Commit 8b700862 authored Jul 11, 2018 by Stefano Brivio Committed by David S. Miller Jul 12, 2018

net: Don't copy pfmemalloc flag in __copy_skb_header()



The pfmemalloc flag indicates that the skb was allocated from
the PFMEMALLOC reserves, and the flag is currently copied on skb
copy and clone.

However, an skb copied from an skb flagged with pfmemalloc
wasn't necessarily allocated from PFMEMALLOC reserves, and on
the other hand an skb allocated that way might be copied from an
skb that wasn't.

So we should not copy the flag on skb copy, and rather decide
whether to allow an skb to be associated with sockets unrelated
to page reclaim depending only on how it was allocated.

Move the pfmemalloc flag before headers_start[0] using an
existing 1-bit hole, so that __copy_skb_header() doesn't copy
it.

When cloning, we'll now take care of this flag explicitly,
contravening to the warning comment of __skb_clone().

While at it, restore the newline usage introduced by commit
b1937227 ("net: reorganize sk_buff for faster
__copy_skb_header()") to visually separate bytes used in
bitfields after headers_start[0], that was gone after commit
a9e419dc ("netfilter: merge ctinfo into nfct pointer storage
area"), and describe the pfmemalloc flag in the kernel-doc
structure comment.

This doesn't change the size of sk_buff or cacheline boundaries,
but consolidates the 15 bits hole before tc_index into a 2 bytes
hole before csum, that could now be filled more easily.

Reported-by: Patrick Talbert <ptalbert@redhat.com>
Fixes: c93bdd0e ("netvm: allow skb allocation to use PFMEMALLOC reserves")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 1ff9c66b

include/linux/skbuff.h

+5 −5

Original line number	Diff line number	Diff line
		@@ -630,6 +630,7 @@ typedef unsigned char *sk_buff_data_t;
		* @hash: the packet hash
		* @queue_mapping: Queue mapping for multiqueue devices
		* @xmit_more: More SKBs are pending for this queue
		* @pfmemalloc: skbuff was allocated from PFMEMALLOC reserves
		* @ndisc_nodetype: router type (from link layer)
		* @ooo_okay: allow the mapping of a socket to a queue to be changed
		* @l4_hash: indicate hash is a canonical 4-tuple hash over transport
		@@ -735,7 +736,7 @@ struct sk_buff {
		peeked:1,
		head_frag:1,
		xmit_more:1,
		__unused:1; /* one bit hole */
		pfmemalloc:1;

		/* fields enclosed in headers_start/headers_end are copied
		* using a single memcpy() in __copy_skb_header()
		@@ -754,31 +755,30 @@ struct sk_buff {

		__u8 __pkt_type_offset[0];
		__u8 pkt_type:3;
		__u8 pfmemalloc:1;
		__u8 ignore_df:1;

		__u8 nf_trace:1;
		__u8 ip_summed:2;
		__u8 ooo_okay:1;

		__u8 l4_hash:1;
		__u8 sw_hash:1;
		__u8 wifi_acked_valid:1;
		__u8 wifi_acked:1;

		__u8 no_fcs:1;
		/* Indicates the inner headers are valid in the skbuff. */
		__u8 encapsulation:1;
		__u8 encap_hdr_csum:1;
		__u8 csum_valid:1;

		__u8 csum_complete_sw:1;
		__u8 csum_level:2;
		__u8 csum_not_inet:1;

		__u8 dst_pending_confirm:1;
		#ifdef CONFIG_IPV6_NDISC_NODETYPE
		__u8 ndisc_nodetype:2;
		#endif
		__u8 ipvs_property:1;

		__u8 inner_protocol_type:1;
		__u8 remcsum_offload:1;
		#ifdef CONFIG_NET_SWITCHDEV

net/core/skbuff.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -858,6 +858,8 @@ static struct sk_buff __skb_clone(struct sk_buff n, struct sk_buff *skb)
		n->cloned = 1;
		n->nohdr = 0;
		n->peeked = 0;
		if (skb->pfmemalloc)
		n->pfmemalloc = 1;
		n->destructor = NULL;
		C(tail);
		C(end);

Admin message