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

[Felix Fietkau]

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@xxxxxxxx> 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