Re: [PATCH v4] mm: slub: move sysfs slab alloc/free interfaces to debugfs

[Faiyaz Mohammed]

On 4/19/2021 5:42 PM, Vlastimil Babka wrote:
> On 4/16/21 4:27 PM, Faiyaz Mohammed wrote:
>> alloc_calls and free_calls implementation in sysfs have two issues,
>> one is PAGE_SIZE limitiation of sysfs and other is it does not adhere
>> to "one value per file" rule.
>>
>> To overcome this issues, move the alloc_calls and free_calls implemeation
>> to debugfs.
>>
>> Signed-off-by: Faiyaz Mohammed <faiyazm@xxxxxxxxxxxxxx>
>> ---
>>  include/linux/slub_def.h |  10 +++
>>  mm/slab_common.c         |   9 +++
>>  mm/slub.c                | 202 ++++++++++++++++++++++++++++++++++++++++++-----
>>  3 files changed, 200 insertions(+), 21 deletions(-)
> 
> The patch doesn't compile, as the bots noticed. Had to add a missing #endif
> after list_locations().
> 
My bad, I think I miss. Updated the patch.

>> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
>> index dcde82a..f8c268d 100644
>> --- a/include/linux/slub_def.h
>> +++ b/include/linux/slub_def.h
>> @@ -110,6 +110,9 @@ struct kmem_cache {
>>  #ifdef CONFIG_SYSFS
>>  	struct kobject kobj;	/* For sysfs */
>>  #endif
>> +#ifdef CONFIG_SLUB_DEBUG
>> +	struct dentry *slab_cache_dentry;
>> +#endif
>>  #ifdef CONFIG_SLAB_FREELIST_HARDENED
>>  	unsigned long random;
>>  #endif
>> @@ -159,6 +162,13 @@ static inline void sysfs_slab_release(struct kmem_cache *s)
>>  }
>>  #endif
>>  
>> +#ifdef CONFIG_DEBUG_FS
>> +void debugfs_slab_release(struct kmem_cache *);
>> +#else
>> +static inline void debugfs_slab_release(struct kmem_cache *s)
>> +{
>> +}
>> +#endif
>>  void object_err(struct kmem_cache *s, struct page *page,
>>  		u8 *object, char *reason);
>>  
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 88e8339..fb28328 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -437,6 +437,9 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>>  #else
>>  		slab_kmem_cache_release(s);
>>  #endif
>> +#ifdef CONFIG_DEBUG_FS
>> +		debugfs_slab_release(s);
>> +#endif
>>  	}
>>  }
>>  
>> @@ -454,6 +457,9 @@ static int shutdown_cache(struct kmem_cache *s)
>>  #ifdef SLAB_SUPPORTS_SYSFS
>>  		sysfs_slab_unlink(s);
>>  #endif
>> +#ifdef CONFIG_DEBUG_FS
>> +		debugfs_slab_release(s);
>> +#endif
>>  		list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
>>  		schedule_work(&slab_caches_to_rcu_destroy_work);
>>  	} else {
>> @@ -464,6 +470,9 @@ static int shutdown_cache(struct kmem_cache *s)
>>  #else
>>  		slab_kmem_cache_release(s);
>>  #endif
>> +#ifdef CONFIG_DEBUG_FS
>> +		debugfs_slab_release(s);
>> +#endif
>>  	}
>>  
>>  	return 0;
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 3021ce9..ab7a0d3 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -36,6 +36,7 @@
>>  #include <linux/memcontrol.h>
>>  #include <linux/random.h>
>>  
>> +#include <linux/debugfs.h>
>>  #include <trace/events/kmem.h>
>>  
>>  #include "internal.h"
>> @@ -225,6 +226,15 @@ static inline int sysfs_slab_alias(struct kmem_cache *s, const char *p)
>>  							{ return 0; }
>>  #endif
>>  
>> +#ifdef CONFIG_DEBUG_FS
>> +static void debugfs_slab_add(struct kmem_cache *);
>> +static int debugfs_slab_alias(struct kmem_cache *, const char *);
>> +#else
>> +static inline void debugfs_slab_add(struct kmem_cache *s) { }
>> +static inline int debugfs_slab_alias(struct kmem_cache *s, const char *p)
>> +							{ return 0; }
>> +#endif
>> +
>>  static inline void stat(const struct kmem_cache *s, enum stat_item si)
>>  {
>>  #ifdef CONFIG_SLUB_STATS
>> @@ -4521,6 +4531,8 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
>>  			s->refcount--;
>>  			s = NULL;
>>  		}
>> +
>> +		debugfs_slab_alias(s, name);
>>  	}
>>  
>>  	return s;
>> @@ -4542,6 +4554,8 @@ int __kmem_cache_create(struct kmem_cache *s, slab_flags_t flags)
>>  	if (err)
>>  		__kmem_cache_release(s);
>>  
>> +	debugfs_slab_add(s);
>> +
>>  	return err;
>>  }
>>  
>> @@ -4682,6 +4696,8 @@ static long validate_slab_cache(struct kmem_cache *s)
>>  
>>  	return count;
>>  }
>> +
>> +#ifdef CONFIG_DEBUG_FS
>>  /*
>>   * Generate lists of code addresses where slabcache objects are allocated
>>   * and freed.
>> @@ -4705,6 +4721,8 @@ struct loc_track {
>>  	struct location *loc;
>>  };
>>  
>> +static struct dentry *slab_debugfs_root;
>> +
>>  static void free_loc_track(struct loc_track *t)
>>  {
>>  	if (t->max)
>> @@ -4822,10 +4840,9 @@ static void process_slab(struct loc_track *t, struct kmem_cache *s,
>>  	put_map(map);
>>  }
>>  
>> -static int list_locations(struct kmem_cache *s, char *buf,
>> +static int list_locations(struct seq_file *seq, struct kmem_cache *s,
>>  			  enum track_item alloc)
>>  {
>> -	int len = 0;
>>  	unsigned long i;
>>  	struct loc_track t = { 0, 0, NULL };
>>  	int node;
>> @@ -4833,7 +4850,8 @@ static int list_locations(struct kmem_cache *s, char *buf,
>>  
>>  	if (!alloc_loc_track(&t, PAGE_SIZE / sizeof(struct location),
>>  			     GFP_KERNEL)) {
>> -		return sysfs_emit(buf, "Out of memory\n");
>> +		seq_puts(seq, "Out of memory\n");
>> +		return -ENOMEM;
>>  	}
>>  	/* Push back cpu slabs */
>>  	flush_all(s);
>> @@ -4856,46 +4874,46 @@ static int list_locations(struct kmem_cache *s, char *buf,
>>  	for (i = 0; i < t.count; i++) {
>>  		struct location *l = &t.loc[i];
>>  
>> -		len += sysfs_emit_at(buf, len, "%7ld ", l->count);
>> +		seq_printf(seq, "%7ld ", l->count);
>>  
>>  		if (l->addr)
>> -			len += sysfs_emit_at(buf, len, "%pS", (void *)l->addr);
>> +			seq_printf(seq, "%pS", (void *)l->addr);
>>  		else
>> -			len += sysfs_emit_at(buf, len, "<not-available>");
>> +			seq_puts(seq, "<not-available>");
>>  
>>  		if (l->sum_time != l->min_time)
>> -			len += sysfs_emit_at(buf, len, " age=%ld/%ld/%ld",
>> +			seq_printf(seq, " age=%ld/%ld/%ld",
>>  					     l->min_time,
>>  					     (long)div_u64(l->sum_time,
>>  							   l->count),
>>  					     l->max_time);
>>  		else
>> -			len += sysfs_emit_at(buf, len, " age=%ld", l->min_time);
>> +			seq_printf(seq, " age=%ld", l->min_time);
>>  
>>  		if (l->min_pid != l->max_pid)
>> -			len += sysfs_emit_at(buf, len, " pid=%ld-%ld",
>> +			seq_printf(seq, " pid=%ld-%ld",
>>  					     l->min_pid, l->max_pid);
>>  		else
>> -			len += sysfs_emit_at(buf, len, " pid=%ld",
>> +			seq_printf(seq, " pid=%ld",
>>  					     l->min_pid);
>>  
>>  		if (num_online_cpus() > 1 &&
>>  		    !cpumask_empty(to_cpumask(l->cpus)))
>> -			len += sysfs_emit_at(buf, len, " cpus=%*pbl",
>> +			seq_printf(seq, " cpus=%*pbl",
>>  					     cpumask_pr_args(to_cpumask(l->cpus)));
>>  
>>  		if (nr_online_nodes > 1 && !nodes_empty(l->nodes))
>> -			len += sysfs_emit_at(buf, len, " nodes=%*pbl",
>> +			seq_printf(seq, " nodes=%*pbl",
>>  					     nodemask_pr_args(&l->nodes));
>>  
>> -		len += sysfs_emit_at(buf, len, "\n");
>> +		seq_puts(seq, "\n");
>>  	}
>>  
>>  	free_loc_track(&t);
>>  	if (!t.count)
>> -		len += sysfs_emit_at(buf, len, "No data\n");
>> +		seq_puts(seq, "No data\n");
>>  
>> -	return len;
>> +	return 0;
>>  }
>>  #endif	/* CONFIG_SLUB_DEBUG */
>>  
>> @@ -5348,17 +5366,23 @@ SLAB_ATTR(validate);
>>  
>>  static ssize_t alloc_calls_show(struct kmem_cache *s, char *buf)
>>  {
>> -	if (!(s->flags & SLAB_STORE_USER))
>> -		return -ENOSYS;
>> -	return list_locations(s, buf, TRACK_ALLOC);
>> +	int len = 0;
>> +
>> +	len += sprintf(buf, "Deprecated, use the equvalent under");
>> +	len += sprintf(buf, "/sys/kernel/debug/slab/%s/alloc_calls", s->name);
> 
> This doesn't work properly, the second printf() overwrites the first one, also
> it's not newline-terminated.
> Also debugfs does not have to be always mounted as /sys/kernel/debug. But maybe
> it's common enough that the suggestion will work most of the time, so we can
> probably leave it.
>
added new line. Updated the patch.

>> +	return len;
>>  }
>>  SLAB_ATTR_RO(alloc_calls);
>>  
>>  static ssize_t free_calls_show(struct kmem_cache *s, char *buf)
>>  {
>> -	if (!(s->flags & SLAB_STORE_USER))
>> -		return -ENOSYS;
>> -	return list_locations(s, buf, TRACK_FREE);
>> +	int len = 0;
>> +
>> +	len += sprintf(buf, "Deprecated, use the equvalent under");
>> +	len += sprintf(buf, "/sys/kernel/debug/slab/%s/free_calls", s->name);
>> +
>> +	return len;
>>  }
>>  SLAB_ATTR_RO(free_calls);
>>  #endif /* CONFIG_SLUB_DEBUG */
>> @@ -5814,6 +5838,142 @@ static int __init slab_sysfs_init(void)
>>  __initcall(slab_sysfs_init);
>>  #endif /* CONFIG_SYSFS */
>>  
>> +#if defined(CONFIG_SLUB_DEBUG) && defined(CONFIG_DEBUG_FS)
>> +static int debugfs_slab_alias(struct kmem_cache *s, const char *name)
>> +{
>> +	struct saved_alias *al;
>> +
>> +	if (slab_state == FULL) {
>> +		/*
>> +		 * If we have a leftover link then remove it.
>> +		 */
>> +		debugfs_remove(s->slab_cache_dentry);
>> +		s->slab_cache_dentry = debugfs_create_symlink(name, slab_debugfs_root, NULL);
>> +		return IS_ERR(s->slab_cache_dentry);
>> +	}
>> +
>> +	al = kmalloc(sizeof(struct saved_alias), GFP_KERNEL);
>> +	if (!al)
>> +		return -ENOMEM;
>> +
>> +	al->s = s;
>> +	al->name = name;
>> +	al->next = alias_list;
>> +	alias_list = al;
>> +	return 0;
>> +}
>> +
>> +static int slab_debug_trace(struct seq_file *seq, void *ignored)
>> +{
>> +	struct kmem_cache *s = seq->private;
>> +
>> +	if (!(s->flags & SLAB_STORE_USER))
>> +		return 0;
>> +
>> +	if (strcmp(seq->file->f_path.dentry->d_name.name, "alloc_trace") == 0)
>> +		return list_locations(seq, s, TRACK_ALLOC);
>> +	else
>> +		return list_locations(seq, s, TRACK_FREE);
>> +
>> +	return 0;
>> +}
>> +
>> +static int slab_debug_trace_open(struct inode *inode, struct file *filp)
>> +{
>> +	return single_open(filp, slab_debug_trace,
>> +				file_inode(filp)->i_private);
> 
> So using single_open() makes things simple, yeah. But IMHO it's still abusing
> the seq_file interface. We do iterate over the loc_track objects so we should
> better use the proper iteration approach as described in
> Documentation/filesystems/seq_file.rst
> and used e.g. in fs/proc/task_mmu.c for /proc/pid/smaps etc.
> 
> Right now this relies on the implementation of traverse() which starts by
> allocating a PAGE_SIZE buffer and if the output doesn't fit, it keeps
> reallocating twice the size and repeating the output, thus the call to
> list_location(). That's suboptimal.
> 
> Also we should probably mimic the return -ENOSYS approach of the sysfs variant
> if tracking is not enabled for the cache.
> 
Updated the interface with seq_file ops.

>> +}
>> +
>> +static const struct file_operations slab_debug_fops = {
>> +	.open    = slab_debug_trace_open,
>> +	.read    = seq_read,
>> +	.llseek  = seq_lseek,
>> +	.release = single_release,
>> +};
>> +
>> +static void debugfs_slab_add(struct kmem_cache *s)
>> +{
>> +	const char *name;
>> +	int unmergeable = slab_unmergeable(s);
>> +
>> +	if (unlikely(!slab_debugfs_root))
>> +		return;
>> +
>> +	if (!unmergeable && disable_higher_order_debug &&
>> +			(slub_debug & DEBUG_METADATA_FLAGS))
>> +		unmergeable = 1;
>> +
>> +	if (unmergeable) {
>> +		/*
>> +		 * Slabcache can never be merged so we can use the name proper.
>> +		 * This is typically the case for debug situations. In that
>> +		 * case we can catch duplicate names easily.
>> +		 */
>> +		debugfs_remove(s->slab_cache_dentry);
>> +		name = s->name;
>> +	} else {
>> +		/*
>> +		 * Create a unique name for the slab as a target
>> +		 * for the symlinks.
>> +		 */
>> +		name = create_unique_id(s);
> 
> You don't seem to be actually using the unique name?
> When testing your patch, I don't see the hierarchy of /sys/kernel/slab mirrored
> in /sys/kernel/debug/slab. There are no directories with the unique names and
> symlinks from normal names, it seems all the caches that are aliases are just
> missing completely from /sys/kernel/debug/slabUpdated the new patch.

>> +	}
>> +
>> +	s->slab_cache_dentry = debugfs_create_dir(s->name, slab_debugfs_root);
>> +	if (!IS_ERR(s->slab_cache_dentry)) {
>> +		debugfs_create_file("alloc_trace", 0400,
>> +			s->slab_cache_dentry, s, &slab_debug_fops);
>> +
>> +		debugfs_create_file("free_trace", 0400,
>> +			s->slab_cache_dentry, s, &slab_debug_fops);
> 
> Was it intentional to rename alloc_calls and free_calls to alloc_trace and
> free_trace? You should say in the changelog why.
> I would agree with the new names as Oliver is in fact working on a patch to add
> actual stack traces to the file instead of just immediate callers. And your
> patch to move these files to debugfs to allow longer output is a required
> prerequisity for that. But IMHO it would be more correct to call the files
> "alloc_traces" and "free_traces".
> 
yes, rename the interfaces alloc/free to alloc_traces/free_traces to be
inline with what it does.

>> +	}
>> +
>> +	if (!unmergeable) {
>> +		/* Setup first alias */
>> +		debugfs_slab_alias(s, s->name);
>> +	}
>> +}
>> +
>> +void debugfs_slab_release(struct kmem_cache *s)
>> +{
>> +	if (slab_state >= FULL)
>> +		debugfs_remove_recursive(s->slab_cache_dentry);
>> +}
>> +
>> +static int __init slab_debugfs_init(void)
>> +{
>> +	struct kmem_cache *s;
>> +	int err;
>> +
>> +	slab_debugfs_root = debugfs_create_dir("slab", NULL);
>> +	if (!IS_ERR(slab_debugfs_root)) {
>> +
>> +		slab_state = FULL;
>> +
>> +		list_for_each_entry(s, &slab_caches, list)
>> +			debugfs_slab_add(s);
>> +	} else {
>> +		pr_err("Cannot create slab debugfs.\n");
>> +		return IS_ERR(slab_debugfs_root);
>> +	}
>> +
>> +	while (alias_list) {
>> +		struct saved_alias *al = alias_list;
>> +
>> +		alias_list = alias_list->next;
>> +
>> +		err = debugfs_slab_alias(al->s, al->name);
>> +		if (err)
>> +			pr_err("SLUB: Unable to add boot slab alias %s to debugfs\n",
>> +			       al->name);
>> +		kfree(al);
>> +	}
>> +
>> +	return 0;
>> +
>> +}
>> +__initcall(slab_debugfs_init);
>> +#endif
>>  /*
>>   * The /proc/slabinfo ABI
>>   */
>>
>