netdev - Re: [mm, net-next v2] mm: net: memcg accounting for TCP rx zerocopy

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Date:   Wed, 17 Mar 2021 20:21:23 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Arjun Roy <arjunroy.kdev@...il.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, arjunroy@...gle.com, shakeelb@...gle.com,
        edumazet@...gle.com, soheil@...gle.com, kuba@...nel.org,
        mhocko@...nel.org, hannes@...xchg.org, shy828301@...il.com,
        guro@...com
Subject: Re: [mm, net-next v2] mm: net: memcg accounting for TCP rx zerocopy

On Mon, 15 Mar 2021 18:30:03 -0700 Arjun Roy <arjunroy.kdev@...il.com> wrote:

> From: Arjun Roy <arjunroy@...gle.com>
> 
> TCP zerocopy receive is used by high performance network applications
> to further scale. For RX zerocopy, the memory containing the network
> data filled by the network driver is directly mapped into the address
> space of high performance applications. To keep the TLB cost low,
> these applications unmap the network memory in big batches. So, this
> memory can remain mapped for long time. This can cause a memory
> isolation issue as this memory becomes unaccounted after getting
> mapped into the application address space. This patch adds the memcg
> accounting for such memory.
> 
> Accounting the network memory comes with its own unique challenges.
> The high performance NIC drivers use page pooling to reuse the pages
> to eliminate/reduce expensive setup steps like IOMMU. These drivers
> keep an extra reference on the pages and thus we can not depend on the
> page reference for the uncharging. The page in the pool may keep a
> memcg pinned for arbitrary long time or may get used by other memcg.
> 
> This patch decouples the uncharging of the page from the refcnt and
> associates it with the map count i.e. the page gets uncharged when the
> last address space unmaps it. Now the question is, what if the driver
> drops its reference while the page is still mapped? That is fine as
> the address space also holds a reference to the page i.e. the
> reference count can not drop to zero before the map count.

What tree were you hoping to get this merged through?  I'd suggest net
- it's more likely to get tested over there.

>
> ...
>
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c

These changes could be inside #ifdef CONFIG_NET.  Although I expect
MEMCG=y&&NET=n is pretty damn rare.