[SOLVED] Allocate swappable memory in linux kernel

Issue

Memory in the Linux kernel is usually unswappable (Do Kernel pages get swapped out?). However, sometimes it is useful to allow memory to be swapped out. Is it possible to explicitly allocate swappable memory inside the Linux kernel? One method I thought of was to create a user space process and use its memory. Is there anything better?

Solution

You can create a file in the internal shm shared memory filesystem.

const char *name = "example";
loff_t size = PAGE_SIZE;
unsigned long flags = 0;
struct file *filp = shmem_file_setup(name, size, flags);
/* assert(!IS_ERR(filp)); */

The file isn't actually linked, so the name isn't visible. The flags may include VM_NORESERVE to skip accounting up-front, instead accounting as pages are allocated. Now you have a shmem file. You can map a page like so:

struct address_space *mapping = filp->f_mapping;
pgoff_t index = 0;
struct page *p = shmem_read_mapping_page(mapping, index);
/* assert(!IS_ERR(filp)); */
void *data = page_to_virt(p);
memset(data, 0, PAGE_SIZE);

There is also shmem_read_mapping_page_gfp(..., gfp_t) to specify how the page is allocated. Don't forget to put the page back when you're done with it.

put_page(p);

Ditto with the file.

fput(filp);

Answered By - ephemient

Answer Checked By - Mildred Charles (WPSolving Admin)