Re: [PATCH 1/1] infiniband/mm: convert put_page() to put_user_page*()

[John Hubbard]

On 5/23/19 10:32 AM, Jason Gunthorpe wrote:
> On Thu, May 23, 2019 at 10:28:52AM -0700, Ira Weiny wrote:
> > >   
> > > @@ -686,8 +686,8 @@ int ib_umem_odp_map_dma_pages(struct ib_umem_odp *umem_odp, u64 user_virt,
> > >   			 * ib_umem_odp_map_dma_single_page().
> > >   			 */
> > >   			if (npages - (j + 1) > 0)
> > > -				release_pages(&local_page_list[j+1],
> > > -					      npages - (j + 1));
> > > +				put_user_pages(&local_page_list[j+1],
> > > +					       npages - (j + 1));
> >
> > I don't know if we discussed this before but it looks like the use of
> > release_pages() was not entirely correct (or at least not necessary) here.  So
> > I think this is ok.
>
> Oh? John switched it from a put_pages loop to release_pages() here:
>
> commit 75a3e6a3c129cddcc683538d8702c6ef998ec589
> Author: John Hubbard <jhubbard@xxxxxxxxxx>
> Date:   Mon Mar 4 11:46:45 2019 -0800
>
>      RDMA/umem: minor bug fix in error handling path
>      
>      1. Bug fix: fix an off by one error in the code that cleans up if it fails
>         to dma-map a page, after having done a get_user_pages_remote() on a
>         range of pages.
>      
>      2. Refinement: for that same cleanup code, release_pages() is better than
>         put_page() in a loop.
>      
>
> And now we are going to back something called put_pages() that
> implements the same for loop the above removed?
>
> Seems like we are going in circles?? John?

put_user_pages() is meant to be a drop-in replacement for release_pages(),
so I made the above change as an interim step in moving the callsite from
a loop, to a single call.

And at some point, it may be possible to find a way to optimize put_user_pages()
in a similar way to the batching that release_pages() does, that was part
of the plan for this.

But I do see what you mean: in the interim, maybe put_user_pages() should
just be calling release_pages(), how does that change sound?


thanks,
--
John Hubbard
NVIDIA