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/b4d8cb09c913d3e34f853736f3f5628abfd7f4b6.1656699567.git.gnault@redhat.com/
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/net/sock.h b/include/net/sock.h
index ecea3dc..fefe1f4 100644 (file)
--- 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
@@ -512,6 +515,7 @@ struct sock {
        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;
@@ -2561,14 +2565,17 @@ 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 d2587d8..f954d58 100644 (file)
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3390,6 +3390,7 @@ 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);