[PATCH] fs/select.c: batch user writes in do_sys_poll

[Daniel Axtens <dja@axtens.net>]

When returning results to userspace, do_sys_poll repeatedly calls
put_user() - once per fd that it's watching.

This means that on architectures that support some form of
kernel-to-userspace access protection, we end up enabling and disabling
access once for each file descripter we're watching. This is inefficent
and we can improve things by batching the accesses together.

To make sure there's not too much happening in the window when user
accesses are permitted, we don't walk the linked list with accesses on.
This leads to some slightly messy code in the loop, unfortunately.

Unscientific benchmarking with the poll2_threads microbenchmark from
will-it-scale, run as `./poll2_threads -t 1 -s 15`:

 - Bare-metal Power9 with KUAP: ~48.8% speed-up
 - VM on amd64 laptop with SMAP: ~25.5% speed-up

Signed-off-by: Daniel Axtens <dja@axtens.net>
---
 fs/select.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 7aef49552d4c..f58976da9e63 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -1015,9 +1015,19 @@ static int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds,
 		struct pollfd *fds = walk->entries;
 		int j;
 
+		if (!user_write_access_begin(ufds, (sizeof(struct pollfd) *
+						    walk->len)))
+			goto out_fds;
+
 		for (j = 0; j < walk->len; j++, ufds++)
-			if (__put_user(fds[j].revents, &ufds->revents))
-				goto out_fds;
+			unsafe_put_user(fds[j].revents, &ufds->revents, loop_fault);
+
+		user_write_access_end();
+		continue;
+
+loop_fault:
+		user_write_access_end();
+		goto out_fds;
   	}
 
 	err = fdcount;
-- 
2.25.1