linux-kernel - Re: [PATCH] net: core: page_pool: add user refcnt and reintroduce page_pool

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Date:   Tue, 2 Jul 2019 21:58:40 +0300
From:   Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        grygorii.strashko@...com, jakub.kicinski@...ronome.com,
        daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
        linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org
Subject: Re: [PATCH] net: core: page_pool: add user refcnt and reintroduce
 page_pool_destroy

On Tue, Jul 02, 2019 at 08:29:07PM +0200, Jesper Dangaard Brouer wrote:
>On Tue, 2 Jul 2019 18:21:13 +0300
>Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org> wrote:
>
>> On Tue, Jul 02, 2019 at 05:10:29PM +0200, Jesper Dangaard Brouer wrote:
>> >On Tue, 2 Jul 2019 17:56:13 +0300
>> >Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org> wrote:
>> >
>> >> On Tue, Jul 02, 2019 at 04:52:30PM +0200, Jesper Dangaard Brouer wrote:
>> >> >On Tue, 2 Jul 2019 17:44:27 +0300
>> >> >Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org> wrote:
>> >> >
>> >> >> On Tue, Jul 02, 2019 at 04:31:39PM +0200, Jesper Dangaard Brouer wrote:
>> >> >> >From: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
>> >> >> >
>> >> >> >Jesper recently removed page_pool_destroy() (from driver invocation) and
>> >> >> >moved shutdown and free of page_pool into xdp_rxq_info_unreg(), in-order to
>> >> >> >handle in-flight packets/pages. This created an asymmetry in drivers
>> >> >> >create/destroy pairs.
>> >> >> >
>> >> >> >This patch add page_pool user refcnt and reintroduce page_pool_destroy.
>> >> >> >This serves two purposes, (1) simplify drivers error handling as driver now
>> >> >> >drivers always calls page_pool_destroy() and don't need to track if
>> >> >> >xdp_rxq_info_reg_mem_model() was unsuccessful. (2) allow special cases
>> >> >> >where a single RX-queue (with a single page_pool) provides packets for two
>> >> >> >net_device'es, and thus needs to register the same page_pool twice with two
>> >> >> >xdp_rxq_info structures.
>> >> >>
>> >> >> As I tend to use xdp level patch there is no more reason to mention (2) case
>> >> >> here. XDP patch serves it better and can prevent not only obj deletion but also
>> >> >> pool flush, so, this one patch I could better leave only for (1) case.
>> >> >
>> >> >I don't understand what you are saying.
>> >> >
>> >> >Do you approve this patch, or do you reject this patch?
>> >> >
>> >> It's not reject, it's proposition to use both, XDP and page pool patches,
>> >> each having its goal.
>> >
>> >Just to be clear, if you want this patch to get accepted you have to
>> >reply with your Signed-off-by (as I wrote).
>> >
>> >Maybe we should discuss it in another thread, about why you want two
>> >solutions to the same problem.
>>
>> If it solves same problem I propose to reject this one and use this:
>> https://lkml.org/lkml/2019/7/2/651
>
>No, I propose using this one, and rejecting the other one.

There is at least several arguments against this one (related (2) purpose)

It allows:
- avoid changes to page_pool/mlx5/netsec
- save not only allocator obj but allocator "page/buffer flush"
- buffer flush can be present not only in page_pool but for other allocators
  that can behave differently and not so simple solution.
- to not limit cpsw/(potentially others) to use "page_pool" allocator only
....

This patch better leave also, as it simplifies error path for page_pool and
have more error prone usage comparing with existent one.

Please, don't limit cpsw and potentially other drivers to use only
page_pool it can be zca or etc... I don't won't to modify each allocator.
I propose to add both as by fact they solve different problems with common
solution.

-- 
Regards,
Ivan Khoronzhuk