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Subject: Re: [PATCH v7] mm: slub: move sysfs slab alloc/free interfaces to
 debugfs
To:     Vlastimil Babka <vbabka@suse.cz>, Greg KH <greg@kroah.com>
Cc:     cl@linux.com, penberg@kernel.org, rientjes@google.com,
        iamjoonsoo.kim@lge.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        glittao@gmail.com, vinmenon@codeaurora.org
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From:   Faiyaz Mohammed <faiyazm@codeaurora.org>
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Date:   Mon, 31 May 2021 12:41:36 +0530
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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
debugfs_slab_release() first and then do other stuff in shutdown_cache().

>> 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
>>
>