[PATCH v2 1/2] mm/slub: enable debugging memory wasting of kmalloc
From: Feng Tang
Date: Mon Jul 25 2022 - 07:20:31 EST
kmalloc's API family is critical for mm, with one shortcoming that
its object size is fixed to be power of 2. When user requests memory
for '2^n + 1' bytes, actually 2^(n+1) bytes will be allocated, so
in worst case, there is around 50% memory space waste.
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 picked just to demo the problem)
So add an way to track each kmalloc's memory waste info, and leverage
the existing SLUB debug framework to show its call stack info, 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+0xa5/0x4a0 [ixgbe] waste=233856/1856 age=1493302/1493830/1494358 pid=1284 cpus=32 nodes=1
__slab_alloc.isra.86+0x52/0x80
__kmalloc_node+0x143/0x350
ixgbe_alloc_q_vector+0xa5/0x4a0 [ixgbe]
ixgbe_init_interrupt_scheme+0x1a6/0x730 [ixgbe]
ixgbe_probe+0xc8e/0x10d0 [ixgbe]
local_pci_probe+0x42/0x80
work_for_cpu_fn+0x13/0x20
process_one_work+0x1c5/0x390
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). And when system starts some real
workload like multiple docker instances, there 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>
---
since v1:
* limit the 'orig_size' to kmalloc objects only, and save
it after track in metadata (Vlastimil Babka)
* fix a offset calculation problem in print_trailer
since RFC:
* fix problems in kmem_cache_alloc_bulk() and records sorting,
improve the print format (Hyeonggon Yoo)
* fix a compiling issue found by 0Day bot
* update the commit log based info from iova developers
include/linux/slab.h | 2 +
mm/slub.c | 96 ++++++++++++++++++++++++++++++++++++--------
2 files changed, 82 insertions(+), 16 deletions(-)
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 0fefdf528e0d..a713b0e5bbcd 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 b1281b8654bd..9763a38bc4f0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -191,6 +191,12 @@ 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 (s->flags & SLAB_KMALLOC &&
+ kmem_cache_debug_flags(s, SLAB_STORE_USER));
+}
+
void *fixup_red_left(struct kmem_cache *s, void *p)
{
if (kmem_cache_debug_flags(s, SLAB_RED_ZONE))
@@ -814,6 +820,36 @@ static void print_slab_info(const struct slab *slab)
pr_err("Slab 0x%p objects=%u used=%u fp=0x%p flags=%pGp\n",
slab, slab->objects, slab->inuse, slab->freelist,
folio_flags(folio, 0));
+
+}
+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 = object + get_info_end(s);
+
+ if (s->flags & SLAB_STORE_USER)
+ 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 = object + get_info_end(s);
+ if (s->flags & SLAB_STORE_USER)
+ p += sizeof(struct track) * 2;
+
+ return *(unsigned int *)p;
}
static void slab_bug(struct kmem_cache *s, char *fmt, ...)
@@ -875,6 +911,9 @@ 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))
@@ -1030,6 +1069,9 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
/* We also have user information there */
off += 2 * sizeof(struct track);
+ if (slub_debug_orig_size(s))
+ off += sizeof(unsigned int);
+
off += kasan_metadata_size(s);
if (size_from_object(s) == off)
@@ -1325,7 +1367,8 @@ 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, unsigned long addr)
+ void *object, unsigned long addr,
+ unsigned int orig_size)
{
if (s->flags & SLAB_CONSISTENCY_CHECKS) {
if (!alloc_consistency_checks(s, slab, object))
@@ -1335,6 +1378,9 @@ static noinline int alloc_debug_processing(struct kmem_cache *s,
/* Success perform special debug activities for allocs */
if (s->flags & SLAB_STORE_USER)
set_track(s, object, TRACK_ALLOC, addr);
+
+ set_orig_size(s, object, orig_size);
+
trace(s, slab, object, 1);
init_object(s, object, SLUB_RED_ACTIVE);
return 1;
@@ -1661,7 +1707,8 @@ 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, unsigned long addr) { return 0; }
+ struct slab *slab, void *object, unsigned long addr,
+ unsigned int orig_size) { return 0; }
static inline int free_debug_processing(
struct kmem_cache *s, struct slab *slab,
@@ -2905,7 +2952,7 @@ 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;
@@ -3048,7 +3095,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
check_new_slab:
if (kmem_cache_debug(s)) {
- if (!alloc_debug_processing(s, slab, freelist, addr)) {
+ if (!alloc_debug_processing(s, slab, freelist, addr, orig_size)) {
/* Slab failed checks. Next slab needed */
goto new_slab;
} else {
@@ -3102,7 +3149,7 @@ 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;
@@ -3115,7 +3162,7 @@ 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
@@ -3206,7 +3253,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l
*/
if (IS_ENABLED(CONFIG_PREEMPT_RT) ||
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);
@@ -3731,7 +3778,7 @@ 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;
@@ -4140,6 +4187,10 @@ static int calculate_sizes(struct kmem_cache *s)
* the object.
*/
size += 2 * sizeof(struct track);
+
+ /* Save the original requested kmalloc size */
+ if (slub_debug_orig_size(s))
+ size += sizeof(unsigned int);
#endif
kasan_cache_create(s, &size, &s->flags);
@@ -4864,7 +4915,7 @@ 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();
@@ -5092,6 +5143,7 @@ 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;
@@ -5138,13 +5190,15 @@ 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);
@@ -5162,11 +5216,13 @@ 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;
@@ -5191,6 +5247,9 @@ 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;
}
@@ -5214,6 +5273,7 @@ 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);
@@ -5232,7 +5292,7 @@ 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 */
@@ -6102,6 +6162,10 @@ 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),
--
2.27.0