linux-stable.git/fs/fs-writeback.c, branch v4.2.1

writeback: sync_inodes_sb() must write out I_DIRTY_TIME inodes and always call wait_sb_inodes()

2015-08-25T20:35:09+00:00

e79729123f63 ("writeback: don't issue wb_writeback_work if clean")
updated writeback path to avoid kicking writeback work items if there
are no inodes to be written out; unfortunately, the avoidance logic
was too aggressive and broke sync_inodes_sb().

* sync_inodes_sb() must write out I_DIRTY_TIME inodes but I_DIRTY_TIME
  inodes dont't contribute to bdi/wb_has_dirty_io() tests and were
  being skipped over.

* inodes are taken off wb->b_dirty/io/more_io lists after writeback
  starts on them.  sync_inodes_sb() skipping wait_sb_inodes() when
  bdi_has_dirty_io() breaks it by making it return while writebacks
  are in-flight.

This patch fixes the breakages by

* Removing bdi_has_dirty_io() shortcut from bdi_split_work_to_wbs().
  The callers are already testing the condition.

* Removing bdi_has_dirty_io() shortcut from sync_inodes_sb() so that
  it always calls into bdi_split_work_to_wbs() and wait_sb_inodes().

* Making bdi_split_work_to_wbs() consider the b_dirty_time list for
  WB_SYNC_ALL writebacks.

Kudos to Eryu, Dave and Jan for tracking down the issue.

Signed-off-by: Tejun Heo 
Fixes: e79729123f63 ("writeback: don't issue wb_writeback_work if clean")
Link: http://lkml.kernel.org/g/20150812101204.GE17933@dhcp-13-216.nay.redhat.com
Reported-and-bisected-by: Eryu Guan 
Cc: Dave Chinner 
Cc: Jan Kara 
Cc: Ted Ts'o 
Signed-off-by: Jens Axboe

block: export bio_associate_*() and wbc_account_io()

2015-07-23T19:36:44+00:00

bio_associate_blkcg(), bio_associate_current() and wbc_account_io()
are used to implement cgroup writeback support for filesystems and
thus need to be exported.  Export them.

Signed-off-by: Tejun Heo 
Reported-by: Stephen Rothwell 
Signed-off-by: Jens Axboe

writeback: do foreign inode detection iff cgroup writeback is enabled

2015-06-17T18:47:37+00:00

Currently, even when a filesystem doesn't set the FS_CGROUP_WRITEBACK
flag, if the filesystem uses wbc_init_bio() and wbc_account_io(), the
foreign inode detection and migration logic still ends up activating
cgroup writeback which is unexpected.  This patch ensures that the
foreign inode detection logic stays disabled when inode_cgwb_enabled()
is false by not associating writeback_control's with bdi_writeback's.

This also avoids unnecessary operations in wbc_init_bio(),
wbc_account_io() and wbc_detach_inode() for filesystems which don't
support cgroup writeback.

Signed-off-by: Tejun Heo 
Signed-off-by: Jens Axboe

writeback: disassociate inodes from dying bdi_writebacks

2015-06-02T14:40:20+00:00

For the purpose of foreign inode detection, wb's (bdi_writeback's) are
identified by the associated memcg ID.  As we create a separate wb for
each memcg, this is enough to identify the active wb's; however, when
blkcg is enabled or disabled higher up in the hierarchy, the mapping
between memcg and blkcg changes which in turn creates a new wb to
service the new mapping.  The old wb is unlinked from index and
released after all references are drained.  The foreign inode
detection logic can't detect this condition because both the old and
new wb's point to the same memcg and thus never decides to move inodes
attached to the old wb to the new one.

This patch adds logic to initiate switching immediately in
wbc_attach_and_unlock_inode() if the associated wb is dying.  We can
make the usual foreign detection logic to distinguish the different
wb's mapped to the memcg but the dying wb is never gonna be in active
service again and there's no point in tracking the usage history and
reaching the switch verdict after enough data points are collected.
It's already known that the wb has to be switched.

Signed-off-by: Tejun Heo 
Cc: Jens Axboe 
Cc: Jan Kara 
Cc: Wu Fengguang 
Cc: Greg Thelen 
Signed-off-by: Jens Axboe

writeback: implement foreign cgroup inode bdi_writeback switching

2015-06-02T14:40:20+00:00

As concurrent write sharing of an inode is expected to be very rare
and memcg only tracks page ownership on first-use basis severely
confining the usefulness of such sharing, cgroup writeback tracks
ownership per-inode.  While the support for concurrent write sharing
of an inode is deemed unnecessary, an inode being written to by
different cgroups at different points in time is a lot more common,
and, more importantly, charging only by first-use can too readily lead
to grossly incorrect behaviors (single foreign page can lead to
gigabytes of writeback to be incorrectly attributed).

To resolve this issue, cgroup writeback detects the majority dirtier
of an inode and transfers the ownership to it.  The previous patches
implemented the foreign condition detection mechanism and laid the
groundwork.  This patch implements the actual switching.

With the previously implemented [unlocked_]inode_to_wb_and_list_lock()
and wb stat transaction, grabbing wb->list_lock, inode->i_lock and
mapping->tree_lock gives us full exclusion against all wb operations
on the target inode.  inode_switch_wb_work_fn() grabs all the locks
and transfers the inode atomically along with its RECLAIMABLE and
WRITEBACK stats.

Signed-off-by: Tejun Heo 
Cc: Jens Axboe 
Cc: Jan Kara 
Cc: Wu Fengguang 
Cc: Greg Thelen 
Signed-off-by: Jens Axboe

writeback: add lockdep annotation to inode_to_wb()

2015-06-02T14:40:20+00:00

With the previous three patches, all operations which acquire wb from
inode are either under one of inode->i_lock, mapping->tree_lock or
wb->list_lock or protected by unlocked_inode_to_wb transaction.  This
will be depended upon by foreign inode wb switching.

This patch adds lockdep assertion to inode_to_wb() so that usages
outside the above list locks can be caught easily.  There are three
exceptions.

* locked_inode_to_wb_and_lock_list() is holding wb->list_lock but the
  wb may not be the inode's.  Ensuring that is the function's role
  after all.  Updated to deref inode->i_wb directly.

* inode_wb_stat_unlocked_begin() is usually protected by combination
  of !I_WB_SWITCH and rcu_read_lock().  Updated to deref inode->i_wb
  directly.

* inode_congested() wants to test whether inode->i_wb is set before
  starting the transaction.  Added inode_to_wb_is_valid() which tests
  inode->i_wb directly.

v5: might_lock() removed.  It annotates that the lock is grabbed w/
    irq enabled which isn't the case and triggering lockdep warning
    spuriously.

v4: might_lock() added to unlocked_inode_to_wb_begin().

v3: inode_congested() conversion added.

v2: locked_inode_to_wb_and_lock_list() was missing in the first
    version.

Signed-off-by: Tejun Heo 
Cc: Jens Axboe 
Cc: Jan Kara 
Cc: Wu Fengguang 
Cc: Greg Thelen 
Signed-off-by: Jens Axboe

writeback: use unlocked_inode_to_wb transaction in inode_congested()

2015-06-02T14:40:20+00:00

Similar to wb stat updates, inode_congested() accesses the associated
wb of an inode locklessly, which will break with foreign inode wb
switching.  This path updates inode_congested() to use unlocked inode
wb access transaction introduced by the previous patch.

Combined with the previous two patches, this makes all wb list and
access operations to be protected by either of inode->i_lock,
wb->list_lock, or mapping->tree_lock while wb switching is in
progress.

Signed-off-by: Tejun Heo 
Cc: Jens Axboe 
Cc: Jan Kara 
Cc: Wu Fengguang 
Cc: Greg Thelen 
Signed-off-by: Jens Axboe

writeback: implement unlocked_inode_to_wb transaction and use it for stat updates

2015-06-02T14:40:20+00:00

The mechanism for detecting whether an inode should switch its wb
(bdi_writeback) association is now in place.  This patch build the
framework for the actual switching.

This patch adds a new inode flag I_WB_SWITCHING, which has two
functions.  First, the easy one, it ensures that there's only one
switching in progress for a give inode.  Second, it's used as a
mechanism to synchronize wb stat updates.

The two stats, WB_RECLAIMABLE and WB_WRITEBACK, aren't event counters
but track the current number of dirty pages and pages under writeback
respectively.  As such, when an inode is moved from one wb to another,
the inode's portion of those stats have to be transferred together;
unfortunately, this is a bit tricky as those stat updates are percpu
operations which are performed without holding any lock in some
places.

This patch solves the problem in a similar way as memcg.  Each such
lockless stat updates are wrapped in transaction surrounded by
unlocked_inode_to_wb_begin/end().  During normal operation, they map
to rcu_read_lock/unlock(); however, if I_WB_SWITCHING is asserted,
mapping->tree_lock is grabbed across the transaction.

In turn, the switching path sets I_WB_SWITCHING and waits for a RCU
grace period to pass before actually starting to switch, which
guarantees that all stat update paths are synchronizing against
mapping->tree_lock.

This patch still doesn't implement the actual switching.

v3: Updated on top of the recent cancel_dirty_page() updates.
    unlocked_inode_to_wb_begin() now nests inside
    mem_cgroup_begin_page_stat() to match the locking order.

v2: The i_wb access transaction will be used for !stat accesses too.
    Function names and comments updated accordingly.

    s/inode_wb_stat_unlocked_{begin|end}/unlocked_inode_to_wb_{begin|end}/
    s/switch_wb/switch_wbs/

Signed-off-by: Tejun Heo 
Cc: Jens Axboe 
Cc: Jan Kara 
Cc: Wu Fengguang 
Cc: Greg Thelen 
Signed-off-by: Jens Axboe

writeback: implement [locked_]inode_to_wb_and_lock_list()

2015-06-02T14:40:20+00:00

cgroup writeback currently assumes that inode to wb association
doesn't change; however, with the planned foreign inode wb switching
mechanism, the association will change dynamically.

When an inode needs to be put on one of the IO lists of its wb, the
current code simply calls inode_to_wb() and locks the returned wb;
however, with the planned wb switching, the association may change
before locking the wb and may even get released.

This patch implements [locked_]inode_to_wb_and_lock_list() which pins
the associated wb while holding i_lock, releases it, acquires
wb->list_lock and verifies that the association hasn't changed
inbetween.  As the association will be protected by both locks among
other things, this guarantees that the wb is the inode's associated wb
until the list_lock is released.

Signed-off-by: Tejun Heo 
Cc: Jens Axboe 
Cc: Jan Kara 
Cc: Wu Fengguang 
Cc: Greg Thelen 
Signed-off-by: Jens Axboe

writeback: implement foreign cgroup inode detection

2015-06-02T14:40:20+00:00

As concurrent write sharing of an inode is expected to be very rare
and memcg only tracks page ownership on first-use basis severely
confining the usefulness of such sharing, cgroup writeback tracks
ownership per-inode.  While the support for concurrent write sharing
of an inode is deemed unnecessary, an inode being written to by
different cgroups at different points in time is a lot more common,
and, more importantly, charging only by first-use can too readily lead
to grossly incorrect behaviors (single foreign page can lead to
gigabytes of writeback to be incorrectly attributed).

To resolve this issue, cgroup writeback detects the majority dirtier
of an inode and will transfer the ownership to it.  To avoid
unnnecessary oscillation, the detection mechanism keeps track of
history and gives out the switch verdict only if the foreign usage
pattern is stable over a certain amount of time and/or writeback
attempts.

The detection mechanism has fairly low space and computation overhead.
It adds 8 bytes to struct inode (one int and two u16's) and minimal
amount of calculation per IO.  The detection mechanism converges to
the correct answer usually in several seconds of IO time when there's
a clear majority dirtier.  Even when there isn't, it can reach an
acceptable answer fairly quickly under most circumstances.

Please see wb_detach_inode() for more details.

This patch only implements detection.  Following patches will
implement actual switching.

v2: wbc_account_io() now checks whether the wbc is associated with a
    wb before dereferencing it.  This can happen when pageout() is
    writing pages directly without going through the usual writeback
    path.  As pageout() path is single-threaded, we don't want it to
    be blocked behind a slow cgroup and ultimately want it to delegate
    actual writing to the usual writeback path.

Signed-off-by: Tejun Heo 
Cc: Jens Axboe 
Cc: Jan Kara 
Cc: Wu Fengguang 
Cc: Greg Thelen 
Signed-off-by: Jens Axboe