[PATCH v6 1/4] mm/slub: enable debugging memory wasting of kmalloc

From: Feng Tang
Date: Tue Sep 13 2022 - 02:55:01 EST

kmalloc's API family is critical for mm, with one nature that it will
round up the request size to a fixed one (mostly power of 2). Say
when user requests memory for '2^n + 1' bytes, actually 2^(n+1) bytes
could be allocated, so in worst case, there is around 50% memory
space waste.

The wastage is not a big issue for requests that get allocated/freed
quickly, but may cause problems with objects that have longer life
time.

We've met a kernel boot OOM panic (v5.10), and from the dumped slab
info:

[ 26.062145] kmalloc-2k 814056KB 814056KB

>From debug we found there are huge number of 'struct iova_magazine',
whose size is 1032 bytes (1024 + 8), so each allocation will waste
1016 bytes. Though the issue was solved by giving the right (bigger)
size of RAM, it is still nice to optimize the size (either use a
kmalloc friendly size or create a dedicated slab for it).

And from lkml archive, there was another crash kernel OOM case [1]
back in 2019, which seems to be related with the similar slab waste
situation, as the log is similar:

[ 4.332648] iommu: Adding device 0000:20:02.0 to group 16
[ 4.338946] swapper/0 invoked oom-killer: gfp_mask=0x6040c0(GFP_KERNEL|__GFP_COMP), nodemask=(null), order=0, oom_score_adj=0
...
[ 4.857565] kmalloc-2048 59164KB 59164KB

The crash kernel only has 256M memory, and 59M is pretty big here.
(Note: the related code has been changed and optimised in recent
kernel [2], these logs are just picked to demo the problem, also
a patch changing its size to 1024 bytes has been merged)

So add an way to track each kmalloc's memory waste info, and
leverage the existing SLUB debug framework (specifically
SLUB_STORE_USER) to show its call stack of original allocation,
so that user can evaluate the waste situation, identify some hot
spots and optimize accordingly, for a better utilization of memory.

The waste info is integrated into existing interface:
'/sys/kernel/debug/slab/kmalloc-xx/alloc_traces', one example of
'kmalloc-4k' after boot is:

126 ixgbe_alloc_q_vector+0xbe/0x830 [ixgbe] waste=233856/1856 age=280763/281414/282065 pid=1330 cpus=32 nodes=1
__kmem_cache_alloc_node+0x11f/0x4e0
__kmalloc_node+0x4e/0x140
ixgbe_alloc_q_vector+0xbe/0x830 [ixgbe]
ixgbe_init_interrupt_scheme+0x2ae/0xc90 [ixgbe]
ixgbe_probe+0x165f/0x1d20 [ixgbe]
local_pci_probe+0x78/0xc0
work_for_cpu_fn+0x26/0x40
...

which means in 'kmalloc-4k' slab, there are 126 requests of
2240 bytes which got a 4KB space (wasting 1856 bytes each
and 233856 bytes in total), from ixgbe_alloc_q_vector().

And when system starts some real workload like multiple docker
instances, there could are more severe waste.

[1]. https://lkml.org/lkml/2019/8/12/266
[2]. https://lore.kernel.org/lkml/2920df89-9975-5785-f79b-257d3052dfaf@xxxxxxxxxx/

[Thanks Hyeonggon for pointing out several bugs about sorting/format]
[Thanks Vlastimil for suggesting way to reduce memory usage of
orig_size and keep it only for kmalloc objects]

Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: John Garry <john.garry@xxxxxxxxxx>
Cc: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
---
Documentation/mm/slub.rst | 33 +++++---
include/linux/slab.h | 2 +
mm/slub.c | 156 ++++++++++++++++++++++++++++----------
3 files changed, 141 insertions(+), 50 deletions(-)

diff --git a/Documentation/mm/slub.rst b/Documentation/mm/slub.rst
index 43063ade737a..4e1578186b4f 100644
--- a/Documentation/mm/slub.rst
+++ b/Documentation/mm/slub.rst
@@ -400,21 +400,30 @@ information:
allocated objects. The output is sorted by frequency of each trace.

Information in the output:
- Number of objects, allocating function, minimal/average/maximal jiffies since alloc,
- pid range of the allocating processes, cpu mask of allocating cpus, and stack trace.
+ Number of objects, allocating function, possible memory wastage of
+ kmalloc objects(total/per-object), minimal/average/maximal jiffies
+ since alloc, pid range of the allocating processes, cpu mask of
+ allocating cpus, numa node mask of origins of memory, and stack trace.

Example:::

- 1085 populate_error_injection_list+0x97/0x110 age=166678/166680/166682 pid=1 cpus=1::
- __slab_alloc+0x6d/0x90
- kmem_cache_alloc_trace+0x2eb/0x300
- populate_error_injection_list+0x97/0x110
- init_error_injection+0x1b/0x71
- do_one_initcall+0x5f/0x2d0
- kernel_init_freeable+0x26f/0x2d7
- kernel_init+0xe/0x118
- ret_from_fork+0x22/0x30
-
+ 338 pci_alloc_dev+0x2c/0xa0 waste=521872/1544 age=290837/291891/293509 pid=1 cpus=106 nodes=0-1
+ __kmem_cache_alloc_node+0x11f/0x4e0
+ kmalloc_trace+0x26/0xa0
+ pci_alloc_dev+0x2c/0xa0
+ pci_scan_single_device+0xd2/0x150
+ pci_scan_slot+0xf7/0x2d0
+ pci_scan_child_bus_extend+0x4e/0x360
+ acpi_pci_root_create+0x32e/0x3b0
+ pci_acpi_scan_root+0x2b9/0x2d0
+ acpi_pci_root_add.cold.11+0x110/0xb0a
+ acpi_bus_attach+0x262/0x3f0
+ device_for_each_child+0xb7/0x110
+ acpi_dev_for_each_child+0x77/0xa0
+ acpi_bus_attach+0x108/0x3f0
+ device_for_each_child+0xb7/0x110
+ acpi_dev_for_each_child+0x77/0xa0
+ acpi_bus_attach+0x108/0x3f0

2. free_traces::

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 9b592e611cb1..6dc495f76644 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -29,6 +29,8 @@
#define SLAB_RED_ZONE ((slab_flags_t __force)0x00000400U)
/* DEBUG: Poison objects */
#define SLAB_POISON ((slab_flags_t __force)0x00000800U)
+/* Indicate a kmalloc slab */
+#define SLAB_KMALLOC ((slab_flags_t __force)0x00001000U)
/* Align objs on cache lines */
#define SLAB_HWCACHE_ALIGN ((slab_flags_t __force)0x00002000U)
/* Use GFP_DMA memory */
diff --git a/mm/slub.c b/mm/slub.c
index fe4fe0e72daf..c8ba16b3a4db 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -194,11 +194,24 @@ DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
#endif
#endif /* CONFIG_SLUB_DEBUG */

+/* Structure holding parameters for get_partial() call chain */
+struct partial_context {
+ struct slab **slab;
+ gfp_t flags;
+ unsigned int orig_size;
+};
+
static inline bool kmem_cache_debug(struct kmem_cache *s)
{
return kmem_cache_debug_flags(s, SLAB_DEBUG_FLAGS);
}

+static inline bool slub_debug_orig_size(struct kmem_cache *s)
+{
+ return (kmem_cache_debug_flags(s, SLAB_STORE_USER) &&
+ (s->flags & SLAB_KMALLOC));
+}
+
void *fixup_red_left(struct kmem_cache *s, void *p)
{
if (kmem_cache_debug_flags(s, SLAB_RED_ZONE))
@@ -785,6 +798,39 @@ static void print_slab_info(const struct slab *slab)
folio_flags(folio, 0));
}

+/*
+ * kmalloc caches has fixed sizes (mostly power of 2), and kmalloc() API
+ * family will round up the real request size to these fixed ones, so
+ * there could be an extra area than what is requested. Save the original
+ * request size in the meta data area, for better debug and sanity check.
+ */
+static inline void set_orig_size(struct kmem_cache *s,
+ void *object, unsigned int orig_size)
+{
+ void *p = kasan_reset_tag(object);
+
+ if (!slub_debug_orig_size(s))
+ return;
+
+ p += get_info_end(s);
+ p += sizeof(struct track) * 2;
+
+ *(unsigned int *)p = orig_size;
+}
+
+static unsigned int get_orig_size(struct kmem_cache *s, void *object)
+{
+ void *p = kasan_reset_tag(object);
+
+ if (!slub_debug_orig_size(s))
+ return s->object_size;
+
+ p += get_info_end(s);
+ p += sizeof(struct track) * 2;
+
+ return *(unsigned int *)p;
+}
+
static void slab_bug(struct kmem_cache *s, char *fmt, ...)
{
struct va_format vaf;
@@ -844,6 +890,9 @@ static void print_trailer(struct kmem_cache *s, struct slab *slab, u8 *p)
if (s->flags & SLAB_STORE_USER)
off += 2 * sizeof(struct track);

+ if (slub_debug_orig_size(s))
+ off += sizeof(unsigned int);
+
off += kasan_metadata_size(s);

if (off != size_from_object(s))
@@ -977,7 +1026,8 @@ static int check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
*
* A. Free pointer (if we cannot overwrite object on free)
* B. Tracking data for SLAB_STORE_USER
- * C. Padding to reach required alignment boundary or at minimum
+ * C. Original request size for kmalloc object (SLAB_STORE_USER enabled)
+ * D. Padding to reach required alignment boundary or at minimum
* one word if debugging is on to be able to detect writes
* before the word boundary.
*
@@ -995,10 +1045,14 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
{
unsigned long off = get_info_end(s); /* The end of info */

- if (s->flags & SLAB_STORE_USER)
+ if (s->flags & SLAB_STORE_USER) {
/* We also have user information there */
off += 2 * sizeof(struct track);

+ if (s->flags & SLAB_KMALLOC)
+ off += sizeof(unsigned int);
+ }
+
off += kasan_metadata_size(s);

if (size_from_object(s) == off)
@@ -1293,7 +1347,7 @@ static inline int alloc_consistency_checks(struct kmem_cache *s,
}

static noinline int alloc_debug_processing(struct kmem_cache *s,
- struct slab *slab, void *object)
+ struct slab *slab, void *object, int orig_size)
{
if (s->flags & SLAB_CONSISTENCY_CHECKS) {
if (!alloc_consistency_checks(s, slab, object))
@@ -1302,6 +1356,7 @@ static noinline int alloc_debug_processing(struct kmem_cache *s,

/* Success. Perform special debug activities for allocs */
trace(s, slab, object, 1);
+ set_orig_size(s, object, orig_size);
init_object(s, object, SLUB_RED_ACTIVE);
return 1;

@@ -1570,7 +1625,10 @@ static inline
void setup_slab_debug(struct kmem_cache *s, struct slab *slab, void *addr) {}

static inline int alloc_debug_processing(struct kmem_cache *s,
- struct slab *slab, void *object) { return 0; }
+ struct slab *slab, void *object, int orig_size) { return 0; }
+
+static inline void set_orig_size(struct kmem_cache *s,
+ void *object, unsigned int orig_size) {}

static inline void free_debug_processing(
struct kmem_cache *s, struct slab *slab,
@@ -1999,7 +2057,7 @@ static inline void remove_partial(struct kmem_cache_node *n,
* it to full list if it was the last free object.
*/
static void *alloc_single_from_partial(struct kmem_cache *s,
- struct kmem_cache_node *n, struct slab *slab)
+ struct kmem_cache_node *n, struct slab *slab, int orig_size)
{
void *object;

@@ -2009,7 +2067,7 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
slab->freelist = get_freepointer(s, object);
slab->inuse++;

- if (!alloc_debug_processing(s, slab, object)) {
+ if (!alloc_debug_processing(s, slab, object, orig_size)) {
remove_partial(n, slab);
return NULL;
}
@@ -2028,7 +2086,7 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
* and put the slab to the partial (or full) list.
*/
static void *alloc_single_from_new_slab(struct kmem_cache *s,
- struct slab *slab)
+ struct slab *slab, int orig_size)
{
int nid = slab_nid(slab);
struct kmem_cache_node *n = get_node(s, nid);
@@ -2040,7 +2098,7 @@ static void *alloc_single_from_new_slab(struct kmem_cache *s,
slab->freelist = get_freepointer(s, object);
slab->inuse = 1;

- if (!alloc_debug_processing(s, slab, object))
+ if (!alloc_debug_processing(s, slab, object, orig_size))
/*
* It's not really expected that this would fail on a
* freshly allocated slab, but a concurrent memory
@@ -2118,7 +2176,7 @@ static inline bool pfmemalloc_match(struct slab *slab, gfp_t gfpflags);
* Try to allocate a partial slab from a specific node.
*/
static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
- struct slab **ret_slab, gfp_t gfpflags)
+ struct partial_context *pc)
{
struct slab *slab, *slab2;
void *object = NULL;
@@ -2138,11 +2196,12 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
list_for_each_entry_safe(slab, slab2, &n->partial, slab_list) {
void *t;

- if (!pfmemalloc_match(slab, gfpflags))
+ if (!pfmemalloc_match(slab, pc->flags))
continue;

if (kmem_cache_debug(s)) {
- object = alloc_single_from_partial(s, n, slab);
+ object = alloc_single_from_partial(s, n, slab,
+ pc->orig_size);
if (object)
break;
continue;
@@ -2153,7 +2212,7 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
break;

if (!object) {
- *ret_slab = slab;
+ *pc->slab = slab;
stat(s, ALLOC_FROM_PARTIAL);
object = t;
} else {
@@ -2177,14 +2236,13 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
/*
* Get a slab from somewhere. Search in increasing NUMA distances.
*/
-static void *get_any_partial(struct kmem_cache *s, gfp_t flags,
- struct slab **ret_slab)
+static void *get_any_partial(struct kmem_cache *s, struct partial_context *pc)
{
#ifdef CONFIG_NUMA
struct zonelist *zonelist;
struct zoneref *z;
struct zone *zone;
- enum zone_type highest_zoneidx = gfp_zone(flags);
+ enum zone_type highest_zoneidx = gfp_zone(pc->flags);
void *object;
unsigned int cpuset_mems_cookie;

@@ -2212,15 +2270,15 @@ static void *get_any_partial(struct kmem_cache *s, gfp_t flags,

do {
cpuset_mems_cookie = read_mems_allowed_begin();
- zonelist = node_zonelist(mempolicy_slab_node(), flags);
+ zonelist = node_zonelist(mempolicy_slab_node(), pc->flags);
for_each_zone_zonelist(zone, z, zonelist, highest_zoneidx) {
struct kmem_cache_node *n;

n = get_node(s, zone_to_nid(zone));

- if (n && cpuset_zone_allowed(zone, flags) &&
+ if (n && cpuset_zone_allowed(zone, pc->flags) &&
n->nr_partial > s->min_partial) {
- object = get_partial_node(s, n, ret_slab, flags);
+ object = get_partial_node(s, n, pc);
if (object) {
/*
* Don't check read_mems_allowed_retry()
@@ -2241,8 +2299,7 @@ static void *get_any_partial(struct kmem_cache *s, gfp_t flags,
/*
* Get a partial slab, lock it and return it.
*/
-static void *get_partial(struct kmem_cache *s, gfp_t flags, int node,
- struct slab **ret_slab)
+static void *get_partial(struct kmem_cache *s, int node, struct partial_context *pc)
{
void *object;
int searchnode = node;
@@ -2250,11 +2307,11 @@ static void *get_partial(struct kmem_cache *s, gfp_t flags, int node,
if (node == NUMA_NO_NODE)
searchnode = numa_mem_id();

- object = get_partial_node(s, get_node(s, searchnode), ret_slab, flags);
+ object = get_partial_node(s, get_node(s, searchnode), pc);
if (object || node != NUMA_NO_NODE)
return object;

- return get_any_partial(s, flags, ret_slab);
+ return get_any_partial(s, pc);
}

#ifdef CONFIG_PREEMPTION
@@ -2974,11 +3031,12 @@ static inline void *get_freelist(struct kmem_cache *s, struct slab *slab)
* already disabled (which is the case for bulk allocation).
*/
static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
- unsigned long addr, struct kmem_cache_cpu *c)
+ unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size)
{
void *freelist;
struct slab *slab;
unsigned long flags;
+ struct partial_context pc;

stat(s, ALLOC_SLOWPATH);

@@ -3092,7 +3150,10 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,

new_objects:

- freelist = get_partial(s, gfpflags, node, &slab);
+ pc.flags = gfpflags;
+ pc.slab = &slab;
+ pc.orig_size = orig_size;
+ freelist = get_partial(s, node, &pc);
if (freelist)
goto check_new_slab;

@@ -3108,7 +3169,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
stat(s, ALLOC_SLAB);

if (kmem_cache_debug(s)) {
- freelist = alloc_single_from_new_slab(s, slab);
+ freelist = alloc_single_from_new_slab(s, slab, orig_size);

if (unlikely(!freelist))
goto new_objects;
@@ -3140,6 +3201,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
*/
if (s->flags & SLAB_STORE_USER)
set_track(s, freelist, TRACK_ALLOC, addr);
+
return freelist;
}

@@ -3182,7 +3244,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
* pointer.
*/
static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
- unsigned long addr, struct kmem_cache_cpu *c)
+ unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size)
{
void *p;

@@ -3195,7 +3257,7 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
c = slub_get_cpu_ptr(s->cpu_slab);
#endif

- p = ___slab_alloc(s, gfpflags, node, addr, c);
+ p = ___slab_alloc(s, gfpflags, node, addr, c, orig_size);
#ifdef CONFIG_PREEMPT_COUNT
slub_put_cpu_ptr(s->cpu_slab);
#endif
@@ -3280,7 +3342,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l

if (!USE_LOCKLESS_FAST_PATH() ||
unlikely(!object || !slab || !node_match(slab, node))) {
- object = __slab_alloc(s, gfpflags, node, addr, c);
+ object = __slab_alloc(s, gfpflags, node, addr, c, orig_size);
} else {
void *next_object = get_freepointer_safe(s, object);

@@ -3747,7 +3809,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
* of re-populating per CPU c->freelist
*/
p[i] = ___slab_alloc(s, flags, NUMA_NO_NODE,
- _RET_IP_, c);
+ _RET_IP_, c, s->object_size);
if (unlikely(!p[i]))
goto error;

@@ -4150,12 +4212,17 @@ static int calculate_sizes(struct kmem_cache *s)
}

#ifdef CONFIG_SLUB_DEBUG
- if (flags & SLAB_STORE_USER)
+ if (flags & SLAB_STORE_USER) {
/*
* Need to store information about allocs and frees after
* the object.
*/
size += 2 * sizeof(struct track);
+
+ /* Save the original kmalloc request size */
+ if (flags & SLAB_KMALLOC)
+ size += sizeof(unsigned int);
+ }
#endif

kasan_cache_create(s, &size, &s->flags);
@@ -4770,7 +4837,7 @@ void __init kmem_cache_init(void)

/* Now we can use the kmem_cache to allocate kmalloc slabs */
setup_kmalloc_cache_index_table();
- create_kmalloc_caches(0);
+ create_kmalloc_caches(SLAB_KMALLOC);

/* Setup random freelists for each cache */
init_freelist_randomization();
@@ -4937,6 +5004,7 @@ struct location {
depot_stack_handle_t handle;
unsigned long count;
unsigned long addr;
+ unsigned long waste;
long long sum_time;
long min_time;
long max_time;
@@ -4983,13 +5051,15 @@ static int alloc_loc_track(struct loc_track *t, unsigned long max, gfp_t flags)
}

static int add_location(struct loc_track *t, struct kmem_cache *s,
- const struct track *track)
+ const struct track *track,
+ unsigned int orig_size)
{
long start, end, pos;
struct location *l;
- unsigned long caddr, chandle;
+ unsigned long caddr, chandle, cwaste;
unsigned long age = jiffies - track->when;
depot_stack_handle_t handle = 0;
+ unsigned int waste = s->object_size - orig_size;

#ifdef CONFIG_STACKDEPOT
handle = READ_ONCE(track->handle);
@@ -5007,11 +5077,13 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
if (pos == end)
break;

- caddr = t->loc[pos].addr;
- chandle = t->loc[pos].handle;
- if ((track->addr == caddr) && (handle == chandle)) {
+ l = &t->loc[pos];
+ caddr = l->addr;
+ chandle = l->handle;
+ cwaste = l->waste;
+ if ((track->addr == caddr) && (handle == chandle) &&
+ (waste == cwaste)) {

- l = &t->loc[pos];
l->count++;
if (track->when) {
l->sum_time += age;
@@ -5036,6 +5108,9 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
end = pos;
else if (track->addr == caddr && handle < chandle)
end = pos;
+ else if (track->addr == caddr && handle == chandle &&
+ waste < cwaste)
+ end = pos;
else
start = pos;
}
@@ -5059,6 +5134,7 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
l->min_pid = track->pid;
l->max_pid = track->pid;
l->handle = handle;
+ l->waste = waste;
cpumask_clear(to_cpumask(l->cpus));
cpumask_set_cpu(track->cpu, to_cpumask(l->cpus));
nodes_clear(l->nodes);
@@ -5077,7 +5153,7 @@ static void process_slab(struct loc_track *t, struct kmem_cache *s,

for_each_object(p, s, addr, slab->objects)
if (!test_bit(__obj_to_index(s, addr, p), obj_map))
- add_location(t, s, get_track(s, p, alloc));
+ add_location(t, s, get_track(s, p, alloc), get_orig_size(s, p));
}
#endif /* CONFIG_DEBUG_FS */
#endif /* CONFIG_SLUB_DEBUG */
@@ -5942,6 +6018,10 @@ static int slab_debugfs_show(struct seq_file *seq, void *v)
else
seq_puts(seq, "<not-available>");

+ if (l->waste)
+ seq_printf(seq, " waste=%lu/%lu",
+ l->count * l->waste, l->waste);
+
if (l->sum_time != l->min_time) {
seq_printf(seq, " age=%ld/%llu/%ld",
l->min_time, div_u64(l->sum_time, l->count),
--
2.34.1