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From:   Alexander Duyck <alexander.duyck@gmail.com>
Date:   Wed, 7 Jul 2021 14:49:30 -0700
Message-ID: <CAKgT0UfPFbAptXMJ4BQyeAadaxyHfkKRfeiwhrVMwafNEM_0cw@mail.gmail.com>
Subject: Re: [PATCH net-next RFC 1/2] page_pool: add page recycling support
 based on elevated refcnt
To:     Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc:     Yunsheng Lin <linyunsheng@huawei.com>,
        David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, linuxarm@openeuler.org,
        yisen.zhuang@huawei.com, Salil Mehta <salil.mehta@huawei.com>,
        thomas.petazzoni@bootlin.com, Marcin Wojtas <mw@semihalf.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        hawk@kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, fenghua.yu@intel.com,
        guro@fb.com, peterx@redhat.com, Feng Tang <feng.tang@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, mcroce@microsoft.com,
        Hugh Dickins <hughd@google.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        Alexander Lobakin <alobakin@pm.me>,
        Willem de Bruijn <willemb@google.com>, wenxu@ucloud.cn,
        cong.wang@bytedance.com, Kevin Hao <haokexin@gmail.com>,
        nogikh@google.com, Marco Elver <elver@google.com>,
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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.