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Subject: Re: [PATCH 3/3] net/xdp: convert put_page() to put_user_page*()
To:     Ira Weiny <ira.weiny@intel.com>, <john.hubbard@gmail.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
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From:   John Hubbard <jhubbard@nvidia.com>
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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: Bj=C3=B6rn T=C3=B6pel <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)
>>  =20
>>   static void xdp_umem_unpin_pages(struct xdp_umem *umem)
>>   {
>> -	unsigned int i;
>> -
>> -	for (i =3D 0; i < umem->npgs; i++) {
>> -		struct page *page =3D umem->pgs[i];
>> -
>> -		set_page_dirty_lock(page);
>> -		put_page(page);
>> -	}
>> +	put_user_pages_dirty_lock(umem->pgs, umem->npgs);
>=20
> What is the difference between this and
>=20
> __put_user_pages(umem->pgs, umem->npgs, PUP_FLAGS_DIRTY_LOCK);
>=20
> ?

No difference.

>=20
> I'm a bit concerned with adding another form of the same interface.  We s=
hould
> either have 1 call with flags (enum in this case) or multiple calls.  Giv=
en 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.

>=20
> So I think on this patch NAK from me.
>=20
> I also don't like having a __* call in the exported interface but there i=
s a
> __get_user_pages_fast() call so I guess there is precedent.  :-/
>=20

I thought about this carefully, and looked at other APIs. And I noticed tha=
t
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 t=
he
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... :)

thanks,
--=20
John Hubbard
NVIDIA