Re: [PATCH v7] mm: slub: move sysfs slab alloc/free interfaces to debugfs

[Faiyaz Mohammed]

On 5/26/2021 5:08 PM, Vlastimil Babka wrote:
> On 5/25/21 9:38 AM, 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.
>>
>> Rename the alloc_calls/free_calls to alloc_traces/free_traces,
>> to be inline with what it does.
>>
>> Reported-by: kernel test robot <lkp@xxxxxxxxx>
>> Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> 
> These were IIRC bot reports for some bugs in the previous versions, so keeping
> the Reported-by: for the whole patch is misleading - these were not reports for
> the sysfs issues this patch fixes by moving the files to debugfs.
> 
Yes, I will update in next patch version.
>> Signed-off-by: Faiyaz Mohammed <faiyazm@xxxxxxxxxxxxxx>
>> ---
>> changes in V7:
>>         - Drop the older alloc_calls and free_calls interface.
>> changes in v6:
>>         - https://lore.kernel.org/linux-mm/1621341949-26762-1-git-send-email-faiyazm@xxxxxxxxxxxxxx/
>>
>> changes in v5:
>>         - https://lore.kernel.org/linux-mm/1620296523-21922-1-git-send-email-faiyazm@xxxxxxxxxxxxxx/
>>
>> changes in v4:
>>         - https://lore.kernel.org/linux-mm/1618583239-18124-1-git-send-email-faiyazm@xxxxxxxxxxxxxx/
>>
>> changes in v3:
>>         - https://lore.kernel.org/linux-mm/1617712064-12264-1-git-send-email-faiyazm@xxxxxxxxxxxxxx/
>>
>> changes in v2:
>>         - https://lore.kernel.org/linux-mm/3ac1d3e6-6207-96ad-16a1-0f5139d8b2b5@xxxxxxxxxxxxxx/
>>
>> changes in v1:
>>         - https://lore.kernel.org/linux-mm/1610443287-23933-1-git-send-email-faiyazm@xxxxxxxxxxxxxx/
>>
>>  include/linux/slub_def.h |   8 ++
>>  mm/slab_common.c         |   9 ++
>>  mm/slub.c                | 353 ++++++++++++++++++++++++++++++++++-------------
>>  3 files changed, 276 insertions(+), 94 deletions(-)
> 
> I don't see any of the symlinks under /sys/kernel/debug/slab/, so I think the
> aliases handling code is wrong, and I can see at least two reasons why it could be:
> 

I think I missed one thing, when CONFIG_SLUB_DEBUG_ON enable or
slub_debug is pass through command line __kmem_cache_alias() will return
null, so no symlinks will be created even if CONFIG_SLAB_MERGE_DEFAULT
is enable and to store user data we need to enable CONFIG_SLUB_DEBUG_ON
or pass slub_debug through command line.

>> @@ -4525,6 +4535,8 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
>>  			s->refcount--;
>>  			s = NULL;
>>  		}
>> +
>> +		debugfs_slab_alias(s, name);
> 
> Here you might be calling debugfs_slab_alias() with NULL if the
> sysfs_slab_alias() above returned true.
> 
I think we can drop debugfs_slab_alias implementation.
>>  	}
>>  
>>  	return s;
> 
> ...
> 
>> +static int __init slab_debugfs_init(void)
>> +{
>> +	struct kmem_cache *s;
>> +
>> +	slab_debugfs_root = debugfs_create_dir("slab", NULL);
>> +
>> +	slab_state = FULL;
>> +
>> +	list_for_each_entry(s, &slab_caches, list)
>> +		debugfs_slab_add(s);
>> +
>> +	while (alias_list) {
>> +		struct saved_alias *al = alias_list;
> 
> alias_list a single list and both slab_sysfs_init() and slab_debugfs_init()
> flush it. So only the init call that happens to be called first, does actually
> find an unflushed list. I think you
> need to use a separate list for debugfs (simpler) or a shared list with both
> sysfs and debugfs processing (probably more complicated).
> 
same here, I think we can drop the debugfs alias change.

> And finally a question, perhaps also for Greg. With sysfs, we hand out the
> lifecycle of struct kmem_cache to sysfs, to ensure we are not reading sysfs
> files of a cache that has been removed.
> 
> But with debugfs, what are the guarantees that things won't blow up when a
> debugfs file is being read while somebody calls kmem_cache_destroy() on the cache?
> 
>> +
>> +		alias_list = alias_list->next;
>> +
>> +		debugfs_slab_alias(al->s, al->name);
>> +
>> +		kfree(al);
>> +	}
>> +
>> +	return 0;
>> +
>> +}
>> +__initcall(slab_debugfs_init);
>> +#endif
>>  /*
>>   * The /proc/slabinfo ABI
>>   */
>>
>