[PATCH AUTOSEL 4.20 50/77] writeback: synchronize sync(2) against cgroup writeback membership switches

Sasha Levin
2019-02-15T02:10:24Z

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 7fc5854f8c6efae9e7624970ab49a1eac2faefb1 ]

sync_inodes_sb() can race against cgwb (cgroup writeback) membership
switches and fail to writeback some inodes. For example, if an inode
switches to another wb while sync_inodes_sb() is in progress, the new
wb might not be visible to bdi_split_work_to_wbs() at all or the inode
might jump from a wb which hasn't issued writebacks yet to one which
already has.

This patch adds backing_dev_info->wb_switch_rwsem to synchronize cgwb
switch path against sync_inodes_sb() so that sync_inodes_sb() is
guaranteed to see all the target wbs and inodes can't jump wbs to
escape syncing.

v2: Fixed misplaced rwsem init. Spotted by Jiufei.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jiufei Xue <xuejiufei@gmail.com>
Link: http://lkml.kernel.org/r/dc694ae2-f07f-61e1-7097-7c8411cee12d@gmail.com
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/fs-writeback.c | 40 ++++++++++++++++++++++++++++++--
include/linux/backing-dev-defs.h | 1 +
mm/backing-dev.c | 1 +
3 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index b40168fcc94a..36855c1f8daf 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -331,11 +331,22 @@ struct inode_switch_wbs_context {
struct work_struct work;
};

+static void bdi_down_write_wb_switch_rwsem(struct backing_dev_info *bdi)
+{
+ down_write(&bdi->wb_switch_rwsem);
+}
+
+static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi)
+{
+ up_write(&bdi->wb_switch_rwsem);
+}
+
static void inode_switch_wbs_work_fn(struct work_struct *work)
{
struct inode_switch_wbs_context *isw =
container_of(work, struct inode_switch_wbs_context, work);
struct inode *inode = isw->inode;
+ struct backing_dev_info *bdi = inode_to_bdi(inode);
struct address_space *mapping = inode->i_mapping;
struct bdi_writeback *old_wb = inode->i_wb;
struct bdi_writeback *new_wb = isw->new_wb;
@@ -343,6 +354,12 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
struct page *page;
bool switched = false;

+ /*
+ * If @inode switches cgwb membership while sync_inodes_sb() is
+ * being issued, sync_inodes_sb() might miss it. Synchronize.
+ */
+ down_read(&bdi->wb_switch_rwsem);
+
/*
* By the time control reaches here, RCU grace period has passed
* since I_WB_SWITCH assertion and all wb stat update transactions
@@ -428,6 +445,8 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
spin_unlock(&new_wb->list_lock);
spin_unlock(&old_wb->list_lock);

+ up_read(&bdi->wb_switch_rwsem);
+
if (switched) {
wb_wakeup(new_wb);
wb_put(old_wb);
@@ -468,9 +487,18 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
if (inode->i_state & I_WB_SWITCH)
return;

+ /*
+ * Avoid starting new switches while sync_inodes_sb() is in
+ * progress. Otherwise, if the down_write protected issue path
+ * blocks heavily, we might end up starting a large number of
+ * switches which will block on the rwsem.
+ */
+ if (!down_read_trylock(&bdi->wb_switch_rwsem))
+ return;
+
isw = kzalloc(sizeof(*isw), GFP_ATOMIC);
if (!isw)
- return;
+ goto out_unlock;

/* find and pin the new wb */
rcu_read_lock();
@@ -504,12 +532,14 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
* Let's continue after I_WB_SWITCH is guaranteed to be visible.
*/
call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
- return;
+ goto out_unlock;

out_free:
if (isw->new_wb)
wb_put(isw->new_wb);
kfree(isw);
+out_unlock:
+ up_read(&bdi->wb_switch_rwsem);
}

/**
@@ -887,6 +917,9 @@ fs_initcall(cgroup_writeback_init);

#else /* CONFIG_CGROUP_WRITEBACK */

+static void bdi_down_write_wb_switch_rwsem(struct backing_dev_info *bdi) { }
+static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi) { }
+
static struct bdi_writeback *
locked_inode_to_wb_and_lock_list(struct inode *inode)
__releases(&inode->i_lock)
@@ -2413,8 +2446,11 @@ void sync_inodes_sb(struct super_block *sb)
return;
WARN_ON(!rwsem_is_locked(&sb->s_umount));

+ /* protect against inode wb switch, see inode_switch_wbs_work_fn() */
+ bdi_down_write_wb_switch_rwsem(bdi);
bdi_split_work_to_wbs(bdi, &work, false);
wb_wait_for_completion(bdi, &done);
+ bdi_up_write_wb_switch_rwsem(bdi);

wait_sb_inodes(sb);
}
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index c31157135598..07e02d6df5ad 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -190,6 +190,7 @@ struct backing_dev_info {
struct radix_tree_root cgwb_tree; /* radix tree of active cgroup wbs */
struct rb_root cgwb_congested_tree; /* their congested states */
struct mutex cgwb_release_mutex; /* protect shutdown of wb structs */
+ struct rw_semaphore wb_switch_rwsem; /* no cgwb switch while syncing */
#else
struct bdi_writeback_congested *wb_congested;
#endif
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 8a8bb8796c6c..72e6d0c55cfa 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -689,6 +689,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC);
bdi->cgwb_congested_tree = RB_ROOT;
mutex_init(&bdi->cgwb_release_mutex);
+ init_rwsem(&bdi->wb_switch_rwsem);

ret = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
if (!ret) {
--
2.19.1