From fb87bd47516d9a26b6d549231aa743b20fd4a569 Mon Sep 17 00:00:00 2001 From: Guillaume Nault Date: Fri, 16 Dec 2022 07:45:26 -0500 Subject: 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 Reviewed-by: Benjamin Coddington Signed-off-by: Jakub Kicinski --- include/net/sock.h | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) (limited to 'include/net') diff --git a/include/net/sock.h b/include/net/sock.h index ecea3dcc2217..fefe1f4abf19 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 @@ -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 ¤t->task_frag; -- cgit v1.2.3 From 08f65892c5ee15806dce7259e06c384b8cd768d7 Mon Sep 17 00:00:00 2001 From: Benjamin Coddington Date: Fri, 16 Dec 2022 07:45:28 -0500 Subject: net: simplify sk_page_frag Now that in-kernel socket users that may recurse during reclaim have benn converted to sk_use_task_frag = false, we can have sk_page_frag() simply check that value. Signed-off-by: Benjamin Coddington Reviewed-by: Guillaume Nault Signed-off-by: Jakub Kicinski --- include/net/sock.h | 9 ++------- 1 file changed, 2 insertions(+), 7 deletions(-) (limited to 'include/net') diff --git a/include/net/sock.h b/include/net/sock.h index fefe1f4abf19..dcd72e6285b2 100644 --- a/include/net/sock.h +++ b/include/net/sock.h @@ -2564,19 +2564,14 @@ static inline void sk_stream_moderate_sndbuf(struct sock *sk) * Both direct reclaim and page faults can nest inside other * 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 or - * explicitly disable sk_use_task_frag. + * when users 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_use_task_frag && - (sk->sk_allocation & (__GFP_DIRECT_RECLAIM | __GFP_MEMALLOC | - __GFP_FS)) == - (__GFP_DIRECT_RECLAIM | __GFP_FS)) + if (sk->sk_use_task_frag) return ¤t->task_frag; return &sk->sk_frag; -- cgit v1.2.3