Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free_traces attribute
From: Vlastimil Babka <hidden>
Date: 2021-11-19 10:41:42
Also in:
linux-s390, lkml
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;
quoted hunk ↗ jump to hunk
Fix it by adding such a check and return NULL. Fixes: 64dd68497be7 ("mm: slub: move sysfs slab alloc/free interfaces to debugfs") Cc: <redacted> # v5.14+ Reported-by: Steffen Maier <redacted> 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; }