Date:   Mon, 11 Jul 2022 19:54:41 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, dave.hansen@...el.com,
        Robin Murphy <robin.murphy@....com>,
        John Garry <john.garry@...wei.com>
Subject: Re: [PATCH v1] mm/slub: enable debugging memory wasting of kmalloc

On Mon, Jul 11, 2022 at 10:15:21AM +0200, Vlastimil Babka wrote:
> On 7/1/22 15:59, Feng Tang wrote:
[...]
> > 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@huawei.com/
> > 
> > Signed-off-by: Feng Tang <feng.tang@...el.com>
> 
> Hi and thanks.
> I would suggest some improvements to consider:
 
Hi Vlastimil,

Thanks for the review and detailed suggestions!

> - don't use the struct track to store orig_size, although it's an obvious
> first choice. It's unused waste for the free_track, and also for any
> non-kmalloc caches. I'd carve out an extra int next to the struct tracks.
> Only for kmalloc caches (probably a new kmem cache flag set on creation will
> be needed to easily distinguish them).
> Besides the saved space, you can then set the field from ___slab_alloc()
> directly and not need to pass the orig_size also to alloc_debug_processing()
> etc.

Do you mean decouple 'track' and the 'orig_size'(waste info), and
add 'orig_size' for each kmalloc object (with the help of flag)?
This solution depends hugely on 'track' framework.

Initially when implementing it, I met several problems:
1. where to save the orig_size(waste)
2. how to calcuate the waste info
3. where to show the waste info
4. how to show the full call stack for users' easy handling

I thought about saving the global waste in kmem_cache, and show it in
'slabinfo', then we have to handle it in all free calls or loop all
objects in-use  when 'cat slabinfo', still it lacks the detail info
of each call stacks (some call points can be called with different
sizes). And the 'track' seems to be ideal to solve them. 

Or based on your suggestion, we still add 'waste' into 'track' but
under a kernel config option 'SLUB_DEBUG_WASTE' for space saving.

Also I checked the struct track, it is well packed, only the
'int cpu' could be hacked, that it could spare 16 bits for storing
the waste info:

struct track {
    ...
    unsigned short cpu; /* 0-65535 */
    unsigned short waste;
    ...
}

> - the knowledge of actual size could be used to improve poisoning checks as
> well, detect cases when there's buffer overrun over the orig_size but not
> cache's size. e.g. if you kmalloc(48) and overrun up to 64 we won't detect
> it now, but with orig_size stored we could?
 
Yes! This could be imporoved. 

Thanks,
Feng

> Thanks!
> Vlastimil
> 
> > ---
> > Changelog:
> > 
> >   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
> > 
> >  mm/slub.c | 52 +++++++++++++++++++++++++++++++++++-----------------
> >  1 file changed, 35 insertions(+), 17 deletions(-)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index b1281b8654bd3..97304ea1e6aa5 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -271,6 +271,7 @@ struct track {
> >  #endif
> >  	int cpu;		/* Was running on cpu */
> >  	int pid;		/* Pid context */
> > +	unsigned long waste;	/* memory waste for a kmalloc-ed object */
> >  	unsigned long when;	/* When did the operation occur */
> >  };
> >  
> > @@ -747,6 +748,7 @@ static inline depot_stack_handle_t set_track_prepare(void)
> >  
> >  static void set_track_update(struct kmem_cache *s, void *object,
> >  			     enum track_item alloc, unsigned long addr,
> > +			     unsigned long waste,
> >  			     depot_stack_handle_t handle)
> >  {
> >  	struct track *p = get_track(s, object, alloc);
> > @@ -758,14 +760,16 @@ static void set_track_update(struct kmem_cache *s, void *object,
> >  	p->cpu = smp_processor_id();
> >  	p->pid = current->pid;
> >  	p->when = jiffies;
> > +	p->waste = waste;
> >  }
> >  
> >  static __always_inline void set_track(struct kmem_cache *s, void *object,
> > -				      enum track_item alloc, unsigned long addr)
> > +				      enum track_item alloc, unsigned long addr,
> > +				      unsigned long waste)
> >  {
> >  	depot_stack_handle_t handle = set_track_prepare();
> >  
> > -	set_track_update(s, object, alloc, addr, handle);
> > +	set_track_update(s, object, alloc, addr, waste, handle);
> >  }
> >  
> >  static void init_tracking(struct kmem_cache *s, void *object)
> > @@ -1325,7 +1329,9 @@ 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 long waste
> > +					)
> >  {
> >  	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> >  		if (!alloc_consistency_checks(s, slab, object))
> > @@ -1334,7 +1340,7 @@ 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_track(s, object, TRACK_ALLOC, addr, waste);
> >  	trace(s, slab, object, 1);
> >  	init_object(s, object, SLUB_RED_ACTIVE);
> >  	return 1;
> > @@ -1418,7 +1424,7 @@ static noinline int free_debug_processing(
> >  	}
> >  
> >  	if (s->flags & SLAB_STORE_USER)
> > -		set_track_update(s, object, TRACK_FREE, addr, handle);
> > +		set_track_update(s, object, TRACK_FREE, addr, 0, handle);
> >  	trace(s, slab, object, 0);
> >  	/* Freepointer not overwritten by init_object(), SLAB_POISON moved it */
> >  	init_object(s, object, SLUB_RED_INACTIVE);
> > @@ -1661,7 +1667,8 @@ 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 long waste) { return 0; }
> >  
> >  static inline int free_debug_processing(
> >  	struct kmem_cache *s, struct slab *slab,
> > @@ -2905,7 +2912,7 @@ 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 +3055,7 @@ 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, s->object_size - orig_size)) {
> >  			/* Slab failed checks. Next slab needed */
> >  			goto new_slab;
> >  		} else {
> > @@ -3102,7 +3109,7 @@ 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 +3122,7 @@ 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 +3213,7 @@ 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 +3738,7 @@ 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;
> >  
> > @@ -5092,6 +5099,7 @@ 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;
> > @@ -5142,7 +5150,7 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
> >  {
> >  	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;
> >  
> > @@ -5162,11 +5170,13 @@ 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) &&
> > +			(track->waste == cwaste)) {
> >  
> > -			l = &t->loc[pos];
> >  			l->count++;
> >  			if (track->when) {
> >  				l->sum_time += age;
> > @@ -5191,6 +5201,9 @@ 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 &&
> > +				track->waste < cwaste)
> > +			end = pos;
> >  		else
> >  			start = pos;
> >  	}
> > @@ -5214,6 +5227,7 @@ 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 = track->waste;
> >  	cpumask_clear(to_cpumask(l->cpus));
> >  	cpumask_set_cpu(track->cpu, to_cpumask(l->cpus));
> >  	nodes_clear(l->nodes);
> > @@ -6102,6 +6116,10 @@ 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),

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