Message-ID: <19121deb-368f-9786-8700-f1c45d227a4c@nbd.name>
Date:   Tue, 24 Jan 2023 18:22:54 +0100
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To:     Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc:     netdev@vger.kernel.org, Jesper Dangaard Brouer <hawk@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        linux-kernel@vger.kernel.org,
        Alexander Duyck <alexander.duyck@gmail.com>,
        Yunsheng Lin <linyunsheng@huawei.com>
References: <20230124124300.94886-1-nbd@nbd.name>
 <CAC_iWjKAEgUB8Z3WNNVgUK8omXD+nwt_VPSVyFn1i4EQzJadog@mail.gmail.com>
From:   Felix Fietkau <nbd@nbd.name>
Subject: Re: [PATCH] net: page_pool: fix refcounting issues with fragmented
 allocation
In-Reply-To: <CAC_iWjKAEgUB8Z3WNNVgUK8omXD+nwt_VPSVyFn1i4EQzJadog@mail.gmail.com>
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On 24.01.23 15:11, Ilias Apalodimas wrote:
> Hi Felix,
> 
> ++cc Alexander and Yunsheng.
> 
> Thanks for the report
> 
> On Tue, 24 Jan 2023 at 14:43, Felix Fietkau <nbd@nbd.name> wrote:
>>
>> While testing fragmented page_pool allocation in the mt76 driver, I was able
>> to reliably trigger page refcount underflow issues, which did not occur with
>> full-page page_pool allocation.
>> It appears to me, that handling refcounting in two separate counters
>> (page->pp_frag_count and page refcount) is racy when page refcount gets
>> incremented by code dealing with skb fragments directly, and
>> page_pool_return_skb_page is called multiple times for the same fragment.
>>
>> Dropping page->pp_frag_count and relying entirely on the page refcount makes
>> these underflow issues and crashes go away.
>>
> 
> This has been discussed here [1].  TL;DR changing this to page
> refcount might blow up in other colorful ways.  Can we look closer and
> figure out why the underflow happens?
I don't see how the approch taken in my patch would blow up. From what I 
can tell, it should be fairly close to how refcount is handled in 
page_frag_alloc. The main improvement it adds is to prevent it from 
blowing up if pool-allocated fragments get shared across multiple skbs 
with corresponding get_page and page_pool_return_skb_page calls.

- Felix