[RFC PATCH bpf-next v1 01/16] mm/vmalloc: Add get_vm_area_align()

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Add a variant of get_vm_area() that takes the alignment of the area's start.
The internal allocator has supported an alignment all along, through
__get_vm_area_node(), but every caller of the public API passes one page, so
a user that needs a naturally aligned region has to over-reserve and skip the
slack in front of the aligned address, wasting up to the alignment in virtual
space per area.

BPF arena maps are about to reserve a 4 GiB kernel-only region next to the
existing arena window, holding one power-of-two slice per program struct
type. Each slice is placed at a multiple of its own size inside the region,
which makes the region's alignment the largest slice size, so that a BPF
program can turn any 64-bit value into an object address in a slice with a
mask and an add. Reserving the region aligned avoids carrying 4 GiB of slack
in every arena map.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/vmalloc.h |  2 ++
 mm/vmalloc.c            | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index aed121d729b0..e8aa955fe062 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -246,6 +246,8 @@ static inline size_t get_vm_area_size(const struct vm_struct *area)
 extern struct vm_struct *get_vm_area(unsigned long size, unsigned long flags);
 extern struct vm_struct *get_vm_area_caller(unsigned long size,
 					unsigned long flags, const void *caller);
+extern struct vm_struct *get_vm_area_align(unsigned long size,
+					unsigned long align, unsigned long flags);
 extern struct vm_struct *__get_vm_area_caller(unsigned long size,
 					unsigned long flags,
 					unsigned long start, unsigned long end,
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index bea9f76ed7e7..b70e4e27ec36 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3301,6 +3301,25 @@ struct vm_struct *get_vm_area_caller(unsigned long size, unsigned long flags,
 				  NUMA_NO_NODE, GFP_KERNEL, caller);
 }
 
+/**
+ * get_vm_area_align - reserve a contiguous kernel virtual area at an aligned start
+ * @size:	 size of the area
+ * @align:	 alignment of the start of the area, a power of two
+ * @flags:	 %VM_IOREMAP for I/O mappings or VM_ALLOC
+ *
+ * Like get_vm_area(), with the start of the area aligned to @align.
+ *
+ * Return: the area descriptor on success or %NULL on failure.
+ */
+struct vm_struct *get_vm_area_align(unsigned long size, unsigned long align,
+				    unsigned long flags)
+{
+	return __get_vm_area_node(size, align, PAGE_SHIFT, flags,
+				  VMALLOC_START, VMALLOC_END,
+				  NUMA_NO_NODE, GFP_KERNEL,
+				  __builtin_return_address(0));
+}
+
 /**
  * find_vm_area - find a continuous kernel virtual area
  * @addr:	  base address
-- 
2.53.0