netdev - Re: [PATCH 1/2] xsk: do not use mmap

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Date:   Mon, 11 Feb 2019 16:33:29 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Bjorn Topel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Netdev <netdev@...r.kernel.org>,
        Davidlohr Bueso <dbueso@...e.de>, dan.j.williams@...el.com
Subject: Re: [PATCH 1/2] xsk: do not use mmap_sem

[ +Dan ]

On 02/07/2019 08:43 AM, Björn Töpel wrote:
> Den tors 7 feb. 2019 kl 06:38 skrev Davidlohr Bueso <dave@...olabs.net>:
>>
>> Holding mmap_sem exclusively for a gup() is an overkill.
>> Lets replace the call for gup_fast() and let the mm take
>> it if necessary.
>>
>> Cc: David S. Miller <davem@...emloft.net>
>> Cc: Bjorn Topel <bjorn.topel@...el.com>
>> Cc: Magnus Karlsson <magnus.karlsson@...el.com>
>> CC: netdev@...r.kernel.org
>> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
>> ---
>>  net/xdp/xdp_umem.c | 6 ++----
>>  1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
>> index 5ab236c5c9a5..25e1e76654a8 100644
>> --- a/net/xdp/xdp_umem.c
>> +++ b/net/xdp/xdp_umem.c
>> @@ -265,10 +265,8 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem)
>>         if (!umem->pgs)
>>                 return -ENOMEM;
>>
>> -       down_write(&current->mm->mmap_sem);
>> -       npgs = get_user_pages(umem->address, umem->npgs,
>> -                             gup_flags, &umem->pgs[0], NULL);
>> -       up_write(&current->mm->mmap_sem);
>> +       npgs = get_user_pages_fast(umem->address, umem->npgs,
>> +                                  gup_flags, &umem->pgs[0]);
>>
> 
> Thanks for the patch!
> 
> The lifetime of the pinning is similar to RDMA umem mapping, so isn't
> gup_longterm preferred?

Seems reasonable from reading what gup_longterm seems to do. Davidlohr
or Dan, any thoughts on the above?

Thanks,
Daniel