From: Arjun Roy <arjunroy@google.com>
To: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
Arjun Roy <arjunroy.kdev@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
David Miller <davem@davemloft.net>,
netdev <netdev@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
Eric Dumazet <edumazet@google.com>,
Soheil Hassas Yeganeh <soheil@google.com>,
Jakub Kicinski <kuba@kernel.org>,
Michal Hocko <mhocko@kernel.org>, Yang Shi <shy828301@gmail.com>,
Roman Gushchin <guro@fb.com>
Subject: Re: [mm, net-next v2] mm: net: memcg accounting for TCP rx zerocopy
Date: Wed, 24 Mar 2021 15:21:25 -0700 [thread overview]
Message-ID: <CAOFY-A0GtpeFUrp+eK1__pOm=gkp3ahNXXkm6rztrz_O2FFfeQ@mail.gmail.com> (raw)
In-Reply-To: <CALvZod527EgYmkqWdoLCARj2BD2=YWVCC9Dk87gfQRG8NViX_A@mail.gmail.com>
On Wed, Mar 24, 2021 at 11:26 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Tue, Mar 23, 2021 at 11:42 AM Arjun Roy <arjunroy@google.com> wrote:
> >
> [...]
> >
> > To summarize then, it seems to me that we're on the same page now.
> > I'll put together a tentative v3 such that:
> > 1. It uses pre-charging, as previously discussed.
> > 2. It uses a page flag to delineate pages of a certain networking sort
> > (ie. this mechanism).
> > 3. It avails itself of up to 4 words of data inside struct page,
> > inside the networking specific struct.
> > 4. And it sets up this opt-in lifecycle notification for drivers that
> > choose to use it, falling back to existing behaviour without.
> >
>
> Arjun, if you don't mind, can you explain how the lifetime of such a
> page will look like?
>
> For example:
>
> Driver:
> page = dev_alloc_page()
> /* page has 1 ref */
Yes, this is the case.
> dev_map_page(page)
> /* I don't think dev_map_page() takes a ref on page, so the ref remains 1. */
>
To be clear, do you mean things like DMA setup here? Or specifically
what do you mean by dev_map_page?
> On incoming traffic the page goes to skb and which then gets assigned
> to a struct sock. Does the kernel increase refcnt of the page on these
> operations?
>
Adding a page to an skb will mean that, when the skb is cleaned up, a
page ref is dropped:
https://github.com/torvalds/linux/blob/master/net/core/skbuff.c#L666
So a driver may bump the refcount for the page, before adding it to the skb:
https://github.com/torvalds/linux/blob/master/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c#L442
> The page gets mapped into user space which increments its refcnt.
>
Yes.
> After processing the data, the application unmaps the page and its
> refcnt will be decremented.
>
Yes.
> __put_page() will be called when refcnt reaches 0, so, the initial
> refcnt which the driver has acquired, has to be transferred to the
> next layer. So, I am trying to understand how that will work?
Ah, I see - there was a miscommunication. Johannes mentioned
__put_page() but I read put_page().
That is where I was planning on adding the interposition for these
network pages.
So in put_page(), if it turns out it's a network page, we do our
handling then as I described in prior emails. Sorry for the confusion.
Thanks,
-Arjun
next prev parent reply other threads:[~2021-03-24 22:21 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-16 4:16 [mm, net-next v2] mm: net: memcg accounting for TCP rx zerocopy Arjun Roy
2021-03-16 4:20 ` Arjun Roy
2021-03-16 4:29 ` Shakeel Butt
2021-03-16 6:22 ` Arjun Roy
2021-03-16 6:28 ` Arjun Roy
2021-03-16 21:02 ` Jakub Kicinski
2021-03-16 10:26 ` Johannes Weiner
2021-03-17 6:05 ` Arjun Roy
2021-03-17 22:12 ` Johannes Weiner
2021-03-22 21:35 ` Arjun Roy
2021-03-23 17:01 ` Johannes Weiner
2021-03-23 18:42 ` Arjun Roy
2021-03-24 18:25 ` Shakeel Butt
2021-03-24 22:21 ` Arjun Roy [this message]
2021-03-23 14:34 ` Michal Hocko
2021-03-23 18:47 ` Arjun Roy
2021-03-24 9:12 ` Michal Hocko
2021-03-24 20:39 ` Arjun Roy
2021-03-24 20:53 ` Shakeel Butt
2021-03-24 21:56 ` Michal Hocko
2021-03-24 21:24 ` Johannes Weiner
2021-03-24 22:49 ` Arjun Roy
2021-03-25 9:02 ` Michal Hocko
2021-03-25 16:47 ` Johannes Weiner
2021-03-25 17:50 ` Michal Hocko
-- strict thread matches above, loose matches on Subject: below --
2021-03-16 1:30 Arjun Roy
2021-03-18 3:21 ` Andrew Morton
2021-03-22 21:19 ` Arjun Roy
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