The phaseout of the mmap() file operation

Summary

By Jonathan CorbetSeptember 25, 2025 The file_operations structure in the kernel is a set of function pointers implementing, as the name would suggest, operations on files. A subsystem that manages objects which can be represented by a file descriptor will provide a file_operations structure providing implementations of the various operations that a user of the file descriptor may want to carry out. The mmap() method, in particular, is invoked when user space calls the mmap() system call to map the object behind a file descriptor into its address space. That method, though, is currently on its way out in a multi-release process that started in 6.17. The file_operations structure was introduced in the 0.95 release in March 1992; at that point it supported the basic read() and write() operations and not much else. Support for mmap() first appeared in 0.98.2 later that year, though it took a while before it actually worked as expected. The interface has evolved a bit over time, of course; in current kernels, its prototype is: int (*mmap) (struct file *, struct vm_area_struct *); The vm_area_struct structure (usually referred to as a VMA) describes a range of a process's address space; in this case, it provides mmap() with information about the offset within the file that is to be mapped, how much is to be mapped, the intended page protections, and the address range where the mapping will be. The driver implementing mmap() is expected to do whatever setup is necessary to make the right thing happen when user space accesses memory within that range. There are hundreds of mmap() implementations within the kernel, some of which are quite complex. As described in this 6.17 commit by Lorenzo Stoakes, though, there are some significant problems with this API. The mmap() method is invoked after the memory-management layer has done much of its setup for the new mapping. If the operation fails at the driver layer, all of that setup must be unwound, which can be a complicated tas...