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From: Mina Almasry <almasrymina@google.com>
Date: Wed, 6 Mar 2024 10:46:45 -0800
Message-ID: <CAHS8izOoO-EovwMwAm9tLYetwikNPxC0FKyVGu1TPJWSz4bGoA@mail.gmail.com>
Subject: Re: [RFC] net: esp: fix bad handling of pages from page_pool
To: Jakub Kicinski <kuba@kernel.org>, Liang Chen <liangchen.linux@gmail.com>, 
	Yunsheng Lin <linyunsheng@huawei.com>
Cc: Dragos Tatulea <dtatulea@nvidia.com>, "davem@davemloft.net" <davem@davemloft.net>, 
	"herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>, Gal Pressman <gal@nvidia.com>, 
	"dsahern@kernel.org" <dsahern@kernel.org>, 
	"steffen.klassert@secunet.com" <steffen.klassert@secunet.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "pabeni@redhat.com" <pabeni@redhat.com>, 
	Leon Romanovsky <leonro@nvidia.com>, "edumazet@google.com" <edumazet@google.com>, 
	"ian.kumlien@gmail.com" <ian.kumlien@gmail.com>, 
	"Anatoli.Chechelnickiy@m.interpipe.biz" <Anatoli.Chechelnickiy@m.interpipe.biz>, 
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
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On Wed, Mar 6, 2024 at 10:41=E2=80=AFAM Mina Almasry <almasrymina@google.co=
m> wrote:
>
> On Wed, Mar 6, 2024 at 9:41=E2=80=AFAM Jakub Kicinski <kuba@kernel.org> w=
rote:
> >
> > On Wed, 6 Mar 2024 17:09:57 +0000 Dragos Tatulea wrote:
> > > > Does the caller need to check skb->pp_recycle? pp_recycle seems lik=
e a
> > > > redundant bit. We can tell whether the page is pp by checking
> > > > is_pp_page(page). the pages in the frag must be pp pages when
> > > > skb->pp_recycle is set and must be non pp pages when the
> > > > skb->pp_recycle is not set, so it all seems redundant to me.
> > > >
> > > AFAIU we don't have to check for pp_recycle, at least not in this spe=
cific case.
> >
> > Definitely not something we assuming in a fix that needs to go
> > to stable.
> >
> > So far, AFAIU, it's legal to have an skb without skb->pp_recycle
> > set, which holds full page refs to a PP page.
>
> Interesting. I apologize then I did not realize this combination is
> legal. But I have a follow up question: what is the ref code supposed
> to do in this combination? AFAIU:
>
> - skb->pp_recycle && is_pp_page()
> ref via page_pool_ref_page()
> unref via page_pool_unref_page()
>
> - !skb->pp_recycle && !is_pp_page()
> ref via get_page()
> unref via put_page()
>
> - !skb->pp_recycle && is_pp_page()
> ref via ?
> unref via?
>
> Also is this combination also legal you think? and if so what to do?
> - skb->pp_recycle && !is_pp_page()
> ref via ?
> unref via?
>
> I'm asking because I'm starting to wonder if this patch has some issue in=
 it:
> https://patchwork.kernel.org/project/netdevbpf/patch/20231215033011.12107=
-4-liangchen.linux@gmail.com/
>
> Because in this patch, if we have a !skb->pp_recycle & is_pp_page()
> combination we end up doing in skb_try_coalesce():
> ref via page_pool_ref_page()
> unref via put_page() via eventual napi_frag_unref()
>
> which seems like an issue, no?
>

Gah, nevermind, skb_pp_frag_ref() actually returns -EINVAL if
!skb->pp_recycle, and in the call site we do a skb_frag_ref() on this
error, so all in all we end up doing a get_page/put_page pair. Sorry
for the noise.

So we're supposed to:
- !skb->pp_recycle && is_pp_page()
ref via get_page
unref via put_page

Very subtle stuff (for me at least). I'll try to propose some cleanup
to make this a bit simpler using helpers that handle all these subtle
details internally so that the call sites don't have to do this
special handling.


--=20
Thanks,
Mina