From d1052d236eddf6aa851434db1897b942e8db9921 Mon Sep 17 00:00:00 2001
From: Jinke Han <hanjinke.666@bytedance.com>
Date: Sat, 3 Sep 2022 09:24:29 +0800
Subject: [PATCH] ext4: place buffer head allocation before handle start
Git-commit: d1052d236eddf6aa851434db1897b942e8db9921
Patch-mainline: v6.1-rc1
References: bsc#1207607

In our product environment, we encounter some jbd hung waiting handles to
stop while several writters were doing memory reclaim for buffer head
allocation in delay alloc write path. Ext4 do buffer head allocation with
holding transaction handle which may be blocked too long if the reclaim
works not so smooth. According to our bcc trace, the reclaim time in
buffer head allocation can reach 258s and the jbd transaction commit also
take almost the same time meanwhile. Except for these extreme cases,
we often see several seconds delays for cgroup memory reclaim on our
servers. This is more likely to happen considering docker environment.

One thing to note, the allocation of buffer heads is as often as page
allocation or more often when blocksize less than page size. Just like
page cache allocation, we should also place the buffer head allocation
before startting the handle.

Cc: stable@kernel.org
Signed-off-by: Jinke Han <hanjinke.666@bytedance.com>
Link: https://lore.kernel.org/r/20220903012429.22555-1-hanjinke.666@bytedance.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: Jan Kara <jack@suse.cz>

---
 fs/ext4/inode.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1177,6 +1177,13 @@ retry_grab:
 	page = grab_cache_page_write_begin(mapping, index, flags);
 	if (!page)
 		return -ENOMEM;
+	/*
+	 * The same as page allocation, we prealloc buffer heads before
+	 * starting the handle.
+	 */
+	if (!page_has_buffers(page))
+		create_empty_buffers(page, inode->i_sb->s_blocksize, 0);
+
 	unlock_page(page);
 
 retry_journal: