[PATCH v8 4/5] RDMA/mlx5: Support dma-buf based userspace memory region

[Xiong, Jianxin]

> -----Original Message-----
> From: Jason Gunthorpe <jgg at ziepe.ca>
> Sent: Friday, November 06, 2020 4:49 AM
> To: Xiong, Jianxin <jianxin.xiong at intel.com>
> Cc: linux-rdma at vger.kernel.org; dri-devel at lists.freedesktop.org; Doug Ledford <dledford at redhat.com>; Leon Romanovsky
> <leon at kernel.org>; Sumit Semwal <sumit.semwal at linaro.org>; Christian Koenig <christian.koenig at amd.com>; Vetter, Daniel
> <daniel.vetter at intel.com>
> Subject: Re: [PATCH v8 4/5] RDMA/mlx5: Support dma-buf based userspace memory region
> 
> On Fri, Nov 06, 2020 at 01:11:38AM +0000, Xiong, Jianxin wrote:
> > > On Thu, Nov 05, 2020 at 02:48:08PM -0800, Jianxin Xiong wrote:
> > > > @@ -966,7 +969,10 @@ static struct mlx5_ib_mr *alloc_mr_from_cache(struct ib_pd *pd,
> > > >  	struct mlx5_ib_mr *mr;
> > > >  	unsigned int page_size;
> > > >
> > > > -	page_size = mlx5_umem_find_best_pgsz(umem, mkc, log_page_size, 0, iova);
> > > > +	if (umem->is_dmabuf)
> > > > +		page_size = ib_umem_find_best_pgsz(umem, PAGE_SIZE, iova);
> > >
> > > You said the sgl is not set here, why doesn't this crash? It is certainly wrong to call this function without a SGL.
> >
> > The sgl is NULL, and nmap is 0. The 'for_each_sg' loop is just skipped and won't crash.
> 
> Just wire this to 4k it is clearer than calling some no-op pgsz

Ok

> 
> 
> > > > +	if (!mr->cache_ent) {
> > > > +		mlx5_core_destroy_mkey(mr->dev->mdev, &mr->mmkey);
> > > > +		WARN_ON(mr->descs);
> > > > +	}
> > > > +}
> > >
> > > I would expect this to call ib_umem_dmabuf_unmap_pages() ?
> > >
> > > Who calls it on the dereg path?
> > >
> > > This looks quite strange to me, it calls ib_umem_dmabuf_unmap_pages() only from the invalidate callback?
> >
> > It is also called from ib_umem_dmabuf_release().
> 
> Hmm, that is no how the other APIs work, the unmap should be paired with the map in the caller, and the sequence for destroy should be
> 
>  invalidate
>  unmap
>  destroy_mkey
>  release_umem
> 
> I have another series coming that makes the other three destroy flows much closer to that ideal.
> 

Can fix that.

> > > I feel uneasy how this seems to assume everything works sanely, we
> > > can have parallel page faults so pagefault_dmabuf_mr() can be called multiple times after an invalidation, and it doesn't protect itself
> against calling ib_umem_dmabuf_map_pages() twice.
> > >
> > > Perhaps the umem code should keep track of the current map state and
> > > exit if there is already a sgl. NULL or not NULL sgl would do and seems quite reasonable.
> >
> > Ib_umem_dmabuf_map() already checks the sgl and will do nothing if it is already set.
> 
> How? What I see in patch 1 is an unconditonal call to
> dma_buf_map_attachment() ?

My bad. I misread the lines. It used to be there (in v3) but somehow got lost. 

> 
> > > > +		if (is_dmabuf_mr(mr))
> > > > +			return pagefault_dmabuf_mr(mr, umem_dmabuf, user_va,
> > > > +						   bcnt, bytes_mapped, flags);
> > >
> > > But this doesn't care about user_va or bcnt it just triggers the whole thing to be remapped, so why calculate it?
> >
> > The range check is still needed, in order to catch application errors
> > of using incorrect address or count in verbs command. Passing the
> > values further in is to allow pagefault_dmabuf_mr to generate return
> > value and set bytes_mapped in a way consistent with the page fault
> > handler chain.
> 
> The HW validates the range. The range check in the ODP case is to protect against a HW bug that would cause the kernel to malfunction.
> For dmabuf you don't need to do it

Ok.  So the handler can simply return 0 (as the number of pages mapped) and leave bytes_mapped untouched?

> 
> Jason