linux-kernel - Re: [PATCH] cma: allow concurrent cma pages allocation for multi-cma areas

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Date:	Sat, 6 Jun 2015 02:19:17 +0900
From:	Michał Nazarewicz <mina86@...a86.com>
To:	Laura Abbott <labbott@...hat.com>
Cc:	Weijie Yang <weijie.yang@...sung.com>, iamjoonsoo.kim@....com,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Weijie Yang <weijie.yang.kh@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cma: allow concurrent cma pages allocation for multi-cma areas

On Fri, Jun 05 2015, Laura Abbott wrote:
> On 06/05/2015 01:01 AM, Weijie Yang wrote:
>> Currently we have to hold the single cma_mutex when alloc cma pages,
>> it is ok when there is only one cma area in system.
>> However, when there are several cma areas, such as in our Android smart
>> phone, the single cma_mutex prevents concurrent cma page allocation.
>>
>> This patch removes the single cma_mutex and uses per-cma area alloc_lock,
>> this allows concurrent cma pages allocation for different cma areas while
>> protects access to the same pageblocks.
>>
>> Signed-off-by: Weijie Yang <weijie.yang@...sung.com>

> Last I knew alloc_contig_range needed to be serialized which is why we
> still had the global CMA mutex. https://lkml.org/lkml/2014/2/18/462
>
> So NAK unless something has changed to allow this.

This patch should be fine.

Change you’ve pointed to would get rid of any serialisation around
alloc_contig_range which is dangerous, but since CMA regions are
pageblock-aligned:

    /*
     * Sanitise input arguments.
     * Pages both ends in CMA area could be merged into adjacent unmovable
     * migratetype page by page allocator's buddy algorithm. In the case,
     * you couldn't get a contiguous memory, which is not what we want.
     */
    alignment = max(alignment,
        (phys_addr_t)PAGE_SIZE << max(MAX_ORDER - 1, pageblock_order));
    base = ALIGN(base, alignment);
    size = ALIGN(size, alignment);
    limit &= ~(alignment - 1);

synchronising allocation in each area should work fine.

>> ---
>>   mm/cma.c |    6 +++---
>>   mm/cma.h |    1 +
>>   2 files changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/cma.c b/mm/cma.c
>> index 3a7a67b..eaf1afe 100644
>> --- a/mm/cma.c
>> +++ b/mm/cma.c
>> @@ -41,7 +41,6 @@
>>
>>   struct cma cma_areas[MAX_CMA_AREAS];
>>   unsigned cma_area_count;
>> -static DEFINE_MUTEX(cma_mutex);
>>
>>   phys_addr_t cma_get_base(const struct cma *cma)
>>   {
>> @@ -128,6 +127,7 @@ static int __init cma_activate_area(struct cma *cma)
>>       } while (--i);
>>
>>       mutex_init(&cma->lock);

Since now we have two mutexes in the structure, rename this one to
bitmap_lock.

>> +    mutex_init(&cma->alloc_lock);
>>
>>   #ifdef CONFIG_CMA_DEBUGFS
>>       INIT_HLIST_HEAD(&cma->mem_head);
>> @@ -398,9 +398,9 @@ struct page *cma_alloc(struct cma *cma, unsigned int count, unsigned int align)
>>           mutex_unlock(&cma->lock);
>>
>>           pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
>> -        mutex_lock(&cma_mutex);
>> +        mutex_lock(&cma->alloc_lock);
>>           ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA);
>> -        mutex_unlock(&cma_mutex);
>> +        mutex_unlock(&cma->alloc_lock);
>>           if (ret == 0) {
>>               page = pfn_to_page(pfn);
>>               break;
>> diff --git a/mm/cma.h b/mm/cma.h
>> index 1132d73..2084c9f 100644
>> --- a/mm/cma.h
>> +++ b/mm/cma.h
>> @@ -7,6 +7,7 @@ struct cma {
>>       unsigned long   *bitmap;
>>       unsigned int order_per_bit; /* Order of pages represented by one bit */
>>       struct mutex    lock;
>> +    struct mutex    alloc_lock;
>>   #ifdef CONFIG_CMA_DEBUGFS
>>       struct hlist_head mem_head;
>>       spinlock_t mem_head_lock;

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +--<mpn@...gle.com>--<xmpp:mina86@...ber.org>--ooO--(_)--Ooo--
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