Messages in this thread Patch in this message | | | Date | Fri, 19 Nov 2021 11:41:38 +0100 | From | Vlastimil Babka <> | Subject | Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free_traces attribute |
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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; > } > >
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