netdev - Re: [PATCH bpf-next 02/15] xsk: add user memory registration support sockopt

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Date:   Tue, 24 Apr 2018 09:01:47 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        michael.lundkvist@...csson.com,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Singhai, Anjali" <anjali.singhai@...el.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>
Subject: Re: [PATCH bpf-next 02/15] xsk: add user memory registration support sockopt

2018-04-23 22:26 GMT+02:00 Michael S. Tsirkin <mst@...hat.com>:
> On Mon, Apr 23, 2018 at 10:15:18PM +0200, Björn Töpel wrote:
>> 2018-04-23 22:11 GMT+02:00 Michael S. Tsirkin <mst@...hat.com>:
>> > On Mon, Apr 23, 2018 at 10:00:15PM +0200, Björn Töpel wrote:
>> >> 2018-04-23 18:18 GMT+02:00 Michael S. Tsirkin <mst@...hat.com>:
>> >>
>> >> [...]
>> >>
>> >> >> +static void xdp_umem_unpin_pages(struct xdp_umem *umem)
>> >> >> +{
>> >> >> +     unsigned int i;
>> >> >> +
>> >> >> +     if (umem->pgs) {
>> >> >> +             for (i = 0; i < umem->npgs; i++)
>> >> >
>> >> > Since you pin them with FOLL_WRITE, I assume these pages
>> >> > are written to.
>> >> > Don't you need set_page_dirty_lock here?
>> >> >
>> >>
>> >> Hmm, I actually *removed* it from the RFC V2, but after doing some
>> >> homework, I think you're right. Thanks for pointing this out!
>> >>
>> >> Thinking more about this; This function is called from sk_destruct,
>> >> and in the Tx case the sk_destruct can be called from interrupt
>> >> context, where set_page_dirty_lock cannot be called.
>> >>
>> >> Are there any preferred ways of solving this? Scheduling the whole
>> >> xsk_destruct call to a workqueue is one way (I think). Any
>> >> cleaner/better way?
>> >>
>> >> [...]
>> >
>> > Defer unpinning pages until the next tx call?
>> >
>>
>> If the sock is released, there wont be another tx call.
>
> unpin them on socket release too?
>

AF_XDP pins all memory up front, and unpins it when the socket is
released (final sock_put), which in this case is in the skb
destructor. So there's no later point from a sock lifetime
perspective.

I'll make a stab at doing umem clean up in a worker queue.

>> Or am I
>> missing something obvious?
>>
>> >
>> >> >> +static int __xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
>> >> >> +{
>> >> >> +     u32 frame_size = mr->frame_size, frame_headroom = mr->frame_headroom;
>> >> >> +     u64 addr = mr->addr, size = mr->len;
>> >> >> +     unsigned int nframes;
>> >> >> +     int size_chk, err;
>> >> >> +
>> >> >> +     if (frame_size < XDP_UMEM_MIN_FRAME_SIZE || frame_size > PAGE_SIZE) {
>> >> >> +             /* Strictly speaking we could support this, if:
>> >> >> +              * - huge pages, or*
>> >> >
>> >> > what does "or*" here mean?
>> >> >
>> >>
>> >> Oops, I'll change to just 'or' in the next revision.
>> >>
>> >>
>> >> Thanks!
>> >> Björn