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

Felix Fietkau
2023-01-24T21:31:15Z

On 24.01.23 22:10, Alexander H Duyck wrote:
> 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.
The driver patches for page pool are here:
https://patchwork.kernel.org/project/linux-wireless/patch/64abb23f4867c075c19d704beaae5a0a2f8e8821.1673963374.git.lorenzo@kernel.org/
https://patchwork.kernel.org/project/linux-wireless/patch/68081e02cbe2afa2d35c8aa93194f0adddbd0f05.1673963374.git.lorenzo@kernel.org/

They are also applied in my mt76 tree at:
https://github.com/nbd168/wireless

- Felix