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

Ilias Apalodimas
2023-01-26T16:05:32Z

On Thu, Jan 26, 2023 at 07:41:15AM -0800, Alexander Duyck wrote:
> On Thu, Jan 26, 2023 at 2:32 AM Ilias Apalodimas
> <ilias.apalodimas@linaro.org> wrote:
> >
> > Hi Alexander,
> >
> > Sorry for being late to the party, was overloaded...
> >
> > On Tue, Jan 24, 2023 at 07:57:35AM -0800, Alexander H Duyck wrote:
> > > On Tue, 2023-01-24 at 16:11 +0200, 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?
> > > >
> > > > [1] https://lore.kernel.org/netdev/1625903002-31619-4-git-send-email-linyunsheng@huawei.com/
> > > >
> > > > Thanks
> > > > /Ilias
> > > >
> > > >
> > >
> > > The logic should be safe in terms of the page pool itself as it should
> > > be holding one reference to the page while the pp_frag_count is non-
> > > zero. That one reference is what keeps the two halfs in sync as the
> > > page shouldn't be able to be freed until we exhaust the pp_frag_count.
> >
> > Do you remember why we decided to go with the fragment counter instead of
> > page references?
>
> The issue has to do with when to destroy the mappings. Basically with
> the fragment counter we destroy the mappings and remove the page from
> the pool when the count hits 0. The reference count is really used for
> the page allocator to do its tracking. If we end up trying to merge
> the two the problem becomes one of lifetimes as we wouldn't know when
> to destroy the DMA mappings as they would have to live the full life
> of the page.

Ah yes thanks! We need that on a comment somewhere, I keep forgetting...
Basically the pp_frag_count is our number of outstanding dma mappings.

>
> > >
> > > To have an underflow there are two possible scenarios. One is that
> > > either put_page or free_page is being called somewhere that the
> > > page_pool freeing functions should be used.
> >
> > Wouldn't that affect the non fragmented path as well? IOW the driver that
> > works with a full page would crash as well.
>
> The problem is the non-fragmented path doesn't get as noisy. Also
> there aren't currently any wireless drivers making use of the page
> pool, or at least that is my understanding. I'm suspecting something
> like the issue we saw in 1effe8ca4e34c ("skbuff: fix coalescing for
> page_pool fragment recycling"). We likely have some corner case where
> we should be taking a page reference and clearing a pp_recycle flag.

Yea, same thinking here. I'll have another closer look tomorrow, but
looking at the wireless internals what happens is
1. They alloc a fragment
2. They create a new SKB, without the recycle bit and refer to the existing
fragments. Since the recyle bit is off *that* skb will never try to
decrease the frag counter. Instead it bumps the page refcnt which should be
properly decremented one that SKB is freed. I guess somehow an SKB ends up with
the recycle bit set, when it shouldn't.

Regards
/Ilias