Re: [PATCH v3] mm/slub: fix missing debugfs entries for caches created before sysfs init

[Harry Yoo <harry@kernel.org>]

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On 7/29/26 7:18 PM, Li Xiasong wrote:
> slab_debugfs_init() creates the slab debugfs root at device initcall
> time, while slab_sysfs_init() moves slab_state to FULL at late initcall
> time. SLAB_STORE_USER caches created in this window miss their debugfs
> entries because do_kmem_cache_create() skips debugfs_slab_add() when
> slab_state <= UP. This was observed with MPTCP's request_sock_subflow_v6
> cache, whose slab debugfs directory was missing.

Ouch, good catch!

> The affected window is:
> 
>   slab_debugfs_init()
>     slab_debugfs_root = debugfs_create_dir(...)
>     list_for_each_entry(s, &slab_caches, list)
>       debugfs_slab_add(s)
> 
>   kmem_cache_create(..., SLAB_STORE_USER, ...)
>     do_kmem_cache_create()
>       if (slab_state <= UP)
>         return without debugfs entries
> 
>   slab_sysfs_init()
>     slab_state = FULL
> 
> Initialize the debugfs root and add debugfs entries while holding
> slab_mutex, walking slab_caches exactly once and handling both sysfs
> and debugfs entries in the same pass. This gives the sysfs and debugfs
> initialization an explicit order and prevents caches from being
> created between the debugfs scan and slab_state reaching FULL.
> 
> Gate the new slab_late_init() on either sysfs or debugfs being enabled,
> with the slab_kset creation and alias_list processing factored into
> helpers that have empty no-sysfs variants, as suggested by Vlastimil
> Babka. On slab_kset_init() failure, slab_state stays below FULL so
> kmem_cache_create() keeps taking the early-boot path, matching prior
> behavior.
> 
> Guard debugfs_slab_release() against an uninitialized debugfs root,
> since the root is now created later and a cache may be released before
> it exists.
> 
> Fixes: 1a5ad30b89b4 ("mm: slub: make slab_sysfs_init() a late_initcall")
> Cc: stable@vger.kernel.org
> Suggested-by: Vlastimil Babka <vbabka@kernel.org>
> Signed-off-by: Li Xiasong <lixiasong1@huawei.com>
> ---

Looks good to me,
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>

-- 
Cheers,
Harry / Hyeonggon

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