Re: [PATCH v12] mm: slub: move sysfs slab alloc/free interfaces to debugfs

[Vlastimil Babka]

On 6/17/21 8:32 AM, Faiyaz Mohammed wrote:
> 
> 
> On 6/16/2021 9:47 PM, Vlastimil Babka wrote:
>> On 6/16/21 5:50 PM, Faiyaz Mohammed wrote:
>>>
>>>
>>> On 6/16/2021 4:35 PM, Vlastimil Babka wrote:
>>>> On 6/15/21 5:58 PM, Qian Cai wrote:
>>>>>
>>>>>
>>>>> On 6/11/2021 3:03 PM, Faiyaz Mohammed wrote:
>>>>>> alloc_calls and free_calls implementation in sysfs have two issues,
>>>>>> one is PAGE_SIZE limitation 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
>>>>>> implementation to debugfs.
>>>>>>
>>>>>> Debugfs cache will be created if SLAB_STORE_USER flag is set.
>>>>>>
>>>>>> Rename the alloc_calls/free_calls to alloc_traces/free_traces,
>>>>>> to be inline with what it does.
>>>>>>
>>>>>> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>>>>>> Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>>>>> Signed-off-by: Faiyaz Mohammed <faiyazm@xxxxxxxxxxxxxx>
>>>>>
>>>>> Reverting this commit on today's linux-next fixed all leaks (hundreds) reported by kmemleak like below,
>>>>>
>>>>> unreferenced object 0xffff00091ae1b540 (size 64):
>>>>>   comm "lsbug", pid 1607, jiffies 4294958291 (age 1476.340s)
>>>>>   hex dump (first 32 bytes):
>>>>>     02 00 00 00 00 00 00 00 6b 6b 6b 6b 6b 6b 6b 6b  ........kkkkkkkk
>>>>>     6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
>>>>>   backtrace:
>>>>>     [<ffff8000106b06b8>] slab_post_alloc_hook+0xa0/0x418
>>>>>     [<ffff8000106b5c7c>] kmem_cache_alloc_trace+0x1e4/0x378
>>>>>     [<ffff8000106b5e40>] slab_debugfs_start+0x30/0x50
>>>>>     slab_debugfs_start at /usr/src/linux-next/mm/slub.c:5831
>>>>>     [<ffff8000107b3dbc>] seq_read_iter+0x214/0xd50
>>>>>     [<ffff8000107b4b84>] seq_read+0x28c/0x418
>>>>>     [<ffff8000109560b4>] full_proxy_read+0xdc/0x148
>>>>>     [<ffff800010738f24>] vfs_read+0x104/0x340
>>>>>     [<ffff800010739ee0>] ksys_read+0xf8/0x1e0
>>>>>     [<ffff80001073a03c>] __arm64_sys_read+0x74/0xa8
>>>>>     [<ffff8000100358d4>] invoke_syscall.constprop.0+0xdc/0x1d8
>>>>>     [<ffff800010035ab4>] do_el0_svc+0xe4/0x298
>>>>>     [<ffff800011138528>] el0_svc+0x20/0x30
>>>>>     [<ffff800011138b08>] el0t_64_sync_handler+0xb0/0xb8
>>>>>     [<ffff80001001259c>] el0t_64_sync+0x178/0x17c
>>>>>
>>>>
>>>> I think the problem is here:
>>>>
>>>>>> +static void slab_debugfs_stop(struct seq_file *seq, void *v)
>>>>>> +{
>>>>>> +	kfree(v);
>>>>>> +}
>>>>>> +
>>>>>> +static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
>>>>>> +{
>>>>>> +	loff_t *spos = v;
>>>>>> +	struct loc_track *t = seq->private;
>>>>>> +
>>>>>> +	if (*ppos < t->count) {
>>>>>> +		*ppos = ++*spos;
>>>>>> +		return spos;
>>>>>> +	}
>>>>>> +	*ppos = ++*spos;
>>>>>> +	return NULL;
>>>>>> +}
>>>>
>>>> If we return NULL, then NULL is passed to slab_debugfs_stop and thus we don't
>>>> kfree ppos. kfree(NULL) is silently ignored.
>>>>
>>> I think yes, if NULL passed to kfree, it simply do return.
>>>> I think as we have private struct loc_track, we can add a pos field there and
>>>> avoid the kmaloc/kfree altogether.
>>>>
>>> Hmm, yes we can add pos field "or" we can use argument "v" mean we can
>>> update v with pos in ->next() and use in ->show() to avoid the leak
>>> (kmalloc/kfree).
>> 
>> Can you explain the "or" part more. It's exactly what we already do, no?I am thinking if we simplly do ppos return from slab_debugfs_start() and
> in slab_debugfs_next() assign ppos to "v", update it and return if
> records are there. something like below (approach 1):
> ...
> static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
> {
> ...
>         v = ppos;
>         if (*ppos < t->count) {
>                  ++*ppos;
>                 return v;
>         }
> 
>         ++*ppos;
>         return NULL;
> }
> ...
> static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos)
> {
>         return ppos;
> }

OK maybe that works too. Bonus points if some other code does that. And then it
might be another reason to update the Documentation file.

> ...
> 
>> "v" as you said. The problem is, if next(); returns NULL, then stop() gets the
>> NULL as "v". It's just what I see in the code of seq_read_iter() and traverse()
>> in fs/seq_file.c. I don't see another way to say there are no more records to
>> print - only to return NULL in next().
>> Ah, ok so we could maybe do the kfree() in next() then before returning NULL,
>> which is the last moment we have the pointer. But really, if we already have a
>> loc_track in private, why kmalloc an additional loff_t.
>> 
> Yes, we can do kfree() before returning NULL, but better to add ppos in
> lock_track. (approach 2)
> 
>> Anyway it seems to me also that
>> Documentation/filesystems/seq_file.rst should be updated, as the kfree() in
>> stop() is exactly what it suggests, and it doesn't show how next() indicates
>> that there are no more records by returning NULL, and what to do about kfree() then.
> 
> Can you please suggest me which approach would be good to avoid the
> leak?. I will update in next patch version.

I guess your approach is simpler as we don't track two pos variables.