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

[Jason Gunthorpe <jgg@ziepe.ca>]

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


> > > +	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.

> > 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() ?

> > > +		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

Jason