linux.git/fs/super.c, branch v4.5

writeback: flush inode cgroup wb switches instead of pinning super_block

2016-03-03T21:42:50+00:00

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 
Reported-by: Tahsin Erdogan 
Cc: Jan Kara 
Cc: Al Viro 
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@vger.kernel.org #v4.5
Reviewed-by: Jan Kara 
Tested-by: Tahsin Erdogan 
Signed-off-by: Jens Axboe

Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial

2016-01-15T01:04:19+00:00

Pull trivial tree updates from Jiri Kosina.

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial:
  floppy: make local variable non-static
  exynos: fixes an incorrect header guard
  dt-bindings: fixes some incorrect header guards
  cpufreq-dt: correct dead link in documentation
  cpufreq: ARM big LITTLE: correct dead link in documentation
  treewide: Fix typos in printk
  Documentation: filesystem: Fix typo in fs/eventfd.c
  fs/super.c: use && instead of & for warn_on condition
  Documentation: fix sysfs-ptp
  lib: scatterlist: fix Kconfig description

fs: use block_device name vsprintf helper

2016-01-06T18:03:18+00:00

Signed-off-by: Dmitry Monakhov 
Signed-off-by: Al Viro

fs/super.c: use && instead of & for warn_on condition

2015-12-08T13:50:57+00:00

This fixes the following sparse warning:

  fs/super.c:1202:9: warning: dubious: x & !y

Bitwise and logical and are equivalent here, but logical was intended.
The generated code is identical, with and without CONFIG_LOCKDEP.

Signed-off-by: Vincent Stehlé 
Acked-by: Oleg Nesterov 
Signed-off-by: Jiri Kosina

Merge branch 'superblock-scaling' of git://git.kernel.org/pub/scm/linux/kernel/git/josef/btrfs-next into for-next

2015-08-21T06:31:20+00:00

Conflicts:
	include/linux/fs.h

sync: serialise per-superblock sync operations

2015-08-17T22:39:47+00:00

When competing sync(2) calls walk the same filesystem, they need to
walk the list of inodes on the superblock to find all the inodes
that we need to wait for IO completion on. However, when multiple
wait_sb_inodes() calls do this at the same time, they contend on the
the inode_sb_list_lock and the contention causes system wide
slowdowns. In effect, concurrent sync(2) calls can take longer and
burn more CPU than if they were serialised.

Stop the worst of the contention by adding a per-sb mutex to wrap
around wait_sb_inodes() so that we only execute one sync(2) IO
completion walk per superblock superblock at a time and hence avoid
contention being triggered by concurrent sync(2) calls.

Signed-off-by: Dave Chinner 
Signed-off-by: Josef Bacik 
Reviewed-by: Jan Kara 
Reviewed-by: Christoph Hellwig 
Tested-by: Dave Chinner

inode: convert inode_sb_list_lock to per-sb

2015-08-17T22:39:46+00:00

The process of reducing contention on per-superblock inode lists
starts with moving the locking to match the per-superblock inode
list. This takes the global lock out of the picture and reduces the
contention problems to within a single filesystem. This doesn't get
rid of contention as the locks still have global CPU scope, but it
does isolate operations on different superblocks form each other.

Signed-off-by: Dave Chinner 
Signed-off-by: Josef Bacik 
Reviewed-by: Jan Kara 
Reviewed-by: Christoph Hellwig 
Tested-by: Dave Chinner

change sb_writers to use percpu_rw_semaphore

2015-08-15T11:52:13+00:00

We can remove everything from struct sb_writers except frozen
and add the array of percpu_rw_semaphore's instead.

This patch doesn't remove sb_writers->wait_unfrozen yet, we keep
it for get_super_thawed(). We will probably remove it later.

This change tries to address the following problems:

	- Firstly, __sb_start_write() looks simply buggy. It does
	  __sb_end_write() if it sees ->frozen, but if it migrates
	  to another CPU before percpu_counter_dec(), sb_wait_write()
	  can wrongly succeed if there is another task which holds
	  the same "semaphore": sb_wait_write() can miss the result
	  of the previous percpu_counter_inc() but see the result
	  of this percpu_counter_dec().

	- As Dave Hansen reports, it is suboptimal. The trivial
	  microbenchmark that writes to a tmpfs file in a loop runs
	  12% faster if we change this code to rely on RCU and kill
	  the memory barriers.

	- This code doesn't look simple. It would be better to rely
	  on the generic locking code.

	  According to Dave, this change adds the same performance
	  improvement.

Note: with this change both freeze_super() and thaw_super() will do
synchronize_sched_expedited() 3 times. This is just ugly. But:

	- This will be "fixed" by the rcu_sync changes we are going
	  to merge. After that freeze_super()->percpu_down_write()
	  will use synchronize_sched(), and thaw_super() won't use
	  synchronize() at all.

	  This doesn't need any changes in fs/super.c.

	- Once we merge rcu_sync changes, we can also change super.c
	  so that all wb_write->rw_sem's will share the single ->rss
	  in struct sb_writes, then freeze_super() will need only one
	  synchronize_sched().

Signed-off-by: Oleg Nesterov 
Reviewed-by: Jan Kara

shift percpu_counter_destroy() into destroy_super_work()

2015-08-15T11:52:11+00:00

Of course, this patch is ugly as hell. It will be (partially)
reverted later. We add it to ensure that other WIP changes in
percpu_rw_semaphore won't break fs/super.c.

We do not even need this change right now, percpu_free_rwsem()
is fine in atomic context. But we are going to change this, it
will be might_sleep() after we merge the rcu_sync() patches.

And even after that we do not really need destroy_super_work(),
we will kill it in any case. Instead, destroy_super_rcu() should
just check that rss->cb_state == CB_IDLE and do call_rcu() again
in the (very unlikely) case this is not true.

So this is just the temporary kludge which helps us to avoid the
conflicts with the changes which will be (hopefully) routed via
rcu tree.

Signed-off-by: Oleg Nesterov 
Reviewed-by: Jan Kara

document rwsem_release() in sb_wait_write()

2015-08-15T11:52:09+00:00

Not only we need to avoid the warning from lockdep_sys_exit(), the
caller of freeze_super() can never release this lock. Another thread
can do this, so there is another reason for rwsem_release().

Plus the comment should explain why we have to fool lockdep.

Signed-off-by: Oleg Nesterov 
Reviewed-by: Jan Kara