net: Introduce sk_use_task_frag in struct sock.

Sockets that can be used while recursing into memory reclaim, like
those used by network block devices and file systems, mustn't use
current->task_frag: if the current process is already using it, then
the inner memory reclaim call would corrupt the task_frag structure.

To avoid this, sk_page_frag() uses ->sk_allocation to detect sockets
that mustn't use current->task_frag, assuming that those used during
memory reclaim had their allocation constraints reflected in
->sk_allocation.

This unfortunately doesn't cover all cases: in an attempt to remove all
usage of GFP_NOFS and GFP_NOIO, sunrpc stopped setting these flags in
->sk_allocation, and used memalloc_nofs critical sections instead.
This breaks the sk_page_frag() heuristic since the allocation
constraints are now stored in current->flags, which sk_page_frag()
can't read without risking triggering a cache miss and slowing down
TCP's fast path.

This patch creates a new field in struct sock, named sk_use_task_frag,
which sockets with memory reclaim constraints can set to false if they
can't safely use current->task_frag. In such cases, sk_page_frag() now
always returns the socket's page_frag (->sk_frag). The first user is
sunrpc, which needs to avoid using current->task_frag but can keep
->sk_allocation set to GFP_KERNEL otherwise.

Eventually, it might be possible to simplify sk_page_frag() by only
testing ->sk_use_task_frag and avoid relying on the ->sk_allocation
heuristic entirely (assuming other sockets will set ->sk_use_task_frag
according to their constraints in the future).

The new ->sk_use_task_frag field is placed in a hole in struct sock and
belongs to a cache line shared with ->sk_shutdown. Therefore it should
be hot and shouldn't have negative performance impacts on TCP's fast
path (sk_shutdown is tested just before the while() loop in
tcp_sendmsg_locked()).

Link: https://lore.kernel.org/netdev/[e-mail]/
Signed-off-by: Guillaume Nault <[e-mail]>
Reviewed-by: Benjamin Coddington <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/sock.h

···
   *	@sk_stamp: time stamp of last packet received
   *	@sk_stamp_seq: lock for accessing sk_stamp on 32 bit architectures only
   *	@sk_tsflags: SO_TIMESTAMPING flags
+  *	@sk_use_task_frag: allow sk_page_frag() to use current->task_frag.
+  *			   Sockets that can be used under memory reclaim should
+  *			   set this to false.
   *	@sk_bind_phc: SO_TIMESTAMPING bind PHC index of PTP virtual clock
   *	              for timestamping
   *	@sk_tskey: counter to disambiguate concurrent tstamp requests
···
 	u8			sk_txtime_deadline_mode : 1,
 				sk_txtime_report_errors : 1,
 				sk_txtime_unused : 6;
+	bool			sk_use_task_frag;
 
 	struct socket		*sk_socket;
 	void			*sk_user_data;
···
  * socket operations and end up recursing into sk_page_frag()
  * while it's already in use: explicitly avoid task page_frag
  * usage if the caller is potentially doing any of them.
- * This assumes that page fault handlers use the GFP_NOFS flags.
+ * This assumes that page fault handlers use the GFP_NOFS flags or
+ * explicitly disable sk_use_task_frag.
  *
  * Return: a per task page_frag if context allows that,
  * otherwise a per socket one.
  */
 static inline struct page_frag *sk_page_frag(struct sock *sk)
 {
-	if ((sk->sk_allocation & (__GFP_DIRECT_RECLAIM | __GFP_MEMALLOC | __GFP_FS)) ==
+	if (sk->sk_use_task_frag &&
+	    (sk->sk_allocation & (__GFP_DIRECT_RECLAIM | __GFP_MEMALLOC |
+				  __GFP_FS)) ==
 	    (__GFP_DIRECT_RECLAIM | __GFP_FS))
 		return &current->task_frag;
 

net/core/sock.c

···
 	sk->sk_rcvbuf		=	READ_ONCE(sysctl_rmem_default);
 	sk->sk_sndbuf		=	READ_ONCE(sysctl_wmem_default);
 	sk->sk_state		=	TCP_CLOSE;
+	sk->sk_use_task_frag	=	true;
 	sk_set_socket(sk, sock);
 
 	sock_set_flag(sk, SOCK_ZAPPED);