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

[John Hubbard <jhubbard@nvidia.com>]

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... :)

thanks,
-- 
John Hubbard
NVIDIA