Re: [PATCH 3/3] net/xdp: convert put_page() to put_user_page*()

[Ira Weiny]

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@xxxxxxxxx wrote:
> > > From: John Hubbard <jhubbard@xxxxxxxxxx>
> > > 
> > > 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@xxxxxxxxx>
> > > Cc: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
> > > Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> > > Cc: netdev@xxxxxxxxxxxxxxx
> > > Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> > > ---
> > >   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
>