On 11/19/21 20:59, Gerald Schaefer wrote:

On Fri, 19 Nov 2021 11:41:38 +0100
Vlastimil Babka [off-list ref] wrote:

quoted

On 11/17/21 20:39, Gerald Schaefer wrote:

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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.

Yes, very strange. After a closer look to fs/seq_file.c, especially
seq_read_iter(), it seems that op->next will only be called when m->count == 0,
at least in the first while(1) loop. Printing "No data\n" sets m->count
to 8, so it will continue after Fill:, then call op->next, which returns NULL
and breaks the second while(1) loop, and also calls op->stop. Then it returns
from seq_read_iter(), only to be called again, and again, ...

Only when op->start returns NULL it will end it for good, probably
because seq_read_iter() will then return 0 instead of 8.

Ah, thanks for investigating.

Not sure if
there is a better way to fix this than by adding a second "return NULL"
to op->start, which feels a bit awkward and makes you wonder why the
"return NULL" from op->next is not enough.

I think it's fine to require op->start to return NULL, even if it didn't
cause this infinite loop.

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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.

Ah ok, I wasn't sure about the "t->count > 0" case, i.e. if the check for
"*ppos > t->count" would still be correct there. So apparently it wouldn't,
and we need two checks, like you suggested

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// to show the No data
if (!*ppos && !t->count)
	return ppos;

if (*ppos >= t->count)
	return ppos;

That should be return NULL here, right?

Doh, right.

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return ppos;

Will send a new patch, unless I find a better way after investigating the
endless seq_read_iter() calls mentioned above.
Is there an easy way to test the "t->count > 0" case, i.e. what would need
to be done to get some other reply than "No data"?

Hm the debugfs files alloc_tracess/free_traces for any cache with non-zero
objects (see /proc/slabinfo for that) should have t->count > 0. If the files
are created for a cache, it means the related SLAB_STORE_USER debugging was
enabled both during config and boot-time. If you see only a few caches with
alloc_tracess/free_traces (because they are from e.g. some test module that
adds SLAB_STORE_USER explicitly) and all happen to have 0 objects, boot with
slub_debug=U parameter and then all caches will have this enabled and many
will have >0 objects.