Re: [PATCH] dma-buf: system_heap: No separate allocation for attachment sg_tables

["T.J. Mercier" <tjmercier@google.com>]

On Tue, Apr 22, 2025 at 1:24 AM Christian König
<christian.koenig@amd.com> wrote:
>
> Am 17.04.25 um 20:09 schrieb T.J. Mercier:
> > struct dma_heap_attachment is a separate allocation from the struct
> > sg_table it contains, but there is no reason for this. Let's use the
> > slab allocator just once instead of twice for dma_heap_attachment.
> >
> > Signed-off-by: T.J. Mercier <tjmercier@google.com>
>
> I'm not *that* expert for this code, but looks totally reasonable to me.

I noticed this while reviewing Maxime Ripard's recent carveout heap
patches, where I was confused about sg_free_table() until I realized
it doesn't free the underlying allocation. Then I started looking at
other heaps and found that most of them do it this way (including the
cma heap), and figured it was a nice cleanup here.

> Reviewed-by: Christian König <christian.koenig@amd.com>
>
> Let me know if I should push that to drm-misc-next.
>
> Regards,
> Christian.

Thanks, yes please!




> > ---
> >  drivers/dma-buf/heaps/system_heap.c | 43 ++++++++++++-----------------
> >  1 file changed, 17 insertions(+), 26 deletions(-)
> >
> > diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
> > index 26d5dc89ea16..bee10c400cf0 100644
> > --- a/drivers/dma-buf/heaps/system_heap.c
> > +++ b/drivers/dma-buf/heaps/system_heap.c
> > @@ -35,7 +35,7 @@ struct system_heap_buffer {
> >
> >  struct dma_heap_attachment {
> >       struct device *dev;
> > -     struct sg_table *table;
> > +     struct sg_table table;
> >       struct list_head list;
> >       bool mapped;
> >  };
> > @@ -54,29 +54,22 @@ static gfp_t order_flags[] = {HIGH_ORDER_GFP, HIGH_ORDER_GFP, LOW_ORDER_GFP};
> >  static const unsigned int orders[] = {8, 4, 0};
> >  #define NUM_ORDERS ARRAY_SIZE(orders)
> >
> > -static struct sg_table *dup_sg_table(struct sg_table *table)
> > +static int dup_sg_table(struct sg_table *from, struct sg_table *to)
> >  {
> > -     struct sg_table *new_table;
> > -     int ret, i;
> >       struct scatterlist *sg, *new_sg;
> > +     int ret, i;
> >
> > -     new_table = kzalloc(sizeof(*new_table), GFP_KERNEL);
> > -     if (!new_table)
> > -             return ERR_PTR(-ENOMEM);
> > -
> > -     ret = sg_alloc_table(new_table, table->orig_nents, GFP_KERNEL);
> > -     if (ret) {
> > -             kfree(new_table);
> > -             return ERR_PTR(-ENOMEM);
> > -     }
> > +     ret = sg_alloc_table(to, from->orig_nents, GFP_KERNEL);
> > +     if (ret)
> > +             return ret;
> >
> > -     new_sg = new_table->sgl;
> > -     for_each_sgtable_sg(table, sg, i) {
> > +     new_sg = to->sgl;
> > +     for_each_sgtable_sg(from, sg, i) {
> >               sg_set_page(new_sg, sg_page(sg), sg->length, sg->offset);
> >               new_sg = sg_next(new_sg);
> >       }
> >
> > -     return new_table;
> > +     return 0;
> >  }
> >
> >  static int system_heap_attach(struct dma_buf *dmabuf,
> > @@ -84,19 +77,18 @@ static int system_heap_attach(struct dma_buf *dmabuf,
> >  {
> >       struct system_heap_buffer *buffer = dmabuf->priv;
> >       struct dma_heap_attachment *a;
> > -     struct sg_table *table;
> > +     int ret;
> >
> >       a = kzalloc(sizeof(*a), GFP_KERNEL);
> >       if (!a)
> >               return -ENOMEM;
> >
> > -     table = dup_sg_table(&buffer->sg_table);
> > -     if (IS_ERR(table)) {
> > +     ret = dup_sg_table(&buffer->sg_table, &a->table);
> > +     if (ret) {
> >               kfree(a);
> > -             return -ENOMEM;
> > +             return ret;
> >       }
> >
> > -     a->table = table;
> >       a->dev = attachment->dev;
> >       INIT_LIST_HEAD(&a->list);
> >       a->mapped = false;
> > @@ -120,8 +112,7 @@ static void system_heap_detach(struct dma_buf *dmabuf,
> >       list_del(&a->list);
> >       mutex_unlock(&buffer->lock);
> >
> > -     sg_free_table(a->table);
> > -     kfree(a->table);
> > +     sg_free_table(&a->table);
> >       kfree(a);
> >  }
> >
> > @@ -129,7 +120,7 @@ static struct sg_table *system_heap_map_dma_buf(struct dma_buf_attachment *attac
> >                                               enum dma_data_direction direction)
> >  {
> >       struct dma_heap_attachment *a = attachment->priv;
> > -     struct sg_table *table = a->table;
> > +     struct sg_table *table = &a->table;
> >       int ret;
> >
> >       ret = dma_map_sgtable(attachment->dev, table, direction, 0);
> > @@ -164,7 +155,7 @@ static int system_heap_dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
> >       list_for_each_entry(a, &buffer->attachments, list) {
> >               if (!a->mapped)
> >                       continue;
> > -             dma_sync_sgtable_for_cpu(a->dev, a->table, direction);
> > +             dma_sync_sgtable_for_cpu(a->dev, &a->table, direction);
> >       }
> >       mutex_unlock(&buffer->lock);
> >
> > @@ -185,7 +176,7 @@ static int system_heap_dma_buf_end_cpu_access(struct dma_buf *dmabuf,
> >       list_for_each_entry(a, &buffer->attachments, list) {
> >               if (!a->mapped)
> >                       continue;
> > -             dma_sync_sgtable_for_device(a->dev, a->table, direction);
> > +             dma_sync_sgtable_for_device(a->dev, &a->table, direction);
> >       }
> >       mutex_unlock(&buffer->lock);
> >
> >
> > base-commit: 8ffd015db85fea3e15a77027fda6c02ced4d2444
>