What is Ioremap?

ioremap: ioremap is used to map physical memory into virtual address space of the kernel. In most of the system now a days, the devices are memory mapped to the system. That means, kernel can access these device registers by writing directly into these physical memory addresses.

What is Remap_pfn_range?

remap_pfn_range() maps physical memory (by means of kernel logical address) to a user space process. It is particularly useful for implementing the mmap() system call. The process’s VMA list is updated with a new VMA entry (with appropriate attributes) , which will use PTE to access the same memory.

Is mmap a system call?

In computing, mmap(2) is a POSIX-compliant Unix system call that maps files or devices into memory. It is a method of memory-mapped file I/O. It implements demand paging because file contents are not read from disk directly and initially do not use physical RAM at all.

Why is Ioremap required?

ioremap() function is used to map the physical addres of an I/O device to the kernel virtual address. Kernel creates a page table i.e mapping of virtual address to the physical address requested.

What does Ioremap return?

A successful call to ioremap() returns a kernel virtual address corresponding to start of the requested physical address range. This address is not normally meant to be dereferenced directly, though, for a number of (often architecture-specific) reasons.

How are mmap and malloc difference?

When malloc is running, it gathers all available system facilities. ‘mmap’ on the other hand is a system call that takes charge and requests the kernel to find an unused and contiguous region in an application’s address that is large enough to allow for the mapping of several pages of memory.

What is struct page?

Overview. Linux manages physical memory by dividing it into PAGE_SIZE pieces. Usually this is the same as the CPU’s page size, between 4KiB and 64KiB. Each page has a small data structure (about 64 bytes) called struct page, which contains various pieces of information about the page.

Can kernel access physical memory?

Even the kernel has no direct access to physical memory in the way your are thinking. Basically the kind of memory you seem to imagine (linear memory with flat addressing) is something that is constructed by the kernel (from physical memory banks) and exported, it doesn’t “exist”.

Does malloc use mmap?

For very large requests, malloc() uses the mmap() system call to find addressable memory space. This process helps reduce the negative effects of memory fragmentation when large blocks of memory are freed but locked by smaller, more recently allocated blocks lying between them and the end of the allocated space.

Is malloc faster than mmap?

Almost always, memory is much faster than disk, and malloc is not what’s costing time. The mmap code is faster because for your program, mmap has resulted in either less disk access, or more efficient disk access, than whatever reads and writes you compared against.