ext4: get discard out of jbd2 commit kthread contex

Right now, discard is issued and waited to be completed in jbd2
commit kthread context after the logs are committed. When large
amount of files are deleted and discard is flooding, jbd2 commit
kthread can be blocked for long time. Then all of the metadata
operations can be blocked to wait the log space.

One case is the page fault path with read mm->mmap_sem held, which
wants to update the file time but has to wait for the log space.
When other threads in the task wants to do mmap, then write mmap_sem
is blocked. Finally all of the following read mmap_sem requirements
are blocked, even the ps command which need to read the /proc/pid/
-cmdline. Our monitor service which needs to read /proc/pid/cmdline
used to be blocked for 5 mins.

This patch frees the blocks back to buddy after commit and then do
discard in a async kworker context in fstrim fashion, namely,
 - mark blocks to be discarded as used if they have not been allocated
 - do discard
 - mark them free
After this, jbd2 commit kthread won't be blocked any more by discard
and we won't get NOSPC even if the discard is slow or throttled.

Link: https://marc.info/?l=linux-kernel&m=162143690731901&w=2
Suggested-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wang Jianchao <wangjianchao@kuaishou.com>
Link: https://lore.kernel.org/r/20210830075246.12516-5-jianchao.wan9@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 3c51e243450d..6b678b968d84 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1536,6 +1536,8 @@ struct ext4_sb_info {
 	unsigned int s_mb_free_pending;
 	struct list_head s_freed_data_list;	/* List of blocks to be freed
 						   after commit completed */
+	struct list_head s_discard_list;
+	struct work_struct s_discard_work;
 	struct rb_root s_mb_avg_fragment_size_root;
 	rwlock_t s_mb_rb_lock;
 	struct list_head *s_mb_largest_free_orders;