The patch titled
Subject: mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two)
has been added to the -mm tree. Its filename is
mm-slb-guarantee-natural-alignment-for-kmallocpower-of-two.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-slb-guarantee-natural-alignment-for-kmallocpower-of-two.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-slb-guarantee-natural-alignment-for-kmallocpower-of-two.patch
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------------------------------------------------------
From: Vlastimil Babka <vbabka@xxxxxxx>
Subject: 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@xxxxxx/
[2] https://lore.kernel.org/linux-fsdevel/20190225040904.5557-1-ming.lei@xxxxxxxxxx/
[3] https://lwn.net/Articles/787740/
Link: http://lkml.kernel.org/r/20190826111627.7505-3-vbabka@xxxxxxx
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: "Darrick J . Wong" <darrick.wong@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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(-)
--- a/Documentation/core-api/memory-allocation.rst~mm-slb-guarantee-natural-alignment-for-kmallocpower-of-two
+++ a/Documentation/core-api/memory-allocation.rst
@@ -98,6 +98,10 @@ limited. The actual limit depends on the
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
--- a/include/linux/slab.h~mm-slb-guarantee-natural-alignment-for-kmallocpower-of-two
+++ a/include/linux/slab.h
@@ -493,6 +493,10 @@ static __always_inline void *kmalloc_lar
* 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>`
--- a/mm/slab_common.c~mm-slb-guarantee-natural-alignment-for-kmallocpower-of-two
+++ a/mm/slab_common.c
@@ -1030,10 +1030,19 @@ void __init create_boot_cache(struct kme
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;
--- a/mm/slob.c~mm-slb-guarantee-natural-alignment-for-kmallocpower-of-two
+++ a/mm/slob.c
@@ -224,6 +224,7 @@ static void slob_free_pages(void *b, int
* @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
* 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
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
/*
* 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
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
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
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
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);
_
Patches currently in -mm which might be from vbabka@xxxxxxx are
mm-slb-improve-memory-accounting.patch
mm-slb-guarantee-natural-alignment-for-kmallocpower-of-two.patch
mm-page_owner-record-page-owner-for-each-subpage.patch
mm-page_owner-keep-owner-info-when-freeing-the-page.patch
mm-page_owner-debug_pagealloc-save-and-dump-freeing-stack-trace.patch
mm-compaction-clear-total_migratefree_scanned-before-scanning-a-new-zone-fix-2.patch
mm-reclaim-cleanup-should_continue_reclaim.patch
mm-compaction-raise-compaction-priority-after-it-withdrawns.patch
