Re: [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
[off-list ref] wrote:

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Hi, Alexander

Thanks for detailed reviewing.

Likewise!
I'll have a look on the entire conversation in a few days...

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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.