¿Qué es Vmalloc Linux?

What is Vmalloc Linux?

Linux provides a mechanism via vmalloc() where non-contiguous physically memory can be used that is contiguous in virtual memory. An area is reserved in the virtual address space between VMALLOC_START and VMALLOC_END.

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. When we do iounmap() this mapping is destroyed.

The most commonly used allocation flag is GFP_KERNEL means that allocation is performed on behalf of a process running in the kernel space. This means that the calling function is executing a system call on behalf of a process.

Malloc always returns virtual address, the reason is that when you call malloc it’s actually a wrapper function which calls a system call (system call is a fancy word for kernel level instructions) and this system call allocates a virtual memory inside of your heap segment.

Linux provides a variety of APIs for memory allocation. You can allocate small chunks using kmalloc or kmem_cache_alloc families, large virtually contiguous areas using vmalloc and its derivatives, or you can directly request pages from the page allocator with alloc_pages .

Logical addresses are placed in “low memory” (in the first gigabyte of physical memory) and are mapped directly to physical addresses (use __pa macro to convert it). This property implies kmalloced memory is continuous memory. In other hand, Vmalloc is able to return virtual addresses from “high memory”.

Like vmalloc, ioremap builds new page tables; unlike vmalloc, however, it doesn’t actually allocate any memory. The return value of ioremap is a special virtual address that can be used to access the specified physical address range; the virtual address obtained is eventually released by calling iounmap.

When a driver needs a real physical address (such as a DMA address, used by peripheral hardware to drive the system’s bus), you can’t easily use vmalloc. The right time to call vmalloc is when you are allocating memory for a large sequential buffer that exists only in software.

It’s important to note that vmalloc has more overhead than __get_free_pages because it must both retrieve the memory and build the page tables. Therefore, it doesn’t make sense to call vmalloc to allocate just one page.