| Message ID | 20211117193932.4049412-1-gerald.schaefer@linux.ibm.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
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. > 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(+) >
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; }
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? 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(+)