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From: Vlastimil Babka <vbabka@suse.cz>
To: Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Faiyaz Mohammed <faiyazm@codeaurora.org>
Cc: linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-s390 <linux-s390@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free_traces attribute
Date: Fri, 19 Nov 2021 11:41:38 +0100	[thread overview]
Message-ID: <9a4367c0-8141-f03c-e5a1-13483794d3e8@suse.cz> (raw)
In-Reply-To: <20211117193932.4049412-2-gerald.schaefer@linux.ibm.com>

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@vger.kernel.org> # v5.14+
> Reported-by: Steffen Maier <maier@linux.ibm.com>
> Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.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;
>  }
>  
> 


  reply	other threads:[~2021-11-19 10:41 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-17 19:39 [RFC PATCH 0/1] mm/slub: " Gerald Schaefer
2021-11-17 19:39 ` [RFC PATCH 1/1] mm/slub: fix " Gerald Schaefer
2021-11-19 10:41   ` Vlastimil Babka [this message]
2021-11-19 19:59     ` Gerald Schaefer
2021-11-22 15:02       ` Vlastimil Babka
2021-11-22 20:14         ` Gerald Schaefer
2021-11-22 20:33           ` Gerald Schaefer
2021-11-23 14:19             ` Vlastimil Babka
2021-11-25 16:13               ` Gerald Schaefer
2021-11-25 20:12                 ` Gerald Schaefer
2021-11-25 22:00                   ` Vlastimil Babka
2021-11-26 17:11                     ` Gerald Schaefer
2021-11-26 17:18                     ` [PATCH] mm/slub: fix endianness bug for alloc/free_traces attributes Gerald Schaefer
2021-11-29 14:26                       ` Vlastimil Babka
2021-11-19  9:43 ` [RFC PATCH 0/1] mm/slub: endless "No data" printing for alloc/free_traces attribute Vlastimil Babka

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