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>
diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
index 7744aa3..939e3df 100644
--- a/Documentation/core-api/memory-allocation.rst
+++ b/Documentation/core-api/memory-allocation.rst
@@ -98,6 +98,10 @@
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
diff --git a/include/linux/slab.h b/include/linux/slab.h
index ab2b98a..4d2a2fa 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -493,6 +493,10 @@
* 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 0a94cf8..c29f03a 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1030,10 +1030,19 @@
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 835088d..fa53e9f 100644
--- a/mm/slob.c
+++ b/mm/slob.c
@@ -224,6 +224,7 @@
* @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 @@
* 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 @@
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 @@
/*
* 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 @@
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 @@
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 @@
__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 @@
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 @@
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);
