[PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two)

[Vlastimil Babka]

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 they need without difficult
workarounds or own reimplementations, so let's make the kmalloc() alignment to
size explicitly guaranteed for power-of-two sizes under all configurations.
What this means for the three available allocators?

* SLAB object layout happens to be mostly unchanged by the patch. The
  implicitly provided alignment could be compromised with CONFIG_DEBUG_SLAB due
  to redzoning, however SLAB disables redzoning for caches with alignment
  larger than unsigned long long. Practically on at least x86 this includes
  kmalloc caches as they use cache line alignment, which is larger than that.
  Still, this patch ensures alignment on all arches and cache sizes.

* SLUB layout is also unchanged unless redzoning is enabled through
  CONFIG_SLUB_DEBUG and boot parameter for the particular kmalloc cache. With
  this patch, explicit alignment is guaranteed with redzoning as well. This
  will result in more memory being wasted, but that should be acceptable in a
  debugging scenario.

* SLOB has no implicit alignment so this patch adds it explicitly for
  kmalloc(). The potential downside is increased fragmentation. While
  pathological allocation scenarios are certainly possible, in my testing,
  after booting a x86_64 kernel+userspace with virtme, around 16MB memory
  was consumed by slab pages both before and after the patch, with difference
  in the noise.

[1] https://lore.kernel.org/linux-btrfs/c3157c8e8e0e7588312b40c853f65c02fe6c957a.1566399731.git.christophe.leroy@xxxxxx/
[2] https://lore.kernel.org/linux-fsdevel/20190225040904.5557-1-ming.lei@xxxxxxxxxx/
[3] https://lwn.net/Articles/787740/

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 Documentation/core-api/memory-allocation.rst |  4 ++
 include/linux/slab.h                         |  4 ++
 mm/slab_common.c                             | 11 ++++-
 mm/slob.c                                    | 42 +++++++++++++++-----
 4 files changed, 49 insertions(+), 12 deletions(-)

diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
index 7744aa3bf2e0..27c54854b508 100644
--- a/Documentation/core-api/memory-allocation.rst
+++ b/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 of power of two bytes, the
+alignment is also guaranteed to be at least to 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
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 56c9c7eed34e..0d4c26395785 100644
--- a/include/linux/slab.h
+++ b/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>`
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 929c02a90fba..b9ba93ad5c7f 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -993,10 +993,19 @@ 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;
 
diff --git a/mm/slob.c b/mm/slob.c
index 3dcde9cf2b17..07a39047aa54 100644
--- a/mm/slob.c
+++ b/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);
-- 
2.22.1