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

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Date:   Mon, 31 May 2021 10:19:22 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Faiyaz Mohammed <faiyazm@...eaurora.org>, Greg KH <greg@...ah.com>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        glittao@...il.com, vinmenon@...eaurora.org
Subject: Re: [PATCH v7] mm: slub: move sysfs slab alloc/free interfaces to
 debugfs

On 5/31/21 9:11 AM, Faiyaz Mohammed wrote:
> 
> 
> On 5/26/2021 5:43 PM, Vlastimil Babka wrote:
>> On 5/26/21 1:48 PM, Greg KH wrote:
>>> On Wed, May 26, 2021 at 01:38:55PM +0200, Vlastimil Babka wrote:
>>>>
>>>> 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).
>>>>
>>>> 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?
>>>
>>> It's much harder, but usually the default debugfs_file_create() will
>>> handle this for you.  See the debugfs_file_create_unsafe() for the
>>> "other" variant where you know you can tear things down "safely".
>> 
>> Right, so IIUC debugfs will guarantee that while somebody reads the files, the
>> debugfs cleanup will block, as debugfs_file_get() comment explains.
>> 
>> In that case I think we have the cleanup order wrong in this patch:
>> 
>> shutdown_cache() should first do debugfs_slab_release() (which would block) and
>> only then proceed with slab_kmem_cache_release() which destroys the fundamental
>> structures such as kmem_cache_node, which are also accessed by the debugfs file
>> handlers.
>>
> If user is trying to read the data during shutdown_cache(), then I think
> it's possible user will get empty data, to avoid that we can call

Empty data is fine, when the cache is going away anyway.

> debugfs_slab_release() first and then do other stuff in shutdown_cache().

Everything above list_del(&s->list) should be OK, it's equivalent to normal
cache operations which the debugfs files must cope with anyway.
list_del(&s->list) is OK as the debugfs handlers don't go through the list. It's
slab_kmem_cache_release() that matters.

>>> That being said, yes there are still issues in this area, be careful
>>> about what tools you expect to be constantly hitting debugfs files.
>> 
>> FWIW, the files are accessible only to root.
>> 
>>> thanks,
>>>
>>> greg k-h
>>>
>> 
>