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 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@c-s.fr/ [2] https://lore.kernel.org/linux-fsdevel/20190225040904.5557-1-ming.lei@redhat.com/ [3] https://lwn.net/Articles/787740/ [akpm@linux-foundation.org: documentation fixlet, per Matthew] Link: http://lkml.kernel.org/r/20190826111627.7505-3-vbabka@suse.cz Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org> Acked-by: Michal Hocko <mhocko@suse.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Christoph Hellwig <hch@lst.de> Cc: David Sterba <dsterba@suse.cz> Cc: Christoph Lameter <cl@linux.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: David Rientjes <rientjes@google.com> Cc: Ming Lei <ming.lei@redhat.com> Cc: Dave Chinner <david@fromorbit.com> Cc: "Darrick J . Wong" <darrick.wong@oracle.com> Cc: Christoph Hellwig <hch@lst.de> Cc: James Bottomley <James.Bottomley@HansenPartnership.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Vlastimil Babka committed 7 years ago

Commit 59bb4791 parent 6a486c04 files · +49 −12Browse files

Unified · Split

4 files

Documentation/core-api/memory-allocation.rst +4 −0
include/linux/slab.h +4 −0
mm/slab_common.c +10 −1
mm/slob.c +31 −11

Documentation/core-api/memory-allocation.rst+4−0

@@ -98,6 +98,10 @@ limited. The actual limit depends on the hardware and the kernel · 97 unmodified lines
98	configuration, but it is a good practice to use `kmalloc` for objects	98	configuration, but it is a good practice to use `kmalloc` for objects
99	smaller than page size.	99	smaller than page size.
100		100
		101	The address of a chunk allocated with `kmalloc` is aligned to at least
		102	ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
		103	alignment is also guaranteed to be at least the respective size.
		104
101	For large allocations you can use :c:func:`vmalloc` and	105	For large allocations you can use :c:func:`vmalloc` and
102	:c:func:`vzalloc`, or directly request pages from the page	106	:c:func:`vzalloc`, or directly request pages from the page
103	allocator. The memory allocated by `vmalloc` and related functions is	107	allocator. The memory allocated by `vmalloc` and related functions is

include/linux/slab.h+4−0

  * 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+10−1

@@ -1030,10 +1030,19 @@ void __init create_boot_cache(struct kmem_cache s, const char name, · 1029 unmodified lines
1030	unsigned int useroffset, unsigned int usersize)	1030	unsigned int useroffset, unsigned int usersize)
1031	{	1031	{
1032	int err;	1032	int err;
		1033	unsigned int align = ARCH_KMALLOC_MINALIGN;
1033		1034
1034	s->name = name;	1035	s->name = name;
1035	s->size = s->object_size = size;	1036	s->size = s->object_size = size;
1036	s->align = calculate_alignment(flags, ARCH_KMALLOC_MINALIGN, size);	1037
		1038	/*
		1039	* For power of two sizes, guarantee natural alignment for kmalloc
		1040	* caches, regardless of SL*B debugging options.
		1041	*/
		1042	if (is_power_of_2(size))
		1043	align = max(align, size);
		1044	s->align = calculate_alignment(flags, align, size);
		1045
1037	s->useroffset = useroffset;	1046	s->useroffset = useroffset;
1038	s->usersize = usersize;	1047	s->usersize = usersize;
1039		1048

mm/slob.c+31−11

  * @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.
  *         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);
 	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? */
 /*
  * 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;
 		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;
 		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);
 	}
 __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;
 	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);
 	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);

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

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