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

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Message-ID: <c1b689bd-a05b-85e9-0ce4-7264c818c2dc@huawei.com>
Date:   Wed, 8 Nov 2023 11:40:41 +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 2023/11/8 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.