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SubjectRe: [PATCH net-next RFC 1/2] page_pool: add page recycling support based on elevated refcnt
On Wed, Jul 7, 2021 at 12:03 PM Ilias Apalodimas
<ilias.apalodimas@linaro.org> wrote:
>
> > > Hi, Alexander
> > >
> > > Thanks for detailed reviewing.
> > >
>
> Likewise!
> I'll have a look on the entire conversation in a few days...
>
> > > >
> > > > So this isn't going to work with the current recycling logic. The
> > > > expectation there is that we can safely unmap the entire page as soon
> > > > as the reference count is greater than 1.
> > >
> > > Yes, the expectation is changed to we can always recycle the page
> > > when the last user has dropped the refcnt that has given to it when
> > > the page is not pfmemalloced.
> > >
> > > The above expectation is based on that the last user will always
> > > call page_pool_put_full_page() in order to do the recycling or do
> > > the resource cleanup(dma unmaping..etc).
> > >
> > > As the skb_free_head() and skb_release_data() have both checked the
> > > skb->pp_recycle to call the page_pool_put_full_page() if needed, I
> > > think we are safe for most case, the one case I am not so sure above
> > > is the rx zero copy, which seems to also bump up the refcnt before
> > > mapping the page to user space, we might need to ensure rx zero copy
> > > is not the last user of the page or if it is the last user, make sure
> > > it calls page_pool_put_full_page() too.
> >
> > Yes, but the skb->pp_recycle value is per skb, not per page. So my
> > concern is that carrying around that value can be problematic as there
> > are a number of possible cases where the pages might be
> > unintentionally recycled. All it would take is for a packet to get
> > cloned a few times and then somebody starts using pskb_expand_head and
> > you would have multiple cases, possibly simultaneously, of entities
> > trying to free the page. I just worry it opens us up to a number of
> > possible races.
>
> Maybe I missde something, but I thought the cloned SKBs would never trigger
> the recycling path, since they are protected by the atomic dataref check in
> skb_release_data(). What am I missing?

Are you talking about the head frag? So normally a clone wouldn't
cause an issue because the head isn't changed. In the case of the
head_frag we should be safe since pskb_expand_head will just kmalloc
the new head and clears head_frag so it won't trigger
page_pool_return_skb_page on the head_frag since the dataref just goes
from 2 to 1.

The problem is that pskb_expand_head memcopies the page frags over and
takes a reference on the pages. At that point you would have two skbs
both pointing to the same set of pages and each one ready to call
page_pool_return_skb_page on the pages at any time and possibly racing
with the other.

I suspect if they both called it at roughly the same time one of them
would trigger a NULL pointer dereference since they would both check
pp_magic first, and then both set pp to NULL. If run on a system where
dma_unmap_page_attrs takes a while it would be very likely to race
since pp_magic doesn't get cleared until after the page is unmapped.

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