From: Guillaume Nault <redacted>

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/b4d8cb09c913d3e34f853736f3f5628abfd7f4b6.1656699567.git.gnault@redhat.com/ (local)
Signed-off-by: Guillaume Nault <redacted>
---
 include/net/sock.h | 11 +++++++++--
 net/core/sock.c    |  1 +
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index d08cfe190a78..ffba9e95470d 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h

@@ -318,6 +318,9 @@ struct sk_filter;
   *	@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

@@ -504,6 +507,7 @@ struct sock {
 #endif
 	u16			sk_tsflags;
 	u8			sk_shutdown;
+	bool			sk_use_task_frag;
 	atomic_t		sk_tskey;
 	atomic_t		sk_zckey;
 

@@ -2536,14 +2540,17 @@ static inline void sk_stream_moderate_sndbuf(struct sock *sk)
  * 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;
 

diff --git a/net/core/sock.c b/net/core/sock.c
index 788c1372663c..1ab781be9fbe 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c

@@ -3314,6 +3314,7 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 	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);

-- 
2.31.1