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

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Date:   Wed, 26 May 2021 14:13:25 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Greg KH <greg@...ah.com>
Cc:     Faiyaz Mohammed <faiyazm@...eaurora.org>, 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 5/26/21 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.

> 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
>