Re: [PATCH v6 1/4] mm/slub: enable debugging memory wasting of kmalloc
From: Vlastimil Babka
Date: Fri Sep 23 2022 - 07:43:47 EST
On 9/13/22 08:54, Feng Tang wrote:
> 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>
Thanks.
Given that the merge window is nearing, and the rest of the series a) has
some changes suggested and b) could be hopefully done in a simpler way with
the proposed ksize() cleanup, I am picking just this patch now to slab.git
(and thus -next), with some small modifications:
...
> +
> +static unsigned int get_orig_size(struct kmem_cache *s, void *object)
Made this inline for consistency.
> +{
> + 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) {}
There's no caller (in this patch alone) for the !SLUB_DEBUG version, so removed.
> 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);
Instead of this, add the flag in the common creation function, so SLAB kmalloc caches are also marked even if there's no use for it there now.
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -649,7 +649,8 @@ struct kmem_cache *__init create_kmalloc_cache(const char *name,
if (!s)
panic("Out of memory when creating slab %s\n", name);
- create_boot_cache(s, name, size, flags, useroffset, usersize);
+ create_boot_cache(s, name, size, flags | SLAB_KMALLOC, useroffset,
+ usersize);
kasan_cache_create_kmalloc(s);
list_add(&s->list, &slab_caches);
s->refcount = 1;
> /* 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));
I think it makes little sense to report waste in the 'free_traces' file?
So adjusted like this to make sure nothing is reported there:
@@ -5356,13 +5353,16 @@ static void process_slab(struct loc_track *t, struct kmem_cache *s,
unsigned long *obj_map)
{
void *addr = slab_address(slab);
+ bool is_alloc = (alloc == TRACK_ALLOC);
void *p;
__fill_map(obj_map, s, slab);
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), get_orig_size(s, p));
+ add_location(t, s, get_track(s, p, alloc),
+ is_alloc ? get_orig_size(s, p) :
+ s->object_size);
> }
> #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),