Messages in this thread Patch in this message | | | From | Gerald Schaefer <> | Subject | [RFC PATCH 0/1] mm/slub: endless "No data" printing for alloc/free_traces attribute | Date | Wed, 17 Nov 2021 20:39:31 +0100 |
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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?
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; } 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
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