netdev - Re: [PATCH] net: page_pool: fix refcounting issues with fragmented allocation

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Date:   Tue, 24 Jan 2023 18:22:54 +0100
From:   Felix Fietkau <nbd@....name>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>
Cc:     netdev@...r.kernel.org, Jesper Dangaard Brouer <hawk@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        linux-kernel@...r.kernel.org,
        Alexander Duyck <alexander.duyck@...il.com>,
        Yunsheng Lin <linyunsheng@...wei.com>
Subject: Re: [PATCH] net: page_pool: fix refcounting issues with fragmented
 allocation

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@....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