linux-kernel - Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free

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Date:   Thu, 25 Nov 2021 23:00:47 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     Faiyaz Mohammed <faiyazm@...eaurora.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for
 alloc/free_traces attribute

On 11/25/21 21:12, Gerald Schaefer wrote:
> On Thu, 25 Nov 2021 17:13:10 +0100
> Gerald Schaefer <gerald.schaefer@...ux.ibm.com> wrote:
> 
>> On Tue, 23 Nov 2021 15:19:49 +0100
>> Vlastimil Babka <vbabka@...e.cz> wrote:
>>
>>> On 11/22/21 21:33, Gerald Schaefer wrote:
>>>> On Mon, 22 Nov 2021 21:14:00 +0100
>>>> Gerald Schaefer <gerald.schaefer@...ux.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?
>>
>> Yes, following the example sounds good, and it would also make proper use
>> of *v in op->next, which might make the code more readable. It also looks
>> like it already does exactly what is needed here, i.e. have a simple
>> iterator that just counts the lines.
>>
>> I don't think the iterator needs to be saved in loc_track. IIUC, it is
>> already passed around like in the example, and can then be simply compared
>> to t->count, similar to the existing code.

Saving it the loc_track doesn't preclude using the pointer that's being
passed around. It however avoids the extra kmalloc and turns out it
should also solve the return NULL from op->next() issue you describe below?

>> This is what I'm currently testing, and it seems to work fine. Will send
>> a new patch, if there are no objections:
> 
> Oh well, I have one objection, returning NULL from op->next will be
> passed to op->stop, and then it will not free the allocated value.
> 
> The example is elegantly avoiding this, by not returning NULL anywhere,
> and also not stopping. Sigh.
> 
> Maybe not return NULL in op->next, but only from op->start, and only
> when no allocation was made or it was freed already? Or free it only/
> already in op->next, when returning NULL?

>From these two probably the "free in op->next". But still inclined to
store it in loc_track.
Why does the API need to be so awkward...