Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free_traces attribute

[Vlastimil Babka <vbabka@suse.cz>]

On 11/19/21 20:59, Gerald Schaefer wrote:
> On Fri, 19 Nov 2021 11:41:38 +0100
> Vlastimil Babka <vbabka@suse.cz> wrote:
> 
>> On 11/17/21 20:39, Gerald Schaefer wrote:
>> > 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.

>> 
>> 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
> 
>> 
>> // 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.

>> 
>> 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.