[PATCH bpf-next] mm: Introduce vm_area_[un]map_pages().

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

From: Alexei Starovoitov <ast@kernel.org>

vmap() API is used to map a set of pages into contiguous kernel virtual space.

BPF would like to extend the vmap API to implement a lazily-populated
contiguous kernel virtual space which size and start address is fixed early.

The vmap API has functions to request and release areas of kernel address space:
get_vm_area() and free_vm_area().

Introduce vm_area_map_pages(area, start_addr, count, pages)
to map a set of pages within a given area.
It has the same sanity checks as vmap() does.
In addition it also checks that get_vm_area() was created with VM_MAP flag
(as all users of vmap() should be doing).

Also add vm_area_unmap_pages() that is a safer alternative to
existing vunmap_range() api.

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 an existing vm area with the following steps:

  area = get_vm_area(area_size, VM_MAP | VM_USERMAP); // at bpf prog verification time
  vm_area_map_pages(area, kaddr, 1, page);            // on demand
  vm_area_unmap_pages(area, kaddr, 1);
  free_vm_area(area);                                 // after bpf prog is unloaded

For BPF use case the area_size will be 4Gbyte plus 64Kbyte of guard pages and
area->addr known and fixed at the program verification time.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 include/linux/vmalloc.h |  3 +++
 mm/vmalloc.c            | 46 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index c720be70c8dd..7d112cc5f2a3 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -232,6 +232,9 @@ static inline bool is_vm_area_hugepages(const void *addr)
 }
 
 #ifdef CONFIG_MMU
+int vm_area_map_pages(struct vm_struct *area, unsigned long addr, unsigned int count,
+		      struct page **pages);
+int vm_area_unmap_pages(struct vm_struct *area, unsigned long addr, unsigned int count);
 void vunmap_range(unsigned long addr, unsigned long end);
 static inline void set_vm_flush_reset_perms(void *addr)
 {
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d12a17fc0c17..d6337d46f1d8 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -635,6 +635,52 @@ static int vmap_pages_range(unsigned long addr, unsigned long end,
 	return err;
 }
 
+/**
+ * vm_area_map_pages - map pages inside given vm_area
+ * @area: vm_area
+ * @addr: start address inside vm_area
+ * @count: number of pages
+ * @pages: pages to map (always PAGE_SIZE pages)
+ */
+int vm_area_map_pages(struct vm_struct *area, unsigned long addr, unsigned int count,
+		      struct page **pages)
+{
+	unsigned long size = ((unsigned long)count) * PAGE_SIZE;
+	unsigned long end = addr + size;
+
+	might_sleep();
+	if (WARN_ON_ONCE(area->flags & VM_FLUSH_RESET_PERMS))
+		return -EINVAL;
+	if (WARN_ON_ONCE(area->flags & VM_NO_GUARD))
+		return -EINVAL;
+	if (WARN_ON_ONCE(!(area->flags & VM_MAP)))
+		return -EINVAL;
+	if (count > totalram_pages())
+		return -E2BIG;
+	if (addr < (unsigned long)area->addr || (void *)end > area->addr + area->size)
+		return -ERANGE;
+
+	return vmap_pages_range(addr, end, PAGE_KERNEL, pages, PAGE_SHIFT);
+}
+
+/**
+ * vm_area_unmap_pages - unmap pages inside given vm_area
+ * @area: vm_area
+ * @addr: start address inside vm_area
+ * @count: number of pages to unmap
+ */
+int vm_area_unmap_pages(struct vm_struct *area, unsigned long addr, unsigned int count)
+{
+	unsigned long size = ((unsigned long)count) * PAGE_SIZE;
+	unsigned long end = addr + size;
+
+	if (addr < (unsigned long)area->addr || (void *)end > area->addr + area->size)
+		return -ERANGE;
+
+	vunmap_range(addr, end);
+	return 0;
+}
+
 int is_vmalloc_or_module_addr(const void *x)
 {
 	/*
-- 
2.34.1