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SubjectRe: mm/percpu.c: use smarter memory allocation for struct pcpu_alloc_info (crisv32 hang)
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On 11/19/2017 12:36 PM, Nicolas Pitre wrote:
> On Sat, 18 Nov 2017, Guenter Roeck wrote:
>
>> Hi,
>>
>> On Tue, Oct 03, 2017 at 06:29:49PM -0400, Nicolas Pitre wrote:
>>> On Tue, 3 Oct 2017, Tejun Heo wrote:
>>>
>>>> On Tue, Oct 03, 2017 at 04:57:44PM -0400, Nicolas Pitre wrote:
>>>>> This can be much smaller than a page on very small memory systems.
>>>>> Always rounding up the size to a page is wasteful in that case, and
>>>>> required alignment is smaller than the memblock default. Let's round
>>>>> things up to a page size only when the actual size is >= page size, and
>>>>> then it makes sense to page-align for a nicer allocation pattern.
>>>>
>>>> Isn't that a temporary area which gets freed later during boot?
>>>
>>> Hmmm...
>>>
>>> It may get freed through 3 different paths where 2 of them are error
>>> paths. What looks like a non-error path is in pcpu_embed_first_chunk()
>>> called from setup_per_cpu_areas(). But there are two versions of
>>> setup_per_cpu_areas(): one for SMP and one for !SMP. And the !SMP case
>>> never calls pcpu_free_alloc_info() currently.
>>>
>>> I'm not sure i understand that code fully, but maybe the following patch
>>> could be a better fit:
>>>
>>> ----- >8
>>> Subject: [PATCH] percpu: don't forget to free the temporary struct pcpu_alloc_info
>>>
>>> Unlike the SMP case, the !SMP case does not free the memory for struct
>>> pcpu_alloc_info allocated in setup_per_cpu_areas(). And to give it a
>>> chance of being reused by the page allocator later, align it to a page
>>> boundary just like its size.
>>>
>>> Signed-off-by: Nicolas Pitre <nico@linaro.org>
>>
>> This patch causes my crisv32 qemu emulation to hang with no console output.
>>
>>>
>>> diff --git a/mm/percpu.c b/mm/percpu.c
>>> index 434844415d..caab63375b 100644
>>> --- a/mm/percpu.c
>>> +++ b/mm/percpu.c
>>> @@ -1416,7 +1416,7 @@ struct pcpu_alloc_info * __init pcpu_alloc_alloc_info(int nr_groups,
>>> __alignof__(ai->groups[0].cpu_map[0]));
>>> ai_size = base_size + nr_units * sizeof(ai->groups[0].cpu_map[0]);
>>>
>>> - ptr = memblock_virt_alloc_nopanic(PFN_ALIGN(ai_size), 0);
>>> + ptr = memblock_virt_alloc_nopanic(PFN_ALIGN(ai_size), PAGE_SIZE);
>>> if (!ptr)
>>> return NULL;
>>> ai = ptr;
>>> @@ -2295,6 +2295,7 @@ void __init setup_per_cpu_areas(void)
>>>
>>> if (pcpu_setup_first_chunk(ai, fc) < 0)
>>> panic("Failed to initialize percpu areas.");
>>> + pcpu_free_alloc_info(ai);
>>
>> This is the culprit. Everything works fine if I remove this line.
>
> Without this line, the memory at the ai pointer is leaked. Maybe this is
> modifying the memory allocation pattern and that triggers a bug later on
> in your case.
>
> At that point the console driver is not yet initialized and any error
> message won't be printed. You should enable the early console mechanism
> in your kernel (see arch/cris/arch-v32/kernel/debugport.c) and see what
> that might tell you.
>

The problem is that BUG() on crisv32 does not yield useful output.
Anyway, here is the culprit.

diff --git a/mm/bootmem.c b/mm/bootmem.c
index 6aef64254203..2bcc8901450c 100644
--- a/mm/bootmem.c
+++ b/mm/bootmem.c
@@ -382,7 +382,8 @@ static int __init mark_bootmem(unsigned long start, unsigned long end,
return 0;
pos = bdata->node_low_pfn;
}
- BUG();
+ WARN(1, "mark_bootmem(): memory range 0x%lx-0x%lx not found\n", start, end);
+ return -ENOMEM;
}

/**
diff --git a/mm/percpu.c b/mm/percpu.c
index 79e3549cab0f..c75622d844f1 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1881,6 +1881,7 @@ struct pcpu_alloc_info * __init pcpu_alloc_alloc_info(int nr_groups,
*/
void __init pcpu_free_alloc_info(struct pcpu_alloc_info *ai)
{
+ printk("pcpu_free_alloc_info(%p (0x%lx))\n", ai, __pa(ai));
memblock_free_early(__pa(ai), ai->__ai_size);
}

results in:
pcpu_free_alloc_info(c0534000 (0x40534000))
------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at mm/bootmem.c:385 mark_bootmem+0x9a/0xaa
mark_bootmem(): memory range 0x2029a-0x2029b not found
and the system keeps booting.

If I drop the __pa() from the memblock_free_early() parameter, everything works
as expected. Off to the cris maintainers ... I have no idea how to fix the problem.

Guenter

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