linux-kernel - Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free

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Date:   Fri, 19 Nov 2021 11:41:38 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Faiyaz Mohammed <faiyazm@...eaurora.org>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for
 alloc/free_traces attribute

On 11/17/21 20:39, Gerald Schaefer wrote:
> Reading from alloc/free_traces attribute in /sys/kernel/debug/slab/ results
> in an endless sequence of "No data". This is because slab_debugfs_start()
> does not check for a "past end of file" condition and return NULL.

I still have no idea how that endless sequence happens.
To get it, we would have to call slab_debugfs_show() repeatedly with such v
that *v == 0. Which should only happen with slab_debugfs_start() with *ppos
== 0. Which your patch won't change because you add a '*ppos > t->count'
condition, so *ppos has to be at least 1 to trigger this.

But yeah, AFAIK we should detect this in slab_debugfs_start() anyway.
But I think the condition should be something like below, because we are
past end of file already with *ppos == t->count. But if both are 0, we want
to proceed for the "No data" output.

// to show the No data
if (!*ppos && !t->count)
	return ppos;

if (*ppos >= t->count)
	return ppos;

return ppos;

> Fix it by adding such a check and return NULL.
> 
> Fixes: 64dd68497be7 ("mm: slub: move sysfs slab alloc/free interfaces to debugfs")
> Cc: <stable@...r.kernel.org> # v5.14+
> Reported-by: Steffen Maier <maier@...ux.ibm.com>
> Signed-off-by: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
> ---
>  mm/slub.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f7368bfffb7a..336609671bc2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6115,6 +6115,11 @@ static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
>  
>  static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos)
>  {
> +	struct loc_track *t = seq->private;
> +
> +	if (*ppos > t->count)
> +		return NULL;
> +
>  	return ppos;
>  }
>  
>