linux-kernel - Re: [RFC PATCH v3 04/12] netdev: support binding dma-buf to netdevice

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Message-ID: <80b4022f-45d1-03e9-56e1-e797c0107786@huawei.com>
Date:   Thu, 9 Nov 2023 17:29:01 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Mina Almasry <almasrymina@...gle.com>
CC:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <linaro-mm-sig@...ts.linaro.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Ahern <dsahern@...nel.org>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jeroen de Borst <jeroendb@...gle.com>,
        Praveen Kaligineedi <pkaligineedi@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [RFC PATCH v3 04/12] netdev: support binding dma-buf to netdevice

On 2023/11/9 10:22, Mina Almasry wrote:
> On Tue, Nov 7, 2023 at 7:40 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>>
>> On 2023/11/8 5:59, Mina Almasry wrote:
>>> On Mon, Nov 6, 2023 at 11:46 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>>>>
>>>> On 2023/11/6 10:44, Mina Almasry wrote:
>>>>> +
>>>>> +void __netdev_devmem_binding_free(struct netdev_dmabuf_binding *binding)
>>>>> +{
>>>>> +     size_t size, avail;
>>>>> +
>>>>> +     gen_pool_for_each_chunk(binding->chunk_pool,
>>>>> +                             netdev_devmem_free_chunk_owner, NULL);
>>>>> +
>>>>> +     size = gen_pool_size(binding->chunk_pool);
>>>>> +     avail = gen_pool_avail(binding->chunk_pool);
>>>>> +
>>>>> +     if (!WARN(size != avail, "can't destroy genpool. size=%lu, avail=%lu",
>>>>> +               size, avail))
>>>>> +             gen_pool_destroy(binding->chunk_pool);
>>>>
>>>>
>>>> Is there any other place calling the gen_pool_destroy() when the above
>>>> warning is triggered? Do we have a leaking for binding->chunk_pool?
>>>>
>>>
>>> gen_pool_destroy BUG_ON() if it's not empty at the time of destroying.
>>> Technically that should never happen, because
>>> __netdev_devmem_binding_free() should only be called when the refcount
>>> hits 0, so all the chunks have been freed back to the gen_pool. But,
>>> just in case, I don't want to crash the server just because I'm
>>> leaking a chunk... this is a bit of defensive programming that is
>>> typically frowned upon, but the behavior of gen_pool is so severe I
>>> think the WARN() + check is warranted here.
>>
>> It seems it is pretty normal for the above to happen nowadays because of
>> retransmits timeouts, NAPI defer schemes mentioned below:
>>
>> https://lkml.kernel.org/netdev/168269854650.2191653.8465259808498269815.stgit@firesoul/
>>
>> And currently page pool core handles that by using a workqueue.
> 
> Forgive me but I'm not understanding the concern here.
> 
> __netdev_devmem_binding_free() is called when binding->ref hits 0.
> 
> binding->ref is incremented when an iov slice of the dma-buf is
> allocated, and decremented when an iov is freed. So,
> __netdev_devmem_binding_free() can't really be called unless all the
> iovs have been freed, and gen_pool_size() == gen_pool_avail(),
> regardless of what's happening on the page_pool side of things, right?

I seems to misunderstand it. In that case, it seems to be about
defensive programming like other checking.

By looking at it more closely, it seems napi_frag_unref() call
page_pool_page_put_many() directly， which means devmem seems to
be bypassing the napi_safe optimization.

Can napi_frag_unref() reuse napi_pp_put_page() in order to reuse
the napi_safe optimization?

>