[PATCH v4 bpf-next 2/2] mm: Introduce VM_SPARSE kind and vm_area_[un]map_pages().

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

From: Alexei Starovoitov <ast@kernel.org>

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>
---
 include/linux/vmalloc.h |  5 ++++
 mm/vmalloc.c            | 59 +++++++++++++++++++++++++++++++++++++++--
 2 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index c720be70c8dd..0f72c85a377b 100644
--- a/include/linux/vmalloc.h
+++ b/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)
 {
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index f42f98a127d5..e5b8c70950bc 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -648,6 +648,58 @@ static int vmap_pages_range(unsigned long addr, unsigned long end,
 	return err;
 }
 
+static int check_sparse_vm_area(struct vm_struct *area, unsigned long start,
+				unsigned long end)
+{
+	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_SPARSE)))
+		return -EINVAL;
+	if ((end - start) >> PAGE_SHIFT > totalram_pages())
+		return -E2BIG;
+	if (start < (unsigned long)area->addr ||
+	    (void *)end > area->addr + get_vm_area_size(area))
+		return -ERANGE;
+	return 0;
+}
+
+/**
+ * vm_area_map_pages - map pages inside given sparse vm_area
+ * @area: vm_area
+ * @start: start address inside vm_area
+ * @end: end address inside vm_area
+ * @pages: pages to map (always PAGE_SIZE pages)
+ */
+int vm_area_map_pages(struct vm_struct *area, unsigned long start,
+		      unsigned long end, struct page **pages)
+{
+	int err;
+
+	err = check_sparse_vm_area(area, start, end);
+	if (err)
+		return err;
+
+	return vmap_pages_range(start, end, PAGE_KERNEL, pages, PAGE_SHIFT);
+}
+
+/**
+ * vm_area_unmap_pages - unmap pages inside given sparse vm_area
+ * @area: vm_area
+ * @start: start address inside vm_area
+ * @end: end address inside vm_area
+ */
+void vm_area_unmap_pages(struct vm_struct *area, unsigned long start,
+			 unsigned long end)
+{
+	if (check_sparse_vm_area(area, start, end))
+		return;
+
+	vunmap_range(start, end);
+}
+
 int is_vmalloc_or_module_addr(const void *x)
 {
 	/*
@@ -3822,9 +3874,9 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
 
 		if (flags & VMAP_RAM)
 			copied = vmap_ram_vread_iter(iter, addr, n, flags);
-		else if (!(vm && (vm->flags & VM_IOREMAP)))
+		else if (!(vm && (vm->flags & (VM_IOREMAP | VM_SPARSE))))
 			copied = aligned_vread_iter(iter, addr, n);
-		else /* IOREMAP area is treated as memory hole */
+		else /* IOREMAP | SPARSE area is treated as memory hole */
 			copied = zero_iter(iter, n);
 
 		addr += copied;
@@ -4415,6 +4467,9 @@ static int s_show(struct seq_file *m, void *p)
 	if (v->flags & VM_IOREMAP)
 		seq_puts(m, " ioremap");
 
+	if (v->flags & VM_SPARSE)
+		seq_puts(m, " sparse");
+
 	if (v->flags & VM_ALLOC)
 		seq_puts(m, " vmalloc");
 
-- 
2.43.0