Incoming packet is dropped silently by sk_filter(), if the skb was
allocated from pfmemalloc reserves and the corresponding socket is
not marked with the SOCK_MEMALLOC flag.

Igb driver allocates pages for DMA with __skb_alloc_page(), which
calls alloc_pages_node() with the __GFP_MEMALLOC flag. So, in case
of OOM condition, igb can get pages with pfmemalloc flag set.

If an incoming packet hits the pfmemalloc page and is large enough
(small packets are copying into the memory, allocated with
netdev_alloc_skb_ip_align(), so they are not affected), it will be
dropped.

This behavior is ok under high memory pressure, but the problem is
that the igb driver reuses these mapped pages. So, packets are still
dropping even if all memory issues are gone and there is a plenty
of free memory.

In my case, some TCP sessions hang on a small percentage (< 0.1%)
of machines days after OOMs.

Fix this by avoiding reuse of such pages.
Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
Tested-by: Aaron Brown "aaron.f.brown@intel.com"
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>