Re: [PATCH v2] sunrpc: Use GFP_NOFS to prevent use of current->task_frag.
From: Guillaume Nault <hidden>
Date: 2022-10-13 12:18:44
[Adding netdev and Eric, who commented on the original RFC.] On Tue, Oct 11, 2022 at 11:14:36PM +0200, Guillaume Nault wrote:
On Tue, Oct 11, 2022 at 11:57:53AM -0400, Trond Myklebust wrote:quoted
On Tue, 2022-10-11 at 17:00 +0200, Guillaume Nault wrote:quoted
On Mon, Oct 10, 2022 at 11:18:19PM -0700, Christoph Hellwig wrote:quoted
On Mon, Oct 10, 2022 at 06:56:50PM +0200, Guillaume Nault wrote:quoted
That's what my RFC patch did. It was rejected because reading current->flags may incur a cache miss thus slowing down TCP fast path. See the discussion in the Link tag: https://lore.kernel.org/netdev/b4d8cb09c913d3e34f853736f3f5628abfd7f4b6.1656699567.git.gnault@redhat.com/As GFP_NOFS/NOIO are on their way out the networking people will have to do this anyway.We can always think of a nicer solution in the future. But right now we have a real bug to fix. Commit a1231fda7e94 ("SUNRPC: Set memalloc_nofs_save() on all rpciod/xprtiod jobs") introduces a bug that crashes the kernel. I can't see anything wrong with a partial revert.How about instead just adding a dedicated flag to the socket that switches between the two page_frag modes? That would remain future proofed, and it would give kernel users a lever with which to do the right thing without unnecessarily constraining the allocation modes.The problem is to find a hole in struct sock, in a cacheline that wouldn't incur a cache miss.
Okay, so I have this patch that adds a flag in struct sock. The cache
line is shared with ->sk_shutdown and should be hot as ->sk_shutdown is
is tested just before the while() loop in tcp_sendmsg_locked().
Still, that looks like net-next material to me. Reverting sunrpc to use
GFP_NOFS looks better for an immediate bug fix.
------------ >8 ------------
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()).
Fixes: a1231fda7e94 ("SUNRPC: Set memalloc_nofs_save() on all rpciod/xprtiod jobs")
Link: https://lore.kernel.org/netdev/b4d8cb09c913d3e34f853736f3f5628abfd7f4b6.1656699567.git.gnault@redhat.com/
diff --git a/include/net/sock.h b/include/net/sock.h
index 08038a385ef2..bd3eef3afb92 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
@@ -505,6 +508,7 @@ struct sock { #endif u16 sk_tsflags; u8 sk_shutdown; + bool sk_use_task_frag; atomic_t sk_tskey; atomic_t sk_zckey;
@@ -2554,14 +2558,17 @@ 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 + * explicitely 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 ¤t->task_frag;
diff --git a/net/core/sock.c b/net/core/sock.c
index a3ba0358c77c..cc113500d442 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c@@ -3368,6 +3368,7 @@ 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);
diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
index e976007f4fd0..d3170b753dfc 100644
--- a/net/sunrpc/xprtsock.c
+++ b/net/sunrpc/xprtsock.c@@ -1882,6 +1882,7 @@ static int xs_local_finish_connecting(struct rpc_xprt *xprt, sk->sk_write_space = xs_udp_write_space; sk->sk_state_change = xs_local_state_change; sk->sk_error_report = xs_error_report; + sk->sk_use_task_frag = false; xprt_clear_connected(xprt);
@@ -2083,6 +2084,7 @@ static void xs_udp_finish_connecting(struct rpc_xprt *xprt, struct socket *sock) sk->sk_user_data = xprt; sk->sk_data_ready = xs_data_ready; sk->sk_write_space = xs_udp_write_space; + sk->sk_use_task_frag = false; xprt_set_connected(xprt);
@@ -2250,6 +2252,7 @@ static int xs_tcp_finish_connecting(struct rpc_xprt *xprt, struct socket *sock) sk->sk_state_change = xs_tcp_state_change; sk->sk_write_space = xs_tcp_write_space; sk->sk_error_report = xs_error_report; + sk->sk_use_task_frag = false; /* socket options */ sock_reset_flag(sk, SOCK_LINGER);