linux-kernel - Re: [PATCH 1/1] infiniband/mm: convert put_page() to put_user

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Date:   Thu, 23 May 2019 16:17:34 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        "john.hubbard@...il.com" <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Doug Ledford <dledford@...hat.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Christian Benvenuti <benve@...co.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 1/1] infiniband/mm: convert put_page() to put_user_page*()

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@...dia.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?

It would have made it more consistent.. But it seems this isn't a
functional problem in this patch

Jason