RFC mm/slub: endless "No data" printing for alloc/free_traces attribute

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

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?

index f7368bfffb7a..0b1832b16f5b 100644
@@ -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;
 }

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(+)

2.25.1