Re: [PATCH rdma-next 2/2] RDMA: Use dma_map_sgtable for map umem pages

[Leon Romanovsky]

On Wed, Jun 23, 2021 at 09:10:05AM -0300, Jason Gunthorpe wrote:
> On Wed, Jun 23, 2021 at 08:23:17AM +0300, Leon Romanovsky wrote:
> > On Tue, Jun 22, 2021 at 10:18:16AM -0300, Jason Gunthorpe wrote:
> > > On Tue, Jun 22, 2021 at 02:39:42PM +0300, Leon Romanovsky wrote:
> > > 
> > > > diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
> > > > index 0eb40025075f..a76ef6a6bac5 100644
> > > > +++ b/drivers/infiniband/core/umem.c
> > > > @@ -51,11 +51,11 @@ static void __ib_umem_release(struct ib_device *dev, struct ib_umem *umem, int d
> > > >  	struct scatterlist *sg;
> > > >  	unsigned int i;
> > > >  
> > > > -	if (umem->nmap > 0)
> > > > -		ib_dma_unmap_sg(dev, umem->sg_head.sgl, umem->sg_nents,
> > > > -				DMA_BIDIRECTIONAL);
> > > > +	if (dirty)
> > > > +		ib_dma_unmap_sgtable_attrs(dev, &umem->sg_head,
> > > > +					   DMA_BIDIRECTIONAL, 0);
> > > >  
> > > > -	for_each_sg(umem->sg_head.sgl, sg, umem->sg_nents, i)
> > > > +	for_each_sgtable_dma_sg(&umem->sg_head, sg, i)
> > > >  		unpin_user_page_range_dirty_lock(sg_page(sg),
> > > >  			DIV_ROUND_UP(sg->length, PAGE_SIZE), make_dirty);
> > > 
> > > This isn't right, can't mix sg_page with a _dma_ API
> > 
> > Jason, why is that?
> > 
> > We use same pages that were passed to __sg_alloc_table_from_pages() in __ib_umem_get().
> 
> A sgl has two lists inside it a 'dma' list and a 'page' list, they are
> not the same length and not interchangable.
> 
> If you use for_each_sgtable_dma_sg() then you iterate over the 'dma'
> list and have to use 'dma' accessors
> 
> If you use for_each_sgtable_sg() then you iterate over the 'page' list
> and have to use 'page' acessors
> 
> Mixing dma iteration with page accessors or vice-versa, like above, is
> always a bug.
> 
> You can see it alos because the old code used umem->sg_nents which is
> the CPU list length while this new code is using the dma list length.

Ohh, I see difference between types now, thank you for the explanation,
will consult with Maor once he returns next week to the office.

Thanks

> 
> Jason