[RFC PATCH] drivers: base: dma-coherent: find free region without alignment

[RFC PATCH] drivers: base: dma-coherent: find free region without alignment

[Jaewon Kim]

dma-coherent uses bitmap API which internally consider align based on the
requested size. Depending on some usage pattern, using align, I think, may
be good for fast search and anti-fragmentation. But with the align, an
allocation may be failed.

This is a example, total size is 30MB, only few memory at front is being
used, and 9MB is being requsted. Then 9MB will be aligned to 16MB. The
first try on offset 0MB will be failed because of others already using. The
second try on offset 16MB will be failed because of ouf of bound.

So if the align is not necessary on dma-coherent, this patch removes the
align policy to allow allocation without increasing the total size.

Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>
---
 drivers/base/dma-coherent.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
index 744f64f43454..b86a96d0cd07 100644
--- a/drivers/base/dma-coherent.c
+++ b/drivers/base/dma-coherent.c
@@ -162,7 +162,7 @@ EXPORT_SYMBOL(dma_mark_declared_memory_occupied);
 static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
 		ssize_t size, dma_addr_t *dma_handle)
 {
-	int order = get_order(size);
+	int nr_page = PAGE_ALIGN(size) >> PAGE_SHIFT;
 	unsigned long flags;
 	int pageno;
 	void *ret;
@@ -172,9 +172,11 @@ static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
 	if (unlikely(size > (mem->size << PAGE_SHIFT)))
 		goto err;
 
-	pageno = bitmap_find_free_region(mem->bitmap, mem->size, order);
-	if (unlikely(pageno < 0))
+	pageno = bitmap_find_next_zero_area(mem->bitmap, mem->size, 0,
+					    nr_page, 0);
+	if (unlikely(pageno >= mem->size)) {
 		goto err;
+	bitmap_set(mem->bitmap, pageno, nr_page);
 
 	/*
 	 * Memory was found in the coherent area.
-- 
2.13.0

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [RFC PATCH] drivers: base: dma-coherent: find free region without alignment

[Marek Szyprowski]

Hi Jaewon,

On 2017-11-14 09:42, Jaewon Kim wrote:
> dma-coherent uses bitmap API which internally consider align based on the
> requested size. Depending on some usage pattern, using align, I think, may
> be good for fast search and anti-fragmentation. But with the align, an
> allocation may be failed.
>
> This is a example, total size is 30MB, only few memory at front is being
> used, and 9MB is being requsted. Then 9MB will be aligned to 16MB. The
> first try on offset 0MB will be failed because of others already using. The
> second try on offset 16MB will be failed because of ouf of bound.
>
> So if the align is not necessary on dma-coherent, this patch removes the
> align policy to allow allocation without increasing the total size.

You are right that keeping strict alignment is waste of memory for large
allocations. However for the smaller ones, typically under 1MiB, it helps
to reduce memory fragmentation. The alignment of the allocated buffers is
de-facto guaranteed by the memory management framework in Linux kernel
and there are drivers that depends on this feature.

Maybe it would make sense to keep alignment for buffers smaller than some
predefined value (like 1MiB), something similar to config
ARM_DMA_IOMMU_ALIGNMENT in arch/arm/Kconfig. Otherwise I would expect that
some drivers will be broken by this patch.

> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>
> ---
>   drivers/base/dma-coherent.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
> index 744f64f43454..b86a96d0cd07 100644
> --- a/drivers/base/dma-coherent.c
> +++ b/drivers/base/dma-coherent.c
> @@ -162,7 +162,7 @@ EXPORT_SYMBOL(dma_mark_declared_memory_occupied);
>   static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
>   		ssize_t size, dma_addr_t *dma_handle)
>   {
> -	int order = get_order(size);
> +	int nr_page = PAGE_ALIGN(size) >> PAGE_SHIFT;
>   	unsigned long flags;
>   	int pageno;
>   	void *ret;
> @@ -172,9 +172,11 @@ static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
>   	if (unlikely(size > (mem->size << PAGE_SHIFT)))
>   		goto err;
>   
> -	pageno = bitmap_find_free_region(mem->bitmap, mem->size, order);
> -	if (unlikely(pageno < 0))
> +	pageno = bitmap_find_next_zero_area(mem->bitmap, mem->size, 0,
> +					    nr_page, 0);
> +	if (unlikely(pageno >= mem->size)) {
>   		goto err;
> +	bitmap_set(mem->bitmap, pageno, nr_page);
>   
>   	/*
>   	 * Memory was found in the coherent area.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [RFC PATCH] drivers: base: dma-coherent: find free region without alignment

[Jaewon Kim]

Hello Marek

On 2017e?? 11i?? 14i? 1/4  20:07, Marek Szyprowski wrote:
> Hi Jaewon,
>
> On 2017-11-14 09:42, Jaewon Kim wrote:
>> dma-coherent uses bitmap API which internally consider align based on the
>> requested size. Depending on some usage pattern, using align, I think, may
>> be good for fast search and anti-fragmentation. But with the align, an
>> allocation may be failed.
>>
>> This is a example, total size is 30MB, only few memory at front is being
>> used, and 9MB is being requsted. Then 9MB will be aligned to 16MB. The
>> first try on offset 0MB will be failed because of others already using. The
>> second try on offset 16MB will be failed because of ouf of bound.
>>
>> So if the align is not necessary on dma-coherent, this patch removes the
>> align policy to allow allocation without increasing the total size.
>
> You are right that keeping strict alignment is waste of memory for large
> allocations. However for the smaller ones, typically under 1MiB, it helps
> to reduce memory fragmentation. The alignment of the allocated buffers is
> de-facto guaranteed by the memory management framework in Linux kernel
> and there are drivers that depends on this feature.
>
> Maybe it would make sense to keep alignment for buffers smaller than some
> predefined value (like 1MiB), something similar to config
> ARM_DMA_IOMMU_ALIGNMENT in arch/arm/Kconfig. Otherwise I would expect that
> some drivers will be broken by this patch.
Thank you for your comment.

I looked ARM_DMA_IOMMU_ALIGNMENT in ARM, it looks similar but it is using
bitmap_find_next_zero_area rather than bitmap_find_free_region. bitmap_find_next_zero_area
apply aligning only onto offset but not onto size. So I think ARM_DMA_IOMMU_ALIGNMENT way
is not perfect on this dma-coherent APIs which tries to align even on size.

Let me say another way where each reserved_mem from device tree can decide if it wants aligning.
This could be implemented like below. I need to change other dma-coherent APIs though.
I will wait for your comment on this.

--- a/drivers/base/dma-coherent.c
+++ b/drivers/base/dma-coherent.c
@@ -17,6 +17,7 @@ struct dma_coherent_mem {
        unsigned long   *bitmap;
        spinlock_t      spinlock;
        bool            use_dev_dma_pfn_offset;
+       bool            no_align;
 };
 
 static struct dma_coherent_mem *dma_coherent_default_memory __ro_after_init;
@@ -162,7 +163,6 @@ EXPORT_SYMBOL(dma_mark_declared_memory_occupied);
 static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
                ssize_t size, dma_addr_t *dma_handle)
 {
-       int order = get_order(size);
        unsigned long flags;
        int pageno;
        void *ret;
@@ -172,9 +172,21 @@ static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
        if (unlikely(size > (mem->size << PAGE_SHIFT)))
                goto err;
 
-       pageno = bitmap_find_free_region(mem->bitmap, mem->size, order);
-       if (unlikely(pageno < 0))
-               goto err;
+       if (mem->no_align) {
+               int nr_page = PAGE_ALIGN(size) >> PAGE_SHIFT;
+
+               pageno = bitmap_find_next_zero_area(mem->bitmap, mem->size, 0,
+                                                   nr_page, 0);
+               if (unlikely(pageno >= mem->size))
+                       goto err;
+               bitmap_set(mem->bitmap, pageno, nr_page);
+       } else {
+               int order = get_order(size);
+
+               pageno = bitmap_find_free_region(mem->bitmap, mem->size, order);
+               if (unlikely(pageno < 0))
+                       goto err;
+       }
 
        /*
         * Memory was found in the coherent area.
@@ -346,6 +358,7 @@ static struct reserved_mem *dma_reserved_default_memory __initdata;
 static int rmem_dma_device_init(struct reserved_mem *rmem, struct device *dev)
 {
        struct dma_coherent_mem *mem = rmem->priv;
+       unsigned long node = rmem->fdt_node;
        int ret;
 
        if (!mem) {
@@ -360,6 +373,8 @@ static int rmem_dma_device_init(struct reserved_mem *rmem, struct device *dev)
        }
        mem->use_dev_dma_pfn_offset = true;
        rmem->priv = mem;
+       if (of_get_flat_dt_prop(node, "no-align", NULL))
+               mem->no_align = true;
        dma_assign_coherent_memory(dev, mem);
        return 0;
 }


>
>> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>
>> ---
>>   drivers/base/dma-coherent.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
>> index 744f64f43454..b86a96d0cd07 100644
>> --- a/drivers/base/dma-coherent.c
>> +++ b/drivers/base/dma-coherent.c
>> @@ -162,7 +162,7 @@ EXPORT_SYMBOL(dma_mark_declared_memory_occupied);
>>   static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
>>           ssize_t size, dma_addr_t *dma_handle)
>>   {
>> -    int order = get_order(size);
>> +    int nr_page = PAGE_ALIGN(size) >> PAGE_SHIFT;
>>       unsigned long flags;
>>       int pageno;
>>       void *ret;
>> @@ -172,9 +172,11 @@ static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
>>       if (unlikely(size > (mem->size << PAGE_SHIFT)))
>>           goto err;
>>   -    pageno = bitmap_find_free_region(mem->bitmap, mem->size, order);
>> -    if (unlikely(pageno < 0))
>> +    pageno = bitmap_find_next_zero_area(mem->bitmap, mem->size, 0,
>> +                        nr_page, 0);
>> +    if (unlikely(pageno >= mem->size)) {
>>           goto err;
>> +    bitmap_set(mem->bitmap, pageno, nr_page);
>>         /*
>>        * Memory was found in the coherent area.
>
> Best regards

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>