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SubjectRe: [PATCH 1/4] net/skbuff: don't waste memory reserves
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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@virtuozzo.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.

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