Re: [RFC PATCH 0/1] mm/slub: endless "No data" printing for alloc/free_traces attribute
From: Vlastimil Babka <hidden>
Date: 2021-11-19 09:43:14
Also in:
linux-s390, lkml
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.
quoted hunk ↗ jump to hunk
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(+)