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From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
To: Alexander Duyck <alexander.duyck@gmail.com>
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>,
	Netdev <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH net-next RFC 1/2] page_pool: add page recycling support based on elevated refcnt
Date: Thu, 8 Jul 2021 18:36:32 +0300	[thread overview]
Message-ID: <YOcbgEKqq/cRBxX9@enceladus> (raw)
In-Reply-To: <CAKgT0UcoLE=MhG+QxS=up5BH_cK5FBSwyMHDvfUg2g8083UM+w@mail.gmail.com>

On Thu, Jul 08, 2021 at 08:29:56AM -0700, Alexander Duyck wrote:
> On Thu, Jul 8, 2021 at 8:17 AM Ilias Apalodimas
> <ilias.apalodimas@linaro.org> wrote:
> >
> > > > >
> > > > > > > > >
> > > > >
> > > > > [...]
> > > > >
> > > > > > > > > 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.
> > > > >
> > > > > Ok let me make sure I get the idea properly.
> > > > > When pskb_expand_head is called, the new dataref will be 1, but the
> > > > > head_frag will be set to 0, in which case the recycling code won't be
> > > > > called for that skb.
> > > > > So you are mostly worried about a race within the context of
> > > > > pskb_expand_skb() between copying the frags, releasing the previous head
> > > > > and preparing the new one (on a cloned skb)?
> > > >
> > > > The race is between freeing the two skbs. So the original and the
> > > > clone w/ the expanded head will have separate instances of the page. I
> > > > am pretty certain there is a race if the two of them start trying to
> > > > free the page frags at the same time.
> > > >
> > >
> > > Right, I completely forgot calling __skb_frag_unref() before releasing the
> > > head ...
> > > You are right, this will be a race.  Let me go back to the original mail
> > > thread and see what we can do
> > >
> >
> > What do you think about resetting pp_recycle bit on pskb_expand_head()?
> 
> I assume you mean specifically in the cloned case?
> 

Yes. Even if we do it unconditionally we'll just loose non-cloned buffers from
the recycling.
I'll send a patch later today.

> > If my memory serves me right Eric wanted that from the beginning. Then the
> > cloned/expanded SKB won't trigger the recycling.  If that skb hits the free
> > path first, we'll end up recycling the fragments eventually.  If the
> > original one goes first, we'll just unmap the page(s) and freeing the cloned
> > one will free all the remaining buffers.
> 
> I *think* that should be fine. Effectively what we are doing is making
> it so that if the original skb is freed first the pages are released,
> and if it is released after the clone/expended skb then it can be
> recycled.

Exactly

> 
> The issue is we have to maintain it so that there will be exactly one
> caller of the recycling function for the pages. So any spot where we
> are updating skb->head we will have to see if there is a clone and if
> so we have to clear the pp_recycle flag on our skb so that it doesn't
> try to recycle the page frags as well.

Correct. I'll keep looking around in case there's something less fragile we
can do 

Thanks
/Ilias

  reply	other threads:[~2021-07-08 15:36 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-30  9:17 [PATCH net-next RFC 0/2] add elevated refcnt support for page pool Yunsheng Lin
2021-06-30  9:17 ` [PATCH net-next RFC 1/2] page_pool: add page recycling support based on elevated refcnt Yunsheng Lin
2021-07-02  9:42   ` Jesper Dangaard Brouer
2021-07-02 10:15     ` Yunsheng Lin
2021-07-06  4:54       ` Ilias Apalodimas
2021-07-06  6:46         ` Yunsheng Lin
2021-07-06  8:18           ` Ilias Apalodimas
2021-07-06 20:45   ` Alexander Duyck
2021-07-07  3:05     ` Yunsheng Lin
2021-07-07 15:01       ` Alexander Duyck
2021-07-07 19:03         ` Ilias Apalodimas
2021-07-07 21:49           ` Alexander Duyck
2021-07-08 14:21             ` Ilias Apalodimas
2021-07-08 14:24               ` Alexander Duyck
2021-07-08 14:50                 ` Ilias Apalodimas
2021-07-08 15:17                   ` Ilias Apalodimas
2021-07-08 15:29                     ` Alexander Duyck
2021-07-08 15:36                       ` Ilias Apalodimas [this message]
2021-07-08 15:41                         ` Alexander Duyck
2021-07-08 15:47                           ` Ilias Apalodimas
2021-07-08  2:27         ` Yunsheng Lin
2021-07-08 15:36           ` Alexander Duyck
2021-07-09  6:26             ` Yunsheng Lin
2021-07-09 14:15               ` Alexander Duyck
2021-07-10  9:16                 ` [Linuxarm] " Yunsheng Lin
2021-06-30  9:17 ` [PATCH net-next RFC 2/2] net: hns3: support skb's frag page recycling based on page pool Yunsheng Lin
2021-07-02  8:36 ` [PATCH net-next RFC 0/2] add elevated refcnt support for " Ilias Apalodimas
2021-07-02 13:39 ` Matteo Croce
2021-07-06 15:51   ` Russell King (Oracle)
2021-07-06 23:19     ` Matteo Croce
2021-07-07 16:50       ` Marcin Wojtas
2021-07-09  4:15         ` Matteo Croce
2021-07-09  6:40           ` Yunsheng Lin
2021-07-09  6:42             ` Ilias Apalodimas

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