[PATCH 3.16, 4.4, 4.9] dm bufio: drop the lock when doing GFP_NOIO allocation

[Mikulas Patocka <mpatocka@xxxxxxxxxx>]

This is backport of the upstream patch 
41c73a49df31151f4ff868f28fe4f129f113fa2c. It should be backported to 
stable kernels because this performance problem was seen on the android 
4.4 kernel.


commit 41c73a49df31151f4ff868f28fe4f129f113fa2c
Author: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Date:   Wed Nov 23 17:04:00 2016 -0500

    dm bufio: drop the lock when doing GFP_NOIO allocation
    
    If the first allocation attempt using GFP_NOWAIT fails, drop the lock
    and retry using GFP_NOIO allocation (lock is dropped because the
    allocation can take some time).
    
    Note that we won't do GFP_NOIO allocation when we loop for the second
    time, because the lock shouldn't be dropped between __wait_for_free_buffer
    and __get_unclaimed_buffer.
    
    Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
    Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index 9ef88f0e2382..1c2e1dd7ca16 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -822,6 +822,7 @@ enum new_flag {
 static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c, enum new_flag nf)
 {
 	struct dm_buffer *b;
+	bool tried_noio_alloc = false;
 
 	/*
 	 * dm-bufio is resistant to allocation failures (it just keeps
@@ -846,6 +847,15 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
 		if (nf == NF_PREFETCH)
 			return NULL;
 
+		if (dm_bufio_cache_size_latch != 1 && !tried_noio_alloc) {
+			dm_bufio_unlock(c);
+			b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
+			dm_bufio_lock(c);
+			if (b)
+				return b;
+			tried_noio_alloc = true;
+		}
+
 		if (!list_empty(&c->reserved_buffers)) {
 			b = list_entry(c->reserved_buffers.next,
 				       struct dm_buffer, lru_list);