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

Felix Fietkau
2023-01-24T17:23:07Z

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