4KBallocations only- in physical memory space
There is no malloc/free in the kernel, it can only kalloc/kfree full pages of memory (4KB).
Free pages are used to form a linked list for finding a free page (kalloc) or adding a previously allocated page (kfree).
This has the following limitations:
- No allocation can be > 4KB as there is no way to guarantee the allocation of free pages with consecutive physical memory locations.
- Even if only a few bytes are needed, each allocation will use up one full 4KB page.
Supports Sv39 on 64-bit and Sv32 on 32-bit RISC V.
One page table fits exactly into one page of 4Kb:
64-bit:
Page table: pagetable_t. An array of 512 pagetable_element which are just 64-bit ints.
32-bit:
Page table: pagetable_t. An array of 1024 pagetable_element which are just 32-bit ints.
All harts use the same kernel page table:
- all memory is mapped to the physical location
- UART, VIRTIO, PLIC are mapped
- the trampoline function gets mapped to the highest address See memory_map_kernel for details.
- one large area of consecutive
4KBallocations per process - in virtual memory space, see memory_map_process
User space applications can only increase or decrease their memory heap with the sbrk system call. The C standard library will manage this heap and provide convenient malloc() and free() calls.
Overview: kernel
Boot: boot_process | init_overview
Subsystems: interrupts | devices | file_system | memory_management processes | scheduling | syscalls