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

[Alexander H Duyck <alexander.duyck@gmail.com>]

On Tue, 2023-01-24 at 18:22 +0100, Felix Fietkau wrote:
> 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
> 

Do you have the patch available to review as an RFC? From what I am
seeing it looks like you are underrunning on the pp_frag_count itself.
I would suspect the issue to be something like starting with a bad
count in terms of the total number of references, or deducing the wrong
amount when you finally free the page assuming you are tracking your
frag count using a non-atomic value in the driver.

Thanks,

- Alex