deep dive into mmap for CPU and GPU mem alloc

mmap.uml

@startuml
title mmap() System Call Flow

actor UserApp
participant KernelVFS as "Kernel VFS"
participant KernelMM as "Kernel Memory Manager"
participant Driver as "Device Driver (if any)"
participant ShmemFS as "Shared Memory Filesystem (shmem)"
participant KernelFS as "Kernel Filesystem"

UserApp -> KernelVFS: mmap(fd, size, prot, flags, offset)

alt fd == -1 (Anonymous Mapping)
    KernelVFS -> KernelMM: Validate flags and permissions
    KernelMM -> KernelMM: Call shmem_zero_setup()
    KernelMM -> ShmemFS: shmem_zero_setup()
    ShmemFS -> KernelMM: Create anonymous inode and VMA
    KernelMM -> KernelMM: Set vm_ops (e.g., shmem_vm_ops)
    KernelMM -> UserApp: Return user-space pointer

    UserApp -> KernelMM: Access memory (page fault)
    KernelMM -> KernelMM: Allocate physical pages
    KernelMM -> KernelMM: Map pages to user space
    KernelMM -> KernelMM: Zero-initialize pages
    KernelMM -> UserApp: Provide access to memory
else fd != -1 (Driver-backed Mapping)
    KernelVFS -> KernelMM: Validate flags and permissions
    KernelMM -> KernelFS: Retrieve file operations
    KernelFS -> Driver: Invoke driver's mmap()
    Driver -> Driver: Validate offset and size
    Driver -> KernelMM: Create VMA and set vm_ops
    KernelMM -> Driver: Return user-space pointer

    UserApp -> KernelMM: Access memory (page fault)
    KernelMM -> Driver: Call vm_ops->fault()
    Driver -> Driver: Resolve physical page
    Driver -> KernelMM: Map device memory to user space
    Driver -> KernelMM: Set appropriate protections
    KernelMM -> UserApp: Provide access to device memory
end

UserApp -> KernelVFS: munmap(ptr, size)
KernelVFS -> KernelMM: Validate address and size
KernelMM -> KernelMM: Unmap pages and free resources
KernelMM -> KernelVFS: Confirm unmapping
KernelVFS -> UserApp: Return success

@enduml