netdev - Re: [PATCH v11 04/25] mm: devmap: refactor 1-based refcounting for ZONE

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Date:   Wed, 18 Dec 2019 22:52:34 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
        Maling list - DRI developers 
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        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v11 04/25] mm: devmap: refactor 1-based refcounting for
 ZONE_DEVICE pages

On Wed, Dec 18, 2019 at 9:51 PM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 12/18/19 9:27 PM, Dan Williams wrote:
> ...
> >> @@ -461,5 +449,5 @@ void __put_devmap_managed_page(struct page *page)
> >>          page->mapping = NULL;
> >>          page->pgmap->ops->page_free(page);
> >>   }
> >> -EXPORT_SYMBOL(__put_devmap_managed_page);
> >> +EXPORT_SYMBOL(free_devmap_managed_page);
> >
> > This patch does not have a module consumer for
> > free_devmap_managed_page(), so the export should move to the patch
> > that needs the new export.
>
> Hi Dan,
>
> OK, I know that's a policy--although it seems quite pointless here given
> that this is definitely going to need an EXPORT.
>
> At the moment, the series doesn't use it in any module at all, so I'll just
> delete the EXPORT for now.
>
> >
> > Also the only reason that put_devmap_managed_page() is EXPORT_SYMBOL
> > instead of EXPORT_SYMBOL_GPL is that there was no practical way to
> > hide the devmap details from evey module in the kernel that did
> > put_page(). I would expect free_devmap_managed_page() to
> > EXPORT_SYMBOL_GPL if it is not inlined into an existing exported
> > static inline api.
> >
>
> Sure, I'll change it to EXPORT_SYMBOL_GPL when the time comes. We do have
> to be careful that we don't shut out normal put_page() types of callers,
> but...glancing through the current callers, that doesn't look to be a problem.
> Good. So it should be OK to do EXPORT_SYMBOL_GPL here.
>
> Are you *sure* you don't want to just pre-emptively EXPORT now, and save
> looking at it again?

I'm positive. There is enough history for "trust me the consumer is
coming" turning out not to be true to justify the hassle in my mind. I
do trust you, but things happen.