. Operating systems which swap kernel memory need to isolate everything which should not be swappd out (e.g. things needed for swap-in, realtime stuff, security sensitive data, ...). This is quite a bit of work. I also guess it is pretty pointless nowadays. Installed memory and is getting so huge that virtual memory developers have a hard time trying to keep cpu-usage overhead for swapping user space memory low.

There is no concept of kernel virtual address?

Kernel memory uses virtual addresses as well. However, these the entire system memory is continuously mapped somewhere in the virtual address space. The drawback is that fragmentation turns allocation of large continuous memory regions into a game of luck. There is also an virtual address area (vmalloc) which is used to dynamically map multiple scattered pages to a continuous region. But this is rather slow and rarely used. You might want to take a look at: http://lwn.net/Kernel/LDD3/ -Michi -- programing a layer 3+4 network protocol for mesh networks see http://michaelblizek.twilightparadox.com