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

[Michal Nazarewicz <mina86@mina86.com>]

On Fri, Dec 30 2016, Michal Hocko wrote:
> On Fri 30-12-16 16:24:46, Jaewon Kim wrote:
> [...]
>> >From 7577cc94da3af27907aa6eec590d2ef51e4b9d80 Mon Sep 17 00:00:00 2001
>> From: Jaewon Kim <jaewon31.kim@samsung.com>
>> Date: Thu, 29 Dec 2016 11:00:16 +0900
>> Subject: [PATCH] mm: cma: print allocation failure reason and bitmap status
>> 
>> There are many reasons of CMA allocation failure such as EBUSY, ENOMEM, EINTR.
>> But we did not know error reason so far. This patch prints the error value.
>> 
>> Additionally if CONFIG_CMA_DEBUG is enabled, this patch shows bitmap status to
>> know available pages. Actually CMA internally try all available regions because
>> some regions can be failed because of EBUSY. Bitmap status is useful to know in
>> detail on both ENONEM and EBUSY;
>>  ENOMEM: not tried at all because of no available region
>>          it could be too small total region or could be fragmentation issue
>>  EBUSY:  tried some region but all failed
>> 
>> This is an ENOMEM example with this patch.
>> [   13.250961]  [1:   Binder:715_1:  846] cma: cma_alloc: alloc failed, req-size: 256 pages, ret: -12
>> Avabile pages also will be shown if CONFIG_CMA_DEBUG is enabled
>> [   13.251052]  [1:   Binder:715_1:  846] cma: number of available pages: 4@572+7@585+7@601+8@632+38@730+166@1114+127@1921=>357 pages, total: 2048 pages
>
> please mention how to interpret this information.
>
> some more style suggestions below
>> 
>> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>

Acked-by: Michal Nazarewicz <mina86@mina86.com>

>> ---
>>  mm/cma.c | 29 ++++++++++++++++++++++++++++-
>>  1 file changed, 28 insertions(+), 1 deletion(-)
>> 
>> diff --git a/mm/cma.c b/mm/cma.c
>> index c960459..1bcd9db 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);

This line should be moved after the ‘if (ret != 0)’ block, i.e. just
before return.

>> +
>> +    if (ret != 0)
>
> you can simply do
> 	if (!ret) {
>
> 		pr_info("%s: alloc failed, req-size: %zu pages, ret: %d\n",
> 			__func__, count, ret);
> 		debug_show_cma_areas();
> 	}
>
> 	return page;
>
> static void debug_show_cma_areas(void)
> {
> #ifdef CONFIG_CMA_DEBUG
> 	unsigned int nr, nr_total = 0;
> 	unsigned long next_set_bit;
>
> 	mutex_lock(&cma->lock);
> 	pr_info("number of available pages: ");
> 	start = 0;
> 	for (;;) {
> 		bitmap_no = find_next_zero_bit(cma->bitmap, cma->count, start);
> 		if (bitmap_no >= cma->count)
> 		break;
> 		next_set_bit = find_next_bit(cma->bitmap, cma->count, bitmap_no);
> 		nr = next_set_bit - bitmap_no;
> 		pr_cont("%s%u@%lu", nr_total ? "+" : "", nr, bitmap_no);
> 		nr_total += nr;
> 		start = bitmap_no + nr;
> 	}
> 	pr_cont("=>%u pages, total: %lu pages\n", nr_total, cma->count);

Perhaps:
	pr_cont("=> %u free of %lu total pages\n", nr_total, cma->count);
or shorter (but more cryptic):
	pr_cont("=> %u/%lu pages\n", nr_total, cma->count);

> 	mutex_unlock(&cma->lock);
> #endif
> }

Actually, Linux style is more like:

#ifdef CONFIG_CMA_DEBUG
static void cma_debug_show_areas()
{
	…
}
#else
static inline void cma_debug_show_areas() { }
#endif

>
> -- 
> 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»

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