netdev - Re: [PATCH net] Revert "virtio_net: replace netdev_alloc_skb_ip_align() with napi_alloc

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Date:   Wed, 13 Jan 2021 10:57:36 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH net] Revert "virtio_net: replace netdev_alloc_skb_ip_align()
 with napi_alloc_skb()"

On Wed, Jan 13, 2021 at 10:53 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Tue, Jan 12, 2021 at 09:12:07PM -0800, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > This reverts commit c67f5db82027ba6d2ea4ac9176bc45996a03ae6a.
> >
> > While using page fragments instead of a kmalloc backed skb->head might give
> > a small performance improvement in some cases, there is a huge risk of
> > memory use under estimation.
> >
> > GOOD_COPY_LEN is 128 bytes. This means that we need a small amount
> > of memory to hold the headers and struct skb_shared_info
> >
> > Yet, napi_alloc_skb() might use a whole 32KB page (or 64KB on PowerPc)
> > for long lived incoming TCP packets.
> >
> > We have been tracking OOM issues on GKE hosts hitting tcp_mem limits
> > but consuming far more memory for TCP buffers than instructed in tcp_mem[2]
>
> Are you using virtio on the host then? Is this with a hardware virtio
> device? These do exist, guest is just more common, so I wanted to
> make sure this is not a mistake.
>
> > Even if we force napi_alloc_skb() to only use order-0 pages, the issue
> > would still be there on arches with PAGE_SIZE >= 32768
> >
> > Using alloc_skb() and thus standard kmallloc() for skb->head allocations
> > will get the benefit of letting other objects in each page being independently
> > used by other skbs, regardless of the lifetime.
> >
> > Note that a similar problem exists for skbs allocated from napi_get_frags(),
> > this is handled in a separate patch.
> >
> > I would like to thank Greg Thelen for his precious help on this matter,
> > analysing crash dumps is always a time consuming task.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Paolo Abeni <pabeni@...hat.com>
> > Cc: Michael S. Tsirkin <mst@...hat.com>
> > Cc: Greg Thelen <gthelen@...gle.com>
>
> Just curious - is the way virtio used napi_alloc_skb wrong somehow?
>
> The idea was to benefit from better batching and less play with irq save
> ...
>
> It would be helpful to improve the comments for napi_alloc_skb
> to make it clearer how to use it correctly.
>
> Are other uses of napi_alloc_skb ok?

Yeah, as I mentioned already, the real problem lies in use of
fragments for skb->head in general,
especially for small buffers.

It is too dangerous to use 32KB pages to allocate ~1KB buffers.

I am working on it.