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

[Ira Weiny <ira.weiny@intel.com>]

On Thu, May 23, 2019 at 10:46:38AM -0700, John Hubbard wrote:
> 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@nvidia.com>
> > 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?

I'm certainly not the expert here but FWICT release_pages() was originally
designed to work with the page cache.

aabfb57296e3  mm: memcontrol: do not kill uncharge batching in free_pages_and_swap_cache

But at some point it was changed to be more general?

ea1754a08476 mm, fs: remove remaining PAGE_CACHE_* and page_cache_{get,release} usage

... and it is exported and used outside of the swapping code... and used at
lease 1 place to directly "put" pages gotten from get_user_pages_fast()
[arch/x86/kvm/svm.c]

From that it seems like it is safe.

But I don't see where release_page() actually calls put_page() anywhere?  What
am I missing?

Ira