netdev - Re: [PATCH 1/4] net/skbuff: don't waste memory reserves

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Date:   Thu, 18 Apr 2019 11:55:16 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Florian Westphal <fw@...len.de>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH 1/4] net/skbuff: don't waste memory reserves



On 04/18/2019 11:05 AM, Andrey Ryabinin wrote:
> In some workloads we have noticed packets being dropped by
> sk_filter_trim_cap() because the 'skb' was allocated from pfmemalloc
> reserves:
> 
> 	/*
> 	 * If the skb was allocated from pfmemalloc reserves, only
> 	 * allow SOCK_MEMALLOC sockets to use it as this socket is
> 	 * helping free memory
> 	 */
> 	if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC)) {
> 		NET_INC_STATS(sock_net(sk), LINUX_MIB_PFMEMALLOCDROP);
> 		return -ENOMEM;
> 	}
> 
> Memalloc sockets are used for stuff like swap over NBD or NFS
> and only memalloc sockets can process memalloc skbs. Since we
> don't have any memalloc sockets in our setups we shouldn't have
> memalloc skbs either. It simply doesn't make any sense to waste
> memory reserves on skb which will be dropped anyway.
> 
> It appears that __dev_alloc_pages() unconditionally uses
> __GFP_MEMALLOC, so unless caller added __GFP_NOMEMALLOC, the
> __dev_alloc_pages() may dive into memory reserves.
> Later build_skb() or __skb_fill_page_desc() sets skb->pfmemalloc = 1
> so this skb always dropped by sk_filter_trim_cap().
> 
> Instead of wasting memory reserves we simply shouldn't use them in the
> case of absence memalloc sockets in the system. Do this by adding
> the  __GFP_MEMALLOC only when such socket is present in the system.
> 
> Fixes: 0614002bb5f7 ("netvm: propagate page->pfmemalloc from skb_alloc_page to skb")
> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> ---
>  include/linux/skbuff.h | 17 ++++++++++++++++-
>  include/net/sock.h     | 15 ---------------
>  2 files changed, 16 insertions(+), 16 deletions(-)
>

Hi Andrey

Are you targeting net or net-next tree ?

AFAIK, drivers allocate skbs way before a frame is actually received,
(at RX ring buffer initialization or refill)

So sk_memalloc_socks() might be false at that time, but true later.