Re: [RFC PATCH 0/1] mm/slub: endless "No data" printing for alloc/free_traces attribute

[Vlastimil Babka]

On 11/17/21 20:39, Gerald Schaefer wrote:
> Steffen reported endless printing of "No data" when reading from
> alloc/free_traces attribute in /sys/kernel/debug/slab/, at least on
> s390, but I don't see how this could be arch-specific.
> 
> I must admit that I am completely confused by the seq_file interface

Me too, everytime I have to deal with it I relearn it.

> in general, but even more so from this specific implementation.

Right.

> First I suspected a bug in slab_debugfs_next(), because of its very
> weird usage of the *v parameter, which seems totally useless, but not
> responsible for this bug. Still, out of curiosity, does anybody have a
> clue why it is done this way, and not e.g. like this?
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f7368bfffb7a..0b1832b16f5b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6105,10 +6105,9 @@ static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
>  {
>         struct loc_track *t = seq->private;
> 
> -       v = ppos;
>         ++*ppos;
>         if (*ppos <= t->count)
> -               return v;
> +               return ppos;
> 
>         return NULL;
>  }

Yeah that should work and make it a bit less confusing.

> 
> Anyway, NULL is returned for the "*ppos > t->count" case, as described
> in Documentation/filesystems/seq_file.rst, for "if the sequence is
> complete", so that should be ok, but still very confusing. Unfortunately,
> the documentation does not seem to explain that *v parameter at all, or
> I missed it, but in this case here it seems to be simply ignored and
> misused w/o any need.
> 
> The documentation does mention that "in most cases the start() function
> should check for a "past end of file" condition and return NULL if need
> be". So I just added a similar check to slab_debugfs_start() and return
> NULL for "*ppos > t->count", which apparently fixed the bug. "No data"
> is now only printed once, like it was before commit 64dd68497be7
> ("mm: slub: move sysfs slab alloc/free interfaces to debugfs").
> 
> However, since I obviously do not really understand the seq_file
> documentation, and also not the alloc/free_traces stuff in general,
> that fix might be wrong or incomplete. Comments are welcome.
> 
> Gerald Schaefer (1):
>   mm/slub: fix endless "No data" printing for alloc/free_traces
>     attribute
> 
>  mm/slub.c | 5 +++++
>  1 file changed, 5 insertions(+)
>