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

[Vlastimil Babka <vbabka@suse.cz>]

On 11/22/21 21:33, Gerald Schaefer wrote:
> On Mon, 22 Nov 2021 21:14:00 +0100
> Gerald Schaefer <gerald.schaefer@linux.ibm.com> wrote:
> 
> [...]
>> 
>> Thanks. While testing this properly, yet another bug showed up. The idx
>> in op->show remains 0 in all iterations, so I always see the same line
>> printed t->count times (or infinitely, ATM). Not sure if this only shows
>> on s390 due to endianness, but the reason is this:
>> 
>>   unsigned int idx = *(unsigned int *)v;

Uh, good catch. I was actually looking suspiciously at how we cast signed to
unsigned, but didn't occur to me that shortening together with endiannes is
the problem.

>> 
>> IIUC, void *v is always the same as loff_t *ppos, and therefore idx also
>> should be *ppos. De-referencing the loff_t * with an unsigned int * only
>> gives the upper 32 bit half of the 64 bit value, which remains 0.
>> 
>> This would be fixed e.g. with
>> 
>>   unsigned int idx = (unsigned int) *(loff_t *) v;

With all this experience I'm now inclined to rather follow more the example
in Documentation/filesystems/seq_file.rst and don't pass around the pointer
that we got as ppos in slab_debugfs_start(), and that seq_file.c points to
m->index.

In that example an own value is kmalloced:

loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);

while we could just make this a field of loc_track?


>> With this fixed, my original patch actually also works for t->count > 0,
>> because then op->show would return w/o printing anything when idx reaches
>> t->count. For t->count > 0, it would even work w/o any extra checks in
>> op->start because of that, only "No data" would be printed infinitely.
> 
> Oh, no, that would actually also fix the "No data" part, as op->show
> will then also return w/o printing in the next iteration, so that op->next
> would correctly end it all.
> 
> This could also explain why it might all have worked fine on x86 (haven't
> verified), and really only showed on big-endian s390.
> 
> Hmm, now I'm not so sure anymore if we really want the additional
> checks and return NULL in op->start, just to make it "double safe".

I guess we don't.

>> 
>> It probably still makes sense to make this explicit in op->start, by
>> checking separately for !*ppos && !t->count, and returning NULL for
>> *ppos >= t->count, as you suggested.
>> 
>> I think I will also make idx an unsigned long again, like it was before
>> commit 64dd68497be7, and similar to t->count. Not sure if it needs to
>> be, and with proper casting unsigned int is also possible, but why
>> change it?
>