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

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Date:   Wed, 26 May 2021 13:38:55 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     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, greg@...ah.com, glittao@...il.com
Cc:     vinmenon@...eaurora.org
Subject: Re: [PATCH v7] mm: slub: move sysfs slab alloc/free interfaces to
 debugfs

On 5/25/21 9:38 AM, Faiyaz Mohammed wrote:
> alloc_calls and free_calls implementation in sysfs have two issues,
> one is PAGE_SIZE limitiation of sysfs and other is it does not adhere
> to "one value per file" rule.
> 
> To overcome this issues, move the alloc_calls and free_calls implemeation
> to debugfs.
> 
> Rename the alloc_calls/free_calls to alloc_traces/free_traces,
> to be inline with what it does.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>

These were IIRC bot reports for some bugs in the previous versions, so keeping
the Reported-by: for the whole patch is misleading - these were not reports for
the sysfs issues this patch fixes by moving the files to debugfs.

> Signed-off-by: Faiyaz Mohammed <faiyazm@...eaurora.org>
> ---
> changes in V7:
>         - Drop the older alloc_calls and free_calls interface.
> changes in v6:
>         - https://lore.kernel.org/linux-mm/1621341949-26762-1-git-send-email-faiyazm@codeaurora.org/
> 
> changes in v5:
>         - https://lore.kernel.org/linux-mm/1620296523-21922-1-git-send-email-faiyazm@codeaurora.org/
> 
> changes in v4:
>         - https://lore.kernel.org/linux-mm/1618583239-18124-1-git-send-email-faiyazm@codeaurora.org/
> 
> changes in v3:
>         - https://lore.kernel.org/linux-mm/1617712064-12264-1-git-send-email-faiyazm@codeaurora.org/
> 
> changes in v2:
>         - https://lore.kernel.org/linux-mm/3ac1d3e6-6207-96ad-16a1-0f5139d8b2b5@codeaurora.org/
> 
> changes in v1:
>         - https://lore.kernel.org/linux-mm/1610443287-23933-1-git-send-email-faiyazm@codeaurora.org/
> 
>  include/linux/slub_def.h |   8 ++
>  mm/slab_common.c         |   9 ++
>  mm/slub.c                | 353 ++++++++++++++++++++++++++++++++++-------------
>  3 files changed, 276 insertions(+), 94 deletions(-)

I don't see any of the symlinks under /sys/kernel/debug/slab/, so I think the
aliases handling code is wrong, and I can see at least two reasons why it could be:

> @@ -4525,6 +4535,8 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
>  			s->refcount--;
>  			s = NULL;
>  		}
> +
> +		debugfs_slab_alias(s, name);

Here you might be calling debugfs_slab_alias() with NULL if the
sysfs_slab_alias() above returned true.

>  	}
>  
>  	return s;

...

> +static int __init slab_debugfs_init(void)
> +{
> +	struct kmem_cache *s;
> +
> +	slab_debugfs_root = debugfs_create_dir("slab", NULL);
> +
> +	slab_state = FULL;
> +
> +	list_for_each_entry(s, &slab_caches, list)
> +		debugfs_slab_add(s);
> +
> +	while (alias_list) {
> +		struct saved_alias *al = alias_list;

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?

> +
> +		alias_list = alias_list->next;
> +
> +		debugfs_slab_alias(al->s, al->name);
> +
> +		kfree(al);
> +	}
> +
> +	return 0;
> +
> +}
> +__initcall(slab_debugfs_init);
> +#endif
>  /*
>   * The /proc/slabinfo ABI
>   */
>