mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two)

In most configurations, kmalloc() happens to return naturally aligned
(i.e.  aligned to the block size itself) blocks for power of two sizes.

That means some kmalloc() users might unknowingly rely on that
alignment, until stuff breaks when the kernel is built with e.g.
CONFIG_SLUB_DEBUG or CONFIG_SLOB, and blocks stop being aligned.  Then
developers have to devise workaround such as own kmem caches with
specified alignment [1], which is not always practical, as recently
evidenced in [2].

The topic has been discussed at LSF/MM 2019 [3].  Adding a
'kmalloc_aligned()' variant would not help with code unknowingly relying
on the implicit alignment.  For slab implementations it would either
require creating more kmalloc caches, or allocate a larger size and only
give back part of it.  That would be wasteful, especially with a generic
alignment parameter (in contrast with a fixed alignment to size).

Ideally we should provide to mm users what ...

Documentation/core-api/memory-allocation.rst

@@ -98,6 +98,10 @@ limited. The actual limit depends on the hardware and the kernel
 configuration, but it is a good practice to use `kmalloc` for objects
 smaller than page size.
 
+The address of a chunk allocated with `kmalloc` is aligned to at least
+ARCH_KMALLOC_MINALIGN bytes.  For sizes which are a power of two, the
+alignment is also guaranteed to be at least the respective size.
+
 For large allocations you can use :c:func:`vmalloc` and
 :c:func:`vzalloc`, or directly request pages from the page
 allocator. The memory allocated by `vmalloc` and related functions is

include/linux/slab.h

@@ -493,6 +493,10 @@ static __always_inline void *kmalloc_large(size_t size, gfp_t flags)
  * kmalloc is the normal method of allocating memory
  * for objects smaller than page size in the kernel.
  *
+ * The allocated object address is aligned to at least ARCH_KMALLOC_MINALIGN
+ * bytes. For @size of power of two bytes, the alignment is also guaranteed
+ * to be at least to the size.
+ *
  * The @flags argument may be one of the GFP flags defined at
  * include/linux/gfp.h and described at
  * :ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>`

mm/slab_common.c

@@ -1030,10 +1030,19 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name,
 		unsigned int useroffset, unsigned int usersize)
 {
 	int err;
+	unsigned int align = ARCH_KMALLOC_MINALIGN;
 
 	s->name = name;
 	s->size = s->object_size = size;
-	s->align = calculate_alignment(flags, ARCH_KMALLOC_MINALIGN, size);
+
+	/*
+	 * For power of two sizes, guarantee natural alignment for kmalloc
+	 * caches, regardless of SL*B debugging options.
+	 */
+	if (is_power_of_2(size))
+		align = max(align, size);
+	s->align = calculate_alignment(flags, align, size);
+
 	s->useroffset = useroffset;
 	s->usersize = usersize;
 

mm/slob.c

@@ -224,6 +224,7 @@ static void slob_free_pages(void *b, int order)
  * @sp: Page to look in.
  * @size: Size of the allocation.
  * @align: Allocation alignment.
+ * @align_offset: Offset in the allocated block that will be aligned.
  * @page_removed_from_list: Return parameter.
  *
  * Tries to find a chunk of memory at least @size bytes big within @page.
@@ -234,7 +235,7 @@ static void slob_free_pages(void *b, int order)
  *         true (set to false otherwise).
  */
 static void *slob_page_alloc(struct page *sp, size_t size, int align,
-			     bool *page_removed_from_list)
+			      int align_offset, bool *page_removed_from_list)
 {
 	slob_t *prev, *cur, *aligned = NULL;
 	int delta = 0, units = SLOB_UNITS(size);
@@ -243,8 +244,17 @@ static void *slob_page_alloc(struct page *sp, size_t size, int align,
 	for (prev = NULL, cur = sp->freelist; ; prev = cur, cur = slob_next(cur)) {
 		slobidx_t avail = slob_units(cur);
 
+		/*
+		 * 'aligned' will hold the address of the slob block so that the
+		 * address 'aligned'+'align_offset' is aligned according to the
+		 * 'align' parameter. This is for kmalloc() which prepends the
+		 * allocated block with its size, so that the block itself is
+		 * aligned when needed.
+		 */
 		if (align) {
-			aligned = (slob_t *)ALIGN((unsigned long)cur, align);
+			aligned = (slob_t *)
+				(ALIGN((unsigned long)cur + align_offset, align)
+				 - align_offset);
 			delta = aligned - cur;
 		}
 		if (avail >= units + delta) { /* room enough? */
@@ -288,7 +298,8 @@ static void *slob_page_alloc(struct page *sp, size_t size, int align,
 /*
  * slob_alloc: entry point into the slob allocator.
  */
-static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
+static void *slob_alloc(size_t size, gfp_t gfp, int align, int node,
+							int align_offset)
 {
 	struct page *sp;
 	struct list_head *slob_list;
@@ -319,7 +330,7 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
 		if (sp->units < SLOB_UNITS(size))
 			continue;
 
-		b = slob_page_alloc(sp, size, align, &page_removed_from_list);
+		b = slob_page_alloc(sp, size, align, align_offset, &page_removed_from_list);
 		if (!b)
 			continue;
 
@@ -356,7 +367,7 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
 		INIT_LIST_HEAD(&sp->slab_list);
 		set_slob(b, SLOB_UNITS(PAGE_SIZE), b + SLOB_UNITS(PAGE_SIZE));
 		set_slob_page_free(sp, slob_list);
-		b = slob_page_alloc(sp, size, align, &_unused);
+		b = slob_page_alloc(sp, size, align, align_offset, &_unused);
 		BUG_ON(!b);
 		spin_unlock_irqrestore(&slob_lock, flags);
 	}
@@ -458,7 +469,7 @@ static __always_inline void *
 __do_kmalloc_node(size_t size, gfp_t gfp, int node, unsigned long caller)
 {
 	unsigned int *m;
-	int align = max_t(size_t, ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
+	int minalign = max_t(size_t, ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
 	void *ret;
 
 	gfp &= gfp_allowed_mask;
@@ -466,19 +477,28 @@ __do_kmalloc_node(size_t size, gfp_t gfp, int node, unsigned long caller)
 	fs_reclaim_acquire(gfp);
 	fs_reclaim_release(gfp);
 
-	if (size < PAGE_SIZE - align) {
+	if (size < PAGE_SIZE - minalign) {
+		int align = minalign;
+
+		/*
+		 * For power of two sizes, guarantee natural alignment for
+		 * kmalloc()'d objects.
+		 */
+		if (is_power_of_2(size))
+			align = max(minalign, (int) size);
+
 		if (!size)
 			return ZERO_SIZE_PTR;
 
-		m = slob_alloc(size + align, gfp, align, node);
+		m = slob_alloc(size + minalign, gfp, align, node, minalign);
 
 		if (!m)
 			return NULL;
 		*m = size;
-		ret = (void *)m + align;
+		ret = (void *)m + minalign;
 
 		trace_kmalloc_node(caller, ret,
-				   size, size + align, gfp, node);
+				   size, size + minalign, gfp, node);
 	} else {
 		unsigned int order = get_order(size);
 
@@ -579,7 +599,7 @@ static void *slob_alloc_node(struct kmem_cache *c, gfp_t flags, int node)
 	fs_reclaim_release(flags);
 
 	if (c->size < PAGE_SIZE) {
-		b = slob_alloc(c->size, flags, c->align, node);
+		b = slob_alloc(c->size, flags, c->align, node, 0);
 		trace_kmem_cache_alloc_node(_RET_IP_, b, c->object_size,
 					    SLOB_UNITS(c->size) * SLOB_UNIT,
 					    flags, node);