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

[Gerald Schaefer]

On Fri, 19 Nov 2021 11:41:38 +0100
Vlastimil Babka <vbabka@xxxxxxx> 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. 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.

> 
> 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?

> 
> 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"?