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

Joonsoo Kim
2014-06-16T05:23:50Z

On Sat, Jun 14, 2014 at 03:46:44PM +0530, Aneesh Kumar K.V wrote:
> Joonsoo Kim <iamjoonsoo.kim@lge.com> 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@mina86.com>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
>
> .....
>
> > +
> > + 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() ?
>

Unlike your psuedo code, for setting bitmap, we do
- grab the mutex
- bitmap_set
- release the mutex

clear_cma_bitmap() handles these things.

Thanks.