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SubjectRe: [PATCH 3/3] net/xdp: convert put_page() to put_user_page*()
On Mon, Jul 22, 2019 at 09:41:34PM -0700, John Hubbard wrote:
> On 7/22/19 5:25 PM, Ira Weiny wrote:
> > On Mon, Jul 22, 2019 at 03:34:15PM -0700, john.hubbard@gmail.com wrote:
> > > From: John Hubbard <jhubbard@nvidia.com>
> > >
> > > For pages that were retained via get_user_pages*(), release those pages
> > > via the new put_user_page*() routines, instead of via put_page() or
> > > release_pages().
> > >
> > > This is part a tree-wide conversion, as described in commit fc1d8e7cca2d
> > > ("mm: introduce put_user_page*(), placeholder versions").
> > >
> > > Cc: Björn Töpel <bjorn.topel@intel.com>
> > > Cc: Magnus Karlsson <magnus.karlsson@intel.com>
> > > Cc: David S. Miller <davem@davemloft.net>
> > > Cc: netdev@vger.kernel.org
> > > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> > > ---
> > > net/xdp/xdp_umem.c | 9 +--------
> > > 1 file changed, 1 insertion(+), 8 deletions(-)
> > >
> > > diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> > > index 83de74ca729a..0325a17915de 100644
> > > --- a/net/xdp/xdp_umem.c
> > > +++ b/net/xdp/xdp_umem.c
> > > @@ -166,14 +166,7 @@ void xdp_umem_clear_dev(struct xdp_umem *umem)
> > > static void xdp_umem_unpin_pages(struct xdp_umem *umem)
> > > {
> > > - unsigned int i;
> > > -
> > > - for (i = 0; i < umem->npgs; i++) {
> > > - struct page *page = umem->pgs[i];
> > > -
> > > - set_page_dirty_lock(page);
> > > - put_page(page);
> > > - }
> > > + put_user_pages_dirty_lock(umem->pgs, umem->npgs);
> >
> > What is the difference between this and
> >
> > __put_user_pages(umem->pgs, umem->npgs, PUP_FLAGS_DIRTY_LOCK);
> >
> > ?
>
> No difference.
>
> >
> > I'm a bit concerned with adding another form of the same interface. We should
> > either have 1 call with flags (enum in this case) or multiple calls. Given the
> > previous discussion lets move in the direction of having the enum but don't
> > introduce another caller of the "old" interface.
>
> I disagree that this is a "problem". There is no maintenance pitfall here; there
> are merely two ways to call the put_user_page*() API. Both are correct, and
> neither one will get you into trouble.
>
> Not only that, but there is ample precedent for this approach in other
> kernel APIs.
>
> >
> > So I think on this patch NAK from me.
> >
> > I also don't like having a __* call in the exported interface but there is a
> > __get_user_pages_fast() call so I guess there is precedent. :-/
> >
>
> I thought about this carefully, and looked at other APIs. And I noticed that
> things like __get_user_pages*() are how it's often done:
>
> * The leading underscores are often used for the more elaborate form of the
> call (as oppposed to decorating the core function name with "_flags", for
> example).
>
> * There are often calls in which you can either call the simpler form, or the
> form with flags and additional options, and yes, you'll get the same result.
>
> Obviously, this stuff is all subject to a certain amount of opinion, but I
> think I'm on really solid ground as far as precedent goes. So I'm pushing
> back on the NAK... :)

Fair enough... However, we have discussed in the past how GUP can be a
confusing interface to use.

So I'd like to see it be more directed. Only using the __put_user_pages()
version allows us to ID callers easier through a grep of PUP_FLAGS_DIRTY_LOCK
in addition to directing users to use that interface rather than having to read
the GUP code to figure out that the 2 calls above are equal. It is not a huge
deal but...

Ira

>
> thanks,
> --
> John Hubbard
> NVIDIA
>

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