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Message-ID: <CA+FuTSergCOgmYCGzT4vrYfoBB_vk-SwF45z2_XEBXNbyHvGUQ@mail.gmail.com>
Date: Wed, 23 Jun 2021 22:21:11 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, netdev@...r.kernel.org,
eric.dumazet@...il.com, dsahern@...il.com, yoshfuji@...ux-ipv6.org,
brouer@...hat.com, Dave Jones <dsj@...com>
Subject: Re: [PATCH net-next v4] net: ip: avoid OOM kills with large UDP sends
over loopback
On Wed, Jun 23, 2021 at 5:44 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> Dave observed number of machines hitting OOM on the UDP send
> path. The workload seems to be sending large UDP packets over
> loopback. Since loopback has MTU of 64k kernel will try to
> allocate an skb with up to 64k of head space. This has a good
> chance of failing under memory pressure. What's worse if
> the message length is <32k the allocation may trigger an
> OOM killer.
>
> This is entirely avoidable, we can use an skb with page frags.
>
> af_unix solves a similar problem by limiting the head
> length to SKB_MAX_ALLOC. This seems like a good and simple
> approach. It means that UDP messages > 16kB will now
> use fragments if underlying device supports SG, if extra
> allocator pressure causes regressions in real workloads
> we can switch to trying the large allocation first and
> falling back.
>
> v4: pre-calculate all the additions to alloclen so
> we can be sure it won't go over order-2
>
> Reported-by: Dave Jones <dsj@...com>
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
> net/ipv4/ip_output.c | 32 ++++++++++++++++++--------------
> net/ipv6/ip6_output.c | 32 +++++++++++++++++---------------
> 2 files changed, 35 insertions(+), 29 deletions(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index c3efc7d658f6..8d8a8da3ae7e 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -1054,7 +1054,7 @@ static int __ip_append_data(struct sock *sk,
> unsigned int datalen;
> unsigned int fraglen;
> unsigned int fraggap;
> - unsigned int alloclen;
> + unsigned int alloclen, alloc_extra;
Separate line?
> unsigned int pagedlen;
> struct sk_buff *skb_prev;
> alloc_new_skb:
> @@ -1074,35 +1074,39 @@ static int __ip_append_data(struct sock *sk,
> fraglen = datalen + fragheaderlen;
> pagedlen = 0;
>
> + alloc_extra = hh_len + 15;
> + alloc_extra += exthdrlen;
> +
> + /* The last fragment gets additional space at tail.
> + * Note, with MSG_MORE we overallocate on fragments,
> + * because we have no idea what fragment will be
> + * the last.
> + */
> + if (datalen == length + fraggap)
> + alloc_extra += rt->dst.trailer_len;
> +
> if ((flags & MSG_MORE) &&
> !(rt->dst.dev->features&NETIF_F_SG))
> alloclen = mtu;
> - else if (!paged)
> + else if (!paged &&
> + (fraglen + alloc_extra < SKB_MAX_ALLOC ||
> + !(rt->dst.dev->features & NETIF_F_SG)))
This perhaps deserves a comment. Something like this?
/* avoid order-3 allocations where possible: replace with frags if
allowed (sg) */
> alloclen = fraglen;
> else {
> alloclen = min_t(int, fraglen, MAX_HEADER);
> pagedlen = fraglen - alloclen;
> }
>
> - alloclen += exthdrlen;
> -
> - /* The last fragment gets additional space at tail.
> - * Note, with MSG_MORE we overallocate on fragments,
> - * because we have no idea what fragment will be
> - * the last.
> - */
> - if (datalen == length + fraggap)
> - alloclen += rt->dst.trailer_len;
> + alloclen += alloc_extra;
>
> if (transhdrlen) {
> - skb = sock_alloc_send_skb(sk,
> - alloclen + hh_len + 15,
> + skb = sock_alloc_send_skb(sk, alloclen,
> (flags & MSG_DONTWAIT), &err);
> } else {
> skb = NULL;
> if (refcount_read(&sk->sk_wmem_alloc) + wmem_alloc_delta <=
> 2 * sk->sk_sndbuf)
> - skb = alloc_skb(alloclen + hh_len + 15,
> + skb = alloc_skb(alloclen,
> sk->sk_allocation);
> if (unlikely(!skb))
> err = -ENOBUFS;
Is there any risk of regressions? If so, would it be preferable to try
regular alloc and only on failure, just below here, do the size and SG
test and if permitted jump back to the last of the three alloc_len
options?
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