[PATCH 1/2] mm: vmalloc: implement vrealloc()

[Danilo Krummrich <dakr@kernel.org>]

Implement vrealloc() analogous to krealloc().

Currently, krealloc() requires the caller to pass the size of the
previous memory allocation, which, instead, should be self-contained.

We attempt to fix this in a subsequent patch which, in order to do so,
requires vrealloc().

Besides that, we need realloc() functions for kernel allocators in Rust
too. With `Vec` or `KVec` respectively, potentially growing (and
shrinking) data structures are rather common.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 include/linux/vmalloc.h |  4 +++
 mm/vmalloc.c            | 58 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index e4a631ec430b..9ff0a8e5c323 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -189,6 +189,10 @@ extern void *__vcalloc_noprof(size_t n, size_t size, gfp_t flags) __alloc_size(1
 extern void *vcalloc_noprof(size_t n, size_t size) __alloc_size(1, 2);
 #define vcalloc(...)		alloc_hooks(vcalloc_noprof(__VA_ARGS__))
 
+extern void * __must_check vrealloc_noprof(const void *p, size_t size,
+					   gfp_t flags) __realloc_size(2);
+#define vrealloc(...)		alloc_hooks(vrealloc_noprof(__VA_ARGS__))
+
 extern void vfree(const void *addr);
 extern void vfree_atomic(const void *addr);
 
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index e34ea860153f..4ec949ac9d9d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4036,6 +4036,64 @@ void *vzalloc_node_noprof(unsigned long size, int node)
 }
 EXPORT_SYMBOL(vzalloc_node_noprof);
 
+/**
+ * vrealloc - reallocate virtually contiguous memory; contents remain unchanged
+ * @p: object to reallocate memory for
+ * @size: the size to reallocate
+ * @flags: the flags for the page level allocator
+ *
+ * The contents of the object pointed to are preserved up to the lesser of the
+ * new and old size (__GFP_ZERO flag is effectively ignored).
+ *
+ * If @p is %NULL, vrealloc() behaves exactly like vmalloc(). If @size is 0 and
+ * @p is not a %NULL pointer, the object pointed to is freed.
+ *
+ * Return: pointer to the allocated memory; %NULL if @size is zero or in case of
+ *         failure
+ */
+void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
+{
+	size_t old_size = 0;
+	void *n;
+
+	if (!size) {
+		vfree(p);
+		return NULL;
+	}
+
+	if (p) {
+		struct vm_struct *vm;
+
+		vm = find_vm_area(p);
+		if (unlikely(!vm)) {
+			WARN(1, "Trying to vrealloc() nonexistent vm area (%p)\n", p);
+			return NULL;
+		}
+
+		old_size = get_vm_area_size(vm);
+	}
+
+	if (size <= old_size) {
+		/* TODO: Can we optimize and shrink the allocation? What would
+		 * be a good metric for when to shrink the vm_area?
+		 */
+		return (void *)p;
+	}
+
+	/* TODO: Can we optimize and extend the existing allocation if we have
+	 * enough contiguous space left in the virtual address space?
+	 */
+	n = __vmalloc_noprof(size, flags);
+
+	if (p) {
+		memcpy(n, p, old_size);
+		vfree(p);
+	}
+
+	return n;
+}
+EXPORT_SYMBOL(vrealloc_noprof);
+
 #if defined(CONFIG_64BIT) && defined(CONFIG_ZONE_DMA32)
 #define GFP_VMALLOC32 (GFP_DMA32 | GFP_KERNEL)
 #elif defined(CONFIG_64BIT) && defined(CONFIG_ZONE_DMA)
-- 
2.45.2