linux-kernel - Re: [PATCH rdma-next 2/2] RDMA: Use dma_map

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Date:   Wed, 23 Jun 2021 09:10:05 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Doug Ledford <dledford@...hat.com>, Christoph Hellwig <hch@....de>,
        Maor Gottlieb <maorg@...dia.com>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        Mike Marciniszyn <mike.marciniszyn@...nelisnetworks.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Zhu Yanjun <zyjzyj2000@...il.com>
Subject: Re: [PATCH rdma-next 2/2] RDMA: Use dma_map_sgtable for map umem
 pages

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.

Jason