From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
To: Eric Dumazet <edumazet@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
Matteo Croce <mcroce@linux.microsoft.com>,
netdev <netdev@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
Ayush Sawal <ayush.sawal@chelsio.com>,
Vinay Kumar Yadav <vinay.yadav@chelsio.com>,
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"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
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Russell King <linux@armlinux.org.uk>,
Mirko Lindner <mlindner@marvell.com>,
Stephen Hemminger <stephen@networkplumber.org>,
Tariq Toukan <tariqt@nvidia.com>,
Jesper Dangaard Brouer <hawk@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
John Fastabend <john.fastabend@gmail.com>,
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Andrew Morton <akpm@linux-foundation.org>,
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Guillaume Nault <gnault@redhat.com>,
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bpf <bpf@vger.kernel.org>, Matthew Wilcox <willy@infradead.org>,
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Lorenzo Bianconi <lorenzo@kernel.org>,
Saeed Mahameed <saeedm@nvidia.com>, Andrew Lunn <andrew@lunn.ch>,
Paolo Abeni <pabeni@redhat.com>,
Sven Auhagen <sven.auhagen@voleatech.de>
Subject: Re: [PATCH net-next v4 2/4] page_pool: Allow drivers to hint on SKB recycling
Date: Wed, 12 May 2021 17:39:33 +0300 [thread overview]
Message-ID: <YJvopUsZHcGb7q24@apalos.home> (raw)
In-Reply-To: <CANn89iLkq0zcbVeRRPGfeb5ZRcnz+e7dR1BCj-RGehNYE1Hzkw@mail.gmail.com>
Hi Eric,
[...]
> > > > + if (skb->pp_recycle && page_pool_return_skb_page(head))
> > >
> > > This probably should be attempted only in the (skb->head_frag) case ?
> >
> > I think the extra check makes sense.
>
> What do you mean here ?
>
I thought you wanted an extra check in the if statement above. So move the
block under the existing if. Something like
if (skb->head_frag) {
#ifdef (CONFIG_PAGE_POOL)
if (skb->pp_recycle && page_pool_return_skb_page(head))
return;
#endif
skb_free_frag(head);
} else {
.....
> >
> > >
> > > Also this patch misses pskb_expand_head()
> >
> > I am not sure I am following. Misses what? pskb_expand_head() will either
> > call skb_release_data() or skb_free_head(), which would either recycle or
> > unmap the buffer for us (depending on the page refcnt)
>
> pskb_expand_head() allocates a new skb->head, from slab.
>
> We should clear skb->pp_recycle for consistency of the skb->head_frag
> clearing we perform there.
Ah right, good catch. I was mostly worried we are not freeing/unmapping
buffers and I completely missed that. I think nothing bad will happen even
if we don't, since the signature will eventually protect us, but it's
definitely the right thing to do.
>
> But then, I now realize you use skb->pp_recycle bit for both skb->head
> and fragments,
> and rely on this PP_SIGNATURE thing (I note that patch 1 changelog
> does not describe why a random page will _not_ have this signature by
> bad luck)
Correct. I've tried to explain in the previous posting as well, but that's
the big difference compared to the initial RFC we sent a few years ago (the
ability to recycle frags as well).
>
> Please document/describe which struct page fields are aliased with
> page->signature ?
>
Sure, any preference on this? Right above page_pool_return_skb_page() ?
Keep in mind the current [1/4] patch is wrong, since it will overlap
pp_signature with mapping. So we'll have interesting results if a page
gets mapped to userspace :).
What Matthew proposed makes sense, we can add something along the lines of:
+ unsigned long pp_magic;
+ struct page_pool *pp;
+ unsigned long _pp_mapping_pad;
+ unsigned long dma_addr[2];
in struct page. In this case page->mapping aliases to pa->_pp_mapping_pad
The first word (that we'll now be using) is used for a pointer or a
compound_head. So as long as pp_magic doesn't resemble a pointer and has
bits 0/1 set to 0 we should be safe.
Thanks!
/Ilias
next prev parent reply other threads:[~2021-05-12 14:39 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-11 13:31 [PATCH net-next v4 0/4] page_pool: recycle buffers Matteo Croce
2021-05-11 13:31 ` [PATCH net-next v4 1/4] mm: add a signature in struct page Matteo Croce
2021-05-11 13:45 ` Matthew Wilcox
2021-05-11 14:11 ` Ilias Apalodimas
2021-05-11 14:18 ` Matthew Wilcox
2021-05-11 14:25 ` Ilias Apalodimas
2021-05-12 15:57 ` Matthew Wilcox
2021-05-12 16:09 ` Eric Dumazet
2021-05-12 16:26 ` Matthew Wilcox
2021-05-12 16:47 ` Ilias Apalodimas
2021-05-13 2:15 ` Yunsheng Lin
2021-05-13 2:35 ` Matthew Wilcox
2021-05-13 3:25 ` Yunsheng Lin
2021-05-11 13:31 ` [PATCH net-next v4 2/4] page_pool: Allow drivers to hint on SKB recycling Matteo Croce
2021-05-11 15:24 ` Eric Dumazet
2021-05-12 9:50 ` Ilias Apalodimas
2021-05-12 14:09 ` Eric Dumazet
2021-05-12 14:39 ` Ilias Apalodimas [this message]
2021-05-11 13:31 ` [PATCH net-next v4 3/4] mvpp2: recycle buffers Matteo Croce
2021-05-11 13:31 ` [PATCH net-next v4 4/4] mvneta: " Matteo Croce
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