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Subject: Re: [PATCH v3 1/8] mm/cma: change cma mutex to irq safe spinlock
To:     Mike Kravetz <mike.kravetz@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Cc:     Roman Gushchin <guro@fb.com>, Michal Hocko <mhocko@suse.com>,
        Shakeel Butt <shakeelb@google.com>,
        Oscar Salvador <osalvador@suse.de>,
        Muchun Song <songmuchun@bytedance.com>,
        David Rientjes <rientjes@google.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        HORIGUCHI NAOYA <naoya.horiguchi@nec.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Waiman Long <longman@redhat.com>, Peter Xu <peterx@redhat.com>,
        Mina Almasry <almasrymina@google.com>,
        Hillf Danton <hdanton@sina.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Barry Song <song.bao.hua@hisilicon.com>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20210331034148.112624-1-mike.kravetz@oracle.com>
 <20210331034148.112624-2-mike.kravetz@oracle.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <4433e570-d492-f617-7c0b-51f00557ea71@redhat.com>
Date:   Wed, 31 Mar 2021 09:52:49 +0200
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On 31.03.21 05:41, Mike Kravetz wrote:
> cma_release is currently a sleepable operatation because the bitmap
> manipulation is protected by cma->lock mutex. Hugetlb code which relies
> on cma_release for CMA backed (giga) hugetlb pages, however, needs to be
> irq safe.
> 
> The lock doesn't protect any sleepable operation so it can be changed to
> a (irq aware) spin lock. The bitmap processing should be quite fast in
> typical case but if cma sizes grow to TB then we will likely need to
> replace the lock by a more optimized bitmap implementation.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>   mm/cma.c       | 18 +++++++++---------
>   mm/cma.h       |  2 +-
>   mm/cma_debug.c |  8 ++++----
>   3 files changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/cma.c b/mm/cma.c
> index b2393b892d3b..2380f2571eb5 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -24,7 +24,6 @@
>   #include <linux/memblock.h>
>   #include <linux/err.h>
>   #include <linux/mm.h>
> -#include <linux/mutex.h>
>   #include <linux/sizes.h>
>   #include <linux/slab.h>
>   #include <linux/log2.h>
> @@ -83,13 +82,14 @@ static void cma_clear_bitmap(struct cma *cma, unsigned long pfn,
>   			     unsigned int count)
>   {
>   	unsigned long bitmap_no, bitmap_count;
> +	unsigned long flags;
>   
>   	bitmap_no = (pfn - cma->base_pfn) >> cma->order_per_bit;
>   	bitmap_count = cma_bitmap_pages_to_bits(cma, count);
>   
> -	mutex_lock(&cma->lock);
> +	spin_lock_irqsave(&cma->lock, flags);
>   	bitmap_clear(cma->bitmap, bitmap_no, bitmap_count);
> -	mutex_unlock(&cma->lock);
> +	spin_unlock_irqrestore(&cma->lock, flags);
>   }
>   
>   static void __init cma_activate_area(struct cma *cma)
> @@ -118,7 +118,7 @@ static void __init cma_activate_area(struct cma *cma)
>   	     pfn += pageblock_nr_pages)
>   		init_cma_reserved_pageblock(pfn_to_page(pfn));
>   
> -	mutex_init(&cma->lock);
> +	spin_lock_init(&cma->lock);
>   
>   #ifdef CONFIG_CMA_DEBUGFS
>   	INIT_HLIST_HEAD(&cma->mem_head);
> @@ -392,7 +392,7 @@ static void cma_debug_show_areas(struct cma *cma)
>   	unsigned long nr_part, nr_total = 0;
>   	unsigned long nbits = cma_bitmap_maxno(cma);
>   
> -	mutex_lock(&cma->lock);
> +	spin_lock_irq(&cma->lock);
>   	pr_info("number of available pages: ");
>   	for (;;) {
>   		next_zero_bit = find_next_zero_bit(cma->bitmap, nbits, start);
> @@ -407,7 +407,7 @@ static void cma_debug_show_areas(struct cma *cma)
>   		start = next_zero_bit + nr_zero;
>   	}
>   	pr_cont("=> %lu free of %lu total pages\n", nr_total, cma->count);
> -	mutex_unlock(&cma->lock);
> +	spin_unlock_irq(&cma->lock);
>   }
>   #else
>   static inline void cma_debug_show_areas(struct cma *cma) { }
> @@ -454,12 +454,12 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>   		goto out;
>   
>   	for (;;) {
> -		mutex_lock(&cma->lock);
> +		spin_lock_irq(&cma->lock);
>   		bitmap_no = bitmap_find_next_zero_area_off(cma->bitmap,
>   				bitmap_maxno, start, bitmap_count, mask,
>   				offset);
>   		if (bitmap_no >= bitmap_maxno) {
> -			mutex_unlock(&cma->lock);
> +			spin_unlock_irq(&cma->lock);
>   			break;
>   		}
>   		bitmap_set(cma->bitmap, bitmap_no, bitmap_count);
> @@ -468,7 +468,7 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>   		 * our exclusive use. If the migration fails we will take the
>   		 * lock again and unmark it.
>   		 */
> -		mutex_unlock(&cma->lock);
> +		spin_unlock_irq(&cma->lock);
>   
>   		pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
>   		ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA,
> diff --git a/mm/cma.h b/mm/cma.h
> index 68ffad4e430d..2c775877eae2 100644
> --- a/mm/cma.h
> +++ b/mm/cma.h
> @@ -15,7 +15,7 @@ struct cma {
>   	unsigned long   count;
>   	unsigned long   *bitmap;
>   	unsigned int order_per_bit; /* Order of pages represented by one bit */
> -	struct mutex    lock;
> +	spinlock_t	lock;
>   #ifdef CONFIG_CMA_DEBUGFS
>   	struct hlist_head mem_head;
>   	spinlock_t mem_head_lock;
> diff --git a/mm/cma_debug.c b/mm/cma_debug.c
> index d5bf8aa34fdc..2e7704955f4f 100644
> --- a/mm/cma_debug.c
> +++ b/mm/cma_debug.c
> @@ -36,10 +36,10 @@ static int cma_used_get(void *data, u64 *val)
>   	struct cma *cma = data;
>   	unsigned long used;
>   
> -	mutex_lock(&cma->lock);
> +	spin_lock_irq(&cma->lock);
>   	/* pages counter is smaller than sizeof(int) */
>   	used = bitmap_weight(cma->bitmap, (int)cma_bitmap_maxno(cma));
> -	mutex_unlock(&cma->lock);
> +	spin_unlock_irq(&cma->lock);
>   	*val = (u64)used << cma->order_per_bit;
>   
>   	return 0;
> @@ -53,7 +53,7 @@ static int cma_maxchunk_get(void *data, u64 *val)
>   	unsigned long start, end = 0;
>   	unsigned long bitmap_maxno = cma_bitmap_maxno(cma);
>   
> -	mutex_lock(&cma->lock);
> +	spin_lock_irq(&cma->lock);
>   	for (;;) {
>   		start = find_next_zero_bit(cma->bitmap, bitmap_maxno, end);
>   		if (start >= bitmap_maxno)
> @@ -61,7 +61,7 @@ static int cma_maxchunk_get(void *data, u64 *val)
>   		end = find_next_bit(cma->bitmap, bitmap_maxno, start);
>   		maxchunk = max(end - start, maxchunk);
>   	}
> -	mutex_unlock(&cma->lock);
> +	spin_unlock_irq(&cma->lock);
>   	*val = (u64)maxchunk << cma->order_per_bit;
>   
>   	return 0;
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb