[PATCH] seq_file: copy as much as possible to user buffer in seq_read()

[David Wang <00107082@163.com>]

seq_read() yields only seq_file->size bytes to userspace, even when user
buffer is prepare to hold more data. This causes lots of extra *read*
syscalls to fetch data from /proc/*.
For example, on an 8-core system, cat /proc/interrupts needs three
*read*:
	$ strace -T -e read cat /proc/interrupts  > /dev/null
	...
	 43 read(3, "            CPU0       CPU1     "..., 131072) = 4082 <0.000068>
	 44 read(3, "  75:   13490876          0     "..., 131072) = 2936 <0.000148>
	 45 read(3, "", 131072)                     = 0 <0.000010>
On a system with hundreds of cpus, it would need tens of more read call.
A more convincing example is /proc/allocinfo, which is available when
CONFIG_MEM_ALLOC_PROFILING=y. When cat /proc/allocinfo, 4k+ lines need ~100
read calls.

This patch try to fill up user buffer as much as possible, extra read
calls can be avoided, and 2%~10% performance improvement would be
observed:
	$ strace -T -e read cat /proc/interrupts  > /dev/null
	...
	 56 read(3, "            CPU0       CPU1     "..., 131072) = 7018 <0.000208>
	 57 read(3, "", 131072)                     = 0 <0.000010>

Signed-off-by: David Wang <00107082@163.com>
---
 fs/seq_file.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 8bbb1ad46335..2cda43aec4a2 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -220,6 +220,7 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		if (m->count)	// hadn't managed to copy everything
 			goto Done;
 	}
+Restart:
 	// get a non-empty record in the buffer
 	m->from = 0;
 	p = m->op->start(m, &m->index);
@@ -282,6 +283,11 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	copied += n;
 	m->count -= n;
 	m->from = n;
+	/*
+	 * Keep reading in case more data could be copied into user buffer.
+	 */
+	if (m->count == 0)
+		goto Restart;
 Done:
 	if (unlikely(!copied)) {
 		copied = m->count ? -EFAULT : err;
-- 
2.39.2