mm: Introduce VM_SPARSE kind and vm_area_[un]map_pages().

vmap/vmalloc APIs are used to map a set of pages into contiguous kernel
virtual space.

get_vm_area() with appropriate flag is used to request an area of kernel
address range. It's used for vmalloc, vmap, ioremap, xen use cases.
- vmalloc use case dominates the usage. Such vm areas have VM_ALLOC flag.
- the areas created by vmap() function should be tagged with VM_MAP.
- ioremap areas are tagged with VM_IOREMAP.

BPF would like to extend the vmap API to implement a lazily-populated
sparse, yet contiguous kernel virtual space. Introduce VM_SPARSE flag
and vm_area_map_pages(area, start_addr, count, pages) API to map a set
of pages within a given area.
It has the same sanity checks as vmap() does.
It also checks that get_vm_area() was created with VM_SPARSE flag
which identifies such areas in /proc/vmallocinfo
and returns zero pages on read through /proc/kcore.

The next commits will introduce bpf_arena which is a sparsely populated
shared memory region between bpf program and user space process. It will
map privately-managed pages into a sparse vm area with the following steps:

  // request virtual memory region during bpf prog verification
  area = get_vm_area(area_size, VM_SPARSE);

  // on demand
  vm_area_map_pages(area, kaddr, kend, pages);
  vm_area_unmap_pages(area, kaddr, kend);

  // after bpf program is detached and unloaded
  free_vm_area(area);

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/bpf/20240305030516.41519-3-alexei.starovoitov@gmail.com

include/linux/vmalloc.h

@@ -35,6 +35,7 @@ struct iov_iter;		/* in uio.h */
 #else
 #define VM_DEFER_KMEMLEAK	0
 #endif
+#define VM_SPARSE		0x00001000	/* sparse vm_area. not all pages are present. */
 
 /* bits [20..32] reserved for arch specific ioremap internals */
 
@@ -232,6 +233,10 @@ static inline bool is_vm_area_hugepages(const void *addr)
 }
 
 #ifdef CONFIG_MMU
+int vm_area_map_pages(struct vm_struct *area, unsigned long start,
+		      unsigned long end, struct page **pages);
+void vm_area_unmap_pages(struct vm_struct *area, unsigned long start,
+			 unsigned long end);
 void vunmap_range(unsigned long addr, unsigned long end);
 static inline void set_vm_flush_reset_perms(void *addr)
 {