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Message-ID: <20160301092011.GA27666@quack.suse.cz>
Date: Tue, 1 Mar 2016 10:20:11 +0100
From: Jan Kara <jack@...e.cz>
To: Tejun Heo <tj@...nel.org>
Cc: Tahsin Erdogan <tahsin@...gle.com>, Jan Kara <jack@...e.cz>,
Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
Theodore Ts'o <tytso@....edu>,
Nauman Rafique <nauman@...gle.com>,
linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>,
Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH v2 block/for-linus] writeback: flush inode cgroup wb
switches instead of pinning super_block
On Mon 29-02-16 18:28:53, Tejun Heo wrote:
> If cgroup writeback is in use, inodes can be scheduled for
> asynchronous wb switching. Before 5ff8eaac1636 ("writeback: keep
> superblock pinned during cgroup writeback association switches"), this
> could race with umount leading to super_block being destroyed while
> inodes are pinned for wb switching. 5ff8eaac1636 fixed it by bumping
> s_active while wb switches are in flight; however, this allowed
> in-flight wb switches to make umounts asynchronous when the userland
> expected synchronosity - e.g. fsck immediately following umount may
> fail because the device is still busy.
>
> This patch removes the problematic super_block pinning and instead
> makes generic_shutdown_super() flush in-flight wb switches. wb
> switches are now executed on a dedicated isw_wq so that they can be
> flushed and isw_nr_in_flight keeps track of the number of in-flight wb
> switches so that flushing can be avoided in most cases.
>
> v2: Move cgroup_writeback_umount() further below and add MS_ACTIVE
> check in inode_switch_wbs() as Jan an Al suggested.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Tahsin Erdogan <tahsin@...gle.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Al Viro <viro@...IV.linux.org.uk>
> Link: http://lkml.kernel.org/g/CAAeU0aNCq7LGODvVGRU-oU_o-6enii5ey0p1c26D1ZzYwkDc5A@mail.gmail.com
> Fixes: 5ff8eaac1636 ("writeback: keep superblock pinned during cgroup writeback association switches")
> Cc: stable@...r.kernel.org #v4.5
The patch looks good to me now. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> ---
> fs/fs-writeback.c | 54 ++++++++++++++++++++++++++++++++++------------
> fs/super.c | 1
> include/linux/writeback.h | 5 ++++
> 3 files changed, 47 insertions(+), 13 deletions(-)
>
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -223,6 +223,9 @@ static void wb_wait_for_completion(struc
> #define WB_FRN_HIST_MAX_SLOTS (WB_FRN_HIST_THR_SLOTS / 2 + 1)
> /* one round can affect upto 5 slots */
>
> +static atomic_t isw_nr_in_flight = ATOMIC_INIT(0);
> +static struct workqueue_struct *isw_wq;
> +
> void __inode_attach_wb(struct inode *inode, struct page *page)
> {
> struct backing_dev_info *bdi = inode_to_bdi(inode);
> @@ -317,7 +320,6 @@ static void inode_switch_wbs_work_fn(str
> struct inode_switch_wbs_context *isw =
> container_of(work, struct inode_switch_wbs_context, work);
> struct inode *inode = isw->inode;
> - struct super_block *sb = inode->i_sb;
> struct address_space *mapping = inode->i_mapping;
> struct bdi_writeback *old_wb = inode->i_wb;
> struct bdi_writeback *new_wb = isw->new_wb;
> @@ -424,8 +426,9 @@ skip_switch:
> wb_put(new_wb);
>
> iput(inode);
> - deactivate_super(sb);
> kfree(isw);
> +
> + atomic_dec(&isw_nr_in_flight);
> }
>
> static void inode_switch_wbs_rcu_fn(struct rcu_head *rcu_head)
> @@ -435,7 +438,7 @@ static void inode_switch_wbs_rcu_fn(stru
>
> /* needs to grab bh-unsafe locks, bounce to work item */
> INIT_WORK(&isw->work, inode_switch_wbs_work_fn);
> - schedule_work(&isw->work);
> + queue_work(isw_wq, &isw->work);
> }
>
> /**
> @@ -471,20 +474,20 @@ static void inode_switch_wbs(struct inod
>
> /* while holding I_WB_SWITCH, no one else can update the association */
> spin_lock(&inode->i_lock);
> -
> - if (inode->i_state & (I_WB_SWITCH | I_FREEING) ||
> - inode_to_wb(inode) == isw->new_wb)
> - goto out_unlock;
> -
> - if (!atomic_inc_not_zero(&inode->i_sb->s_active))
> - goto out_unlock;
> -
> + if (!(inode->i_sb->s_flags & MS_ACTIVE) ||
> + inode->i_state & (I_WB_SWITCH | I_FREEING) ||
> + inode_to_wb(inode) == isw->new_wb) {
> + spin_unlock(&inode->i_lock);
> + goto out_free;
> + }
> inode->i_state |= I_WB_SWITCH;
> spin_unlock(&inode->i_lock);
>
> ihold(inode);
> isw->inode = inode;
>
> + atomic_inc(&isw_nr_in_flight);
> +
> /*
> * In addition to synchronizing among switchers, I_WB_SWITCH tells
> * the RCU protected stat update paths to grab the mapping's
> @@ -494,8 +497,6 @@ static void inode_switch_wbs(struct inod
> call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
> return;
>
> -out_unlock:
> - spin_unlock(&inode->i_lock);
> out_free:
> if (isw->new_wb)
> wb_put(isw->new_wb);
> @@ -847,6 +848,33 @@ restart:
> wb_put(last_wb);
> }
>
> +/**
> + * cgroup_writeback_umount - flush inode wb switches for umount
> + *
> + * This function is called when a super_block is about to be destroyed and
> + * flushes in-flight inode wb switches. An inode wb switch goes through
> + * RCU and then workqueue, so the two need to be flushed in order to ensure
> + * that all previously scheduled switches are finished. As wb switches are
> + * rare occurrences and synchronize_rcu() can take a while, perform
> + * flushing iff wb switches are in flight.
> + */
> +void cgroup_writeback_umount(void)
> +{
> + if (atomic_read(&isw_nr_in_flight)) {
> + synchronize_rcu();
> + flush_workqueue(isw_wq);
> + }
> +}
> +
> +static int __init cgroup_writeback_init(void)
> +{
> + isw_wq = alloc_workqueue("inode_switch_wbs", 0, 0);
> + if (!isw_wq)
> + return -ENOMEM;
> + return 0;
> +}
> +fs_initcall(cgroup_writeback_init);
> +
> #else /* CONFIG_CGROUP_WRITEBACK */
>
> static struct bdi_writeback *
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -415,6 +415,7 @@ void generic_shutdown_super(struct super
> sb->s_flags &= ~MS_ACTIVE;
>
> fsnotify_unmount_inodes(sb);
> + cgroup_writeback_umount();
>
> evict_inodes(sb);
>
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -198,6 +198,7 @@ void wbc_attach_and_unlock_inode(struct
> void wbc_detach_inode(struct writeback_control *wbc);
> void wbc_account_io(struct writeback_control *wbc, struct page *page,
> size_t bytes);
> +void cgroup_writeback_umount(void);
>
> /**
> * inode_attach_wb - associate an inode with its wb
> @@ -301,6 +302,10 @@ static inline void wbc_account_io(struct
> {
> }
>
> +static inline void cgroup_writeback_umount(void)
> +{
> +}
> +
> #endif /* CONFIG_CGROUP_WRITEBACK */
>
> /*
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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