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

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Message-ID: <2fc580b5-13a5-1a54-fe0b-9843ec685e64@codeaurora.org>
Date:   Mon, 31 May 2021 15:20:40 +0530
From:   Faiyaz Mohammed <faiyazm@...eaurora.org>
To:     Vlastimil Babka <vbabka@...e.cz>, 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 5/31/2021 1:49 PM, Vlastimil Babka wrote:
> 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.
> 
okay, I will move debugfs_slab_release() before the
slab_kmem_cache_release() in next patch version.
>>>> 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
>>>>
>>>
>>
>