Re: [PATCH] mm: cma: print allocation failure reason and bitmap status

[Michal Nazarewicz <mina86@mina86.com>]

On Thu, Dec 29 2016, Michal Hocko wrote:
> On Thu 29-12-16 11:28:02, Jaewon Kim wrote:
>> There are many reasons of CMA allocation failure such as EBUSY, ENOMEM, EINTR.
>> This patch prints the error value and bitmap status to know available pages
>> regarding fragmentation.
>> 
>> This is an ENOMEM example with this patch.
>> [   11.616321]  [2:   Binder:711_1:  740] cma: cma_alloc: alloc failed, req-size: 256 pages, ret: -12
>
>> [   11.616365]  [2:   Binder:711_1:  740] number of available pages: 4+7+7+8+38+166+127=>357 pages, total: 2048 pages
>
> Could you be more specific why this part is useful?
>  
>> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>
>> ---
>>  mm/cma.c | 29 ++++++++++++++++++++++++++++-
>>  1 file changed, 28 insertions(+), 1 deletion(-)
>> 
>> diff --git a/mm/cma.c b/mm/cma.c
>> index c960459..535aa39 100644
>> --- a/mm/cma.c
>> +++ b/mm/cma.c
>> @@ -369,7 +369,7 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align)
>>  	unsigned long start = 0;
>>  	unsigned long bitmap_maxno, bitmap_no, bitmap_count;
>>  	struct page *page = NULL;
>> -	int ret;
>> +	int ret = -ENOMEM;
>>  
>>  	if (!cma || !cma->count)
>>  		return NULL;
>> @@ -427,6 +427,33 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align)
>>  	trace_cma_alloc(pfn, page, count, align);
>>  
>>  	pr_debug("%s(): returned %p\n", __func__, page);
>> +
>> +	if (ret != 0) {
>> +		unsigned int nr, nr_total = 0;
>> +		unsigned long next_set_bit;
>> +
>> +		pr_info("%s: alloc failed, req-size: %zu pages, ret: %d\n",
>> +			__func__, count, ret);
>> +		mutex_lock(&cma->lock);
>> +		printk("number of available pages: ");
>> +		start = 0;
>> +		for (;;) {
>> +			bitmap_no = find_next_zero_bit(cma->bitmap, cma->count, start);
>> +			next_set_bit = find_next_bit(cma->bitmap, cma->count, bitmap_no);
>> +			nr = next_set_bit - bitmap_no;
>> +			if (bitmap_no >= cma->count)
>> +				break;

Put this just next to ‘bitmap_no = …’ line.  No need to call
find_next_bit if we’re gonna break anyway.

>> +			if (nr_total == 0)
>> +				printk("%u", nr);
>> +			else
>> +				printk("+%u", nr);

Perhaps also include location of the hole?  Something like:

		pr_cont("%s%u@%u", nr_total ? "+" : "", nr, bitmap_no);

>> +			nr_total += nr;
>> +			start = bitmap_no + nr;
>> +		}
>> +		printk("=>%u pages, total: %lu pages\n", nr_total, cma->count);
>> +		mutex_unlock(&cma->lock);
>> +	}
>> +

I wonder if this should be wrapped in

#ifdef CMA_DEBUG
…
#endif

On one hand it’s relatively expensive (even involving mutex locking) on
the other it’s in allocation failure path.

>>  	return page;
>>  }
>>  
>> -- 
>> 1.9.1
>> 
>> --
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>
> -- 
> Michal Hocko
> SUSE Labs

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»