linux-kernel - Re: Suspicious error for CMA stress test

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Date:	Fri, 18 Mar 2016 00:31:14 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Hanjun Guo <guohanjun@...wei.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>,
	Laura Abbott <labbott@...hat.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	qiuxishi <qiuxishi@...wei.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>,
	dingtinahong <dingtianhong@...wei.com>, chenjie6@...wei.com,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: Suspicious error for CMA stress test

2016-03-17 18:24 GMT+09:00 Hanjun Guo <guohanjun@...wei.com>:
> On 2016/3/17 14:54, Joonsoo Kim wrote:
>> On Wed, Mar 16, 2016 at 05:44:28PM +0800, Hanjun Guo wrote:
>>> On 2016/3/14 15:18, Joonsoo Kim wrote:
>>>> On Mon, Mar 14, 2016 at 08:06:16AM +0100, Vlastimil Babka wrote:
>>>>> On 03/14/2016 07:49 AM, Joonsoo Kim wrote:
>>>>>> On Fri, Mar 11, 2016 at 06:07:40PM +0100, Vlastimil Babka wrote:
>>>>>>> On 03/11/2016 04:00 PM, Joonsoo Kim wrote:
>>>>>>>
>>>>>>> How about something like this? Just and idea, probably buggy (off-by-one etc.).
>>>>>>> Should keep away cost from <pageblock_order iterations at the expense of the
>>>>>>> relatively fewer >pageblock_order iterations.
>>>>>> Hmm... I tested this and found that it's code size is a little bit
>>>>>> larger than mine. I'm not sure why this happens exactly but I guess it would be
>>>>>> related to compiler optimization. In this case, I'm in favor of my
>>>>>> implementation because it looks like well abstraction. It adds one
>>>>>> unlikely branch to the merge loop but compiler would optimize it to
>>>>>> check it once.
>>>>> I would be surprised if compiler optimized that to check it once, as
>>>>> order increases with each loop iteration. But maybe it's smart
>>>>> enough to do something like I did by hand? Guess I'll check the
>>>>> disassembly.
>>>> Okay. I used following slightly optimized version and I need to
>>>> add 'max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1)'
>>>> to yours. Please consider it, too.
>>> Hmm, this one is not work, I still can see the bug is there after applying
>>> this patch, did I miss something?
>> I may find that there is a bug which was introduced by me some time
>> ago. Could you test following change in __free_one_page() on top of
>> Vlastimil's patch?
>>
>> -page_idx = pfn & ((1 << max_order) - 1);
>> +page_idx = pfn & ((1 << MAX_ORDER) - 1);
>
> I tested Vlastimil's patch + your change with stress for more than half hour, the bug
> I reported is gone :)

Good to hear!

> I have some questions, Joonsoo, you provided a patch as following:
>
> diff --git a/mm/cma.c b/mm/cma.c
> index 3a7a67b..952a8a3 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -448,7 +448,10 @@ bool cma_release(struct cma *cma, const struct page *pages, unsigned int count)
>
>         VM_BUG_ON(pfn + count > cma->base_pfn + cma->count);
>
> + mutex_lock(&cma_mutex);
>         free_contig_range(pfn, count);
> + mutex_unlock(&cma_mutex);
> +
>         cma_clear_bitmap(cma, pfn, count);
>         trace_cma_release(pfn, pages, count);
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7f32950..68ed5ae 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1559,7 +1559,8 @@ void free_hot_cold_page(struct page *page, bool cold)
>          * excessively into the page allocator
>          */
>         if (migratetype >= MIGRATE_PCPTYPES) {
> -           if (unlikely(is_migrate_isolate(migratetype))) {
> +         if (is_migrate_cma(migratetype) ||
> +             unlikely(is_migrate_isolate(migratetype))) {
>                         free_one_page(zone, page, pfn, 0, migratetype);
>                         goto out;
>                 }
>
> This patch also works to fix the bug, why not just use this one? is there
> any side effects for this patch? maybe there is performance issue as the
> mutex lock is used, any other issues?

The changes in free_hot_cold_page() would cause unacceptable performance
problem in a big machine, because, with above change,  it takes zone->lock
whenever freeing one page on CMA region.

Thanks.