[RFC PATCH 07/15] epoll: extend epitem struct with new members for polling from userspace

[Roman Penyaev <rpenyaev@suse.de>]

 ->bit
       every epitem has an element inside user item array, this bit is
       actually an index position of that user item array and also a
       bit inside ep->items_bm

 ->ready_events
       received events in the period when descriptor can't be polled
       from userspace and ep->rdllist is used for keeping list of
       ready items

 ->work
      work for offloading polling from task context if epfd is polled
      from userspace but driver does not provide pollflags on wakeup

Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 fs/eventpoll.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index ae288f62aa4c..637b463587c1 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -9,6 +9,8 @@
  *
  *  Davide Libenzi <davidel@xmailserver.org>
  *
+ *  Polling from userspace support by Roman Penyaev <rpenyaev@suse.de>
+ *  (C) Copyright 2019 SUSE, All Rights Reserved
  */
 
 #include <linux/init.h>
@@ -42,6 +44,7 @@
 #include <linux/seq_file.h>
 #include <linux/compat.h>
 #include <linux/rculist.h>
+#include <linux/workqueue.h>
 #include <net/busy_poll.h>
 
 /*
@@ -176,6 +179,18 @@ struct epitem {
 
 	/* The structure that describe the interested events and the source fd */
 	struct epoll_event event;
+
+	/* Bit in user bitmap for user polling */
+	unsigned int bit;
+
+	/*
+	 * Collect ready events for the period when descriptor is polled by user
+	 * but events are routed to klists.
+	 */
+	__poll_t ready_events;
+
+	/* Work for offloading event callback */
+	struct work_struct work;
 };
 
 #define EPOLL_USER_HEADER_SIZE  128
@@ -2557,12 +2572,6 @@ static int __init eventpoll_init(void)
 	ep_nested_calls_init(&poll_safewake_ncalls);
 #endif
 
-	/*
-	 * We can have many thousands of epitems, so prevent this from
-	 * using an extra cache line on 64-bit (and smaller) CPUs
-	 */
-	BUILD_BUG_ON(sizeof(void *) <= 8 && sizeof(struct epitem) > 128);
-
 	/* Allocates slab cache used to allocate "struct epitem" items */
 	epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
 			0, SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT, NULL);
-- 
2.19.1