Re: [PATCH v2 06/10] CMA: generalize CMA reserved area management functionality

[Aneesh Kumar K.V]

Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> writes:

> Currently, there are two users on CMA functionality, one is the DMA
> subsystem and the other is the kvm on powerpc. They have their own code
> to manage CMA reserved area even if they looks really similar.
> From my guess, it is caused by some needs on bitmap management. Kvm side
> wants to maintain bitmap not for 1 page, but for more size. Eventually it
> use bitmap where one bit represents 64 pages.
>
> When I implement CMA related patches, I should change those two places
> to apply my change and it seem to be painful to me. I want to change
> this situation and reduce future code management overhead through
> this patch.
>
> This change could also help developer who want to use CMA in their
> new feature development, since they can use CMA easily without
> copying & pasting this reserved area management code.
>
> In previous patches, we have prepared some features to generalize
> CMA reserved area management and now it's time to do it. This patch
> moves core functions to mm/cma.c and change DMA APIs to use
> these functions.
>
> There is no functional change in DMA APIs.
>
> v2: There is no big change from v1 in mm/cma.c. Mostly renaming.
>
> Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>

.....

> +
> +	mask = cma_bitmap_aligned_mask(cma, align);
> +	bitmap_maxno = cma_bitmap_maxno(cma);
> +	nr_bits = cma_bitmap_pages_to_bits(cma, count);
> +
> +	for (;;) {
> +		mutex_lock(&cma->lock);
> +		bitmapno = bitmap_find_next_zero_area(cma->bitmap,
> +					bitmap_maxno, start, nr_bits, mask);
> +		if (bitmapno >= bitmap_maxno) {
> +			mutex_unlock(&cma->lock);
> +			break;
> +		}
> +		bitmap_set(cma->bitmap, bitmapno, nr_bits);
> +		/*
> +		 * It's safe to drop the lock here. We've marked this region for
> +		 * our exclusive use. If the migration fails we will take the
> +		 * lock again and unmark it.
> +		 */
> +		mutex_unlock(&cma->lock);
> +
> +		pfn = cma->base_pfn + (bitmapno << cma->order_per_bit);
> +		mutex_lock(&cma_mutex);
> +		ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA);
> +		mutex_unlock(&cma_mutex);
> +		if (ret == 0) {
> +			page = pfn_to_page(pfn);
> +			break;
> +		} else if (ret != -EBUSY) {
> +			clear_cma_bitmap(cma, pfn, count);
> +			break;
> +		}
> +		


For setting bit map we do
		bitmap_set(cma->bitmap, bitmapno, nr_bits);
                alloc_contig()..
                if (error)
                        clear_cma_bitmap(cma, pfn, count);

Why ?

why not bitmap_clear() ?

-aneesh

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