[PATCH 05/11] writeback: support > 1 flusher thread per bdi

From: Jens Axboe
Date: Mon May 18 2009 - 08:25:54 EST

Build on the bdi_writeback support by allowing registration of
more than 1 flusher thread. File systems can call bdi_add_flusher_task(bdi)
to add more flusher threads to the device. If they do so, they must also
provide a super_operations function to return the suitable bdi_writeback
struct from any given inode.

Signed-off-by: Jens Axboe <jens.axboe@xxxxxxxxxx>
---
fs/fs-writeback.c | 328 ++++++++++++++++++++++++++++++++-----------
include/linux/backing-dev.h | 31 ++++-
include/linux/fs.h | 3 +
mm/backing-dev.c | 257 ++++++++++++++++++++++++++--------
mm/page-writeback.c | 4 +-
5 files changed, 479 insertions(+), 144 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 50e21e8..efdce88 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -34,84 +34,175 @@
*/
int nr_pdflush_threads;

-/**
- * writeback_acquire - attempt to get exclusive writeback access to a device
- * @bdi: the device's backing_dev_info structure
- *
- * It is a waste of resources to have more than one pdflush thread blocked on
- * a single request queue. Exclusion at the request_queue level is obtained
- * via a flag in the request_queue's backing_dev_info.state.
- *
- * Non-request_queue-backed address_spaces will share default_backing_dev_info,
- * unless they implement their own. Which is somewhat inefficient, as this
- * may prevent concurrent writeback against multiple devices.
+static void generic_sync_wb_inodes(struct bdi_writeback *wb,
+ struct super_block *sb,
+ struct writeback_control *wbc);
+
+/*
+ * Work items for the bdi_writeback threads
*/
-static int writeback_acquire(struct bdi_writeback *wb)
+struct bdi_work {
+ struct list_head list;
+ struct rcu_head rcu_head;
+
+ unsigned long seen;
+ atomic_t pending;
+
+ unsigned long sb_data;
+ unsigned long nr_pages;
+
+ unsigned long state;
+};
+
+static struct super_block *bdi_work_sb(struct bdi_work *work)
{
- struct backing_dev_info *bdi = wb->bdi;
+ return (struct super_block *) (work->sb_data & ~1UL);
+}
+
+static inline bool bdi_work_on_stack(struct bdi_work *work)
+{
+ return work->sb_data & 1UL;
+}
+
+static inline void bdi_work_init(struct bdi_work *work, struct super_block *sb,
+ unsigned long nr_pages)
+{
+ INIT_RCU_HEAD(&work->rcu_head);
+ work->sb_data = (unsigned long) sb;
+ work->nr_pages = nr_pages;
+ work->state = 0;
+}

- return !test_and_set_bit(wb->nr, &bdi->wb_active);
+static inline void bdi_work_init_on_stack(struct bdi_work *work,
+ struct super_block *sb,
+ unsigned long nr_pages)
+{
+ bdi_work_init(work, sb, nr_pages);
+ set_bit(0, &work->state);
+ work->sb_data |= 1UL;
}

/**
* writeback_in_progress - determine whether there is writeback in progress
* @bdi: the device's backing_dev_info structure.
*
- * Determine whether there is writeback in progress against a backing device.
+ * Determine whether there is writeback waiting to be handled against a
+ * backing device.
*/
int writeback_in_progress(struct backing_dev_info *bdi)
{
- return bdi->wb_active != 0;
+ return !list_empty(&bdi->work_list);
}

-/**
- * writeback_release - relinquish exclusive writeback access against a device.
- * @bdi: the device's backing_dev_info structure
- */
-static void writeback_release(struct bdi_writeback *wb)
+static void bdi_work_free(struct rcu_head *head)
{
- struct backing_dev_info *bdi = wb->bdi;
+ struct bdi_work *work = container_of(head, struct bdi_work, rcu_head);

- wb->nr_pages = 0;
- wb->sb = NULL;
- clear_bit(wb->nr, &bdi->wb_active);
+ if (!bdi_work_on_stack(work))
+ kfree(work);
+ else {
+ clear_bit(0, &work->state);
+ wake_up_bit(&work->state, 0);
+ }
}

-static void wb_start_writeback(struct bdi_writeback *wb, struct super_block *sb,
- long nr_pages)
+static void wb_clear_pending(struct bdi_writeback *wb, struct bdi_work *work)
{
- if (!wb_has_dirty_io(wb))
- return;
+ /*
+ * The caller has retrieved the work arguments from this work,
+ * drop our reference. If this is the last ref, delete and free it
+ */
+ if (atomic_dec_and_test(&work->pending)) {
+ struct backing_dev_info *bdi = wb->bdi;

- if (writeback_acquire(wb)) {
- wb->nr_pages = nr_pages;
- wb->sb = sb;
+ spin_lock(&bdi->wb_lock);
+ list_del_rcu(&work->list);
+ spin_unlock(&bdi->wb_lock);
+
+ call_rcu(&work->rcu_head, bdi_work_free);
+ }
+}
+
+static void wb_start_writeback(struct bdi_writeback *wb, struct bdi_work *work)
+{
+ /*
+ * If we failed allocating the bdi work item, wake up the wb thread
+ * always. As a safety precaution, it'll flush out everything
+ */
+ if (!wb_has_dirty_io(wb) && work)
+ wb_clear_pending(wb, work);
+ else
+ wake_up(&wb->wait);
+}
+
+static int bdi_queue_writeback(struct backing_dev_info *bdi,
+ struct bdi_work *work)
+{
+ if (work) {
+ work->seen = bdi->wb_mask;
+ atomic_set(&work->pending, bdi->wb_cnt);

/*
- * make above store seen before the task is woken
+ * Make sure stores are seen before it appears on the list
*/
smp_mb();
- wake_up(&wb->wait);
+
+ spin_lock(&bdi->wb_lock);
+ list_add_tail_rcu(&work->list, &bdi->work_list);
+ spin_unlock(&bdi->wb_lock);
}
-}

-int bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
- long nr_pages)
-{
/*
* This only happens the first time someone kicks this bdi, so put
* it out-of-line.
*/
- if (unlikely(!bdi->wb.task)) {
+ if (unlikely(list_empty_careful(&bdi->wb_list))) {
bdi_add_default_flusher_task(bdi);
return 1;
}

- wb_start_writeback(&bdi->wb, sb, nr_pages);
+ if (!bdi_wblist_needs_lock(bdi))
+ wb_start_writeback(&bdi->wb, work);
+ else {
+ struct bdi_writeback *wb;
+ int idx;
+
+ idx = srcu_read_lock(&bdi->srcu);
+
+ list_for_each_entry_rcu(wb, &bdi->wb_list, list)
+ wb_start_writeback(wb, work);
+
+ srcu_read_unlock(&bdi->srcu, idx);
+ }
+
return 0;
}

/*
+ * Used for on-stack allocated work items. The caller needs to wait until
+ * the wb threads have acked the work before it's safe to continue.
+ */
+static void bdi_wait_on_work_start(struct bdi_work *work)
+{
+ wait_on_bit(&work->state, 0, bdi_sched_wait, TASK_UNINTERRUPTIBLE);
+}
+
+int bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
+ long nr_pages)
+{
+ struct bdi_work work;
+ int ret;
+
+ bdi_work_init_on_stack(&work, sb, nr_pages);
+
+ ret = bdi_queue_writeback(bdi, &work);
+
+ bdi_wait_on_work_start(&work);
+
+ return ret;
+}
+
+/*
* The maximum number of pages to writeout in a single bdi flush/kupdate
* operation. We do this so we don't hold I_SYNC against an inode for
* enormous amounts of time, which would block a userspace task which has
@@ -160,18 +251,15 @@ static void wb_kupdated(struct bdi_writeback *wb)
wbc.more_io = 0;
wbc.encountered_congestion = 0;
wbc.nr_to_write = MAX_WRITEBACK_PAGES;
- generic_sync_bdi_inodes(NULL, &wbc);
+ generic_sync_wb_inodes(wb, NULL, &wbc);
if (wbc.nr_to_write > 0)
break; /* All the old data is written */
nr_to_write -= MAX_WRITEBACK_PAGES;
}
}

-static void generic_sync_wb_inodes(struct bdi_writeback *wb,
- struct super_block *sb,
- struct writeback_control *wbc);
-
-static void wb_writeback(struct bdi_writeback *wb)
+static void __wb_writeback(struct bdi_writeback *wb, long nr_pages,
+ struct super_block *sb)
{
struct writeback_control wbc = {
.bdi = wb->bdi,
@@ -179,10 +267,10 @@ static void wb_writeback(struct bdi_writeback *wb)
.older_than_this = NULL,
.range_cyclic = 1,
};
- long nr_pages = wb->nr_pages;

for (;;) {
unsigned long background_thresh, dirty_thresh;
+
get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
if ((global_page_state(NR_FILE_DIRTY) +
global_page_state(NR_UNSTABLE_NFS) < background_thresh) &&
@@ -193,7 +281,7 @@ static void wb_writeback(struct bdi_writeback *wb)
wbc.encountered_congestion = 0;
wbc.nr_to_write = MAX_WRITEBACK_PAGES;
wbc.pages_skipped = 0;
- generic_sync_wb_inodes(wb, wb->sb, &wbc);
+ generic_sync_wb_inodes(wb, sb, &wbc);
nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
/*
* If we ran out of stuff to write, bail unless more_io got set
@@ -207,69 +295,135 @@ static void wb_writeback(struct bdi_writeback *wb)
}

/*
+ * Return the next bdi_work struct that hasn't been processed by this
+ * wb thread yet
+ */
+static struct bdi_work *get_next_work_item(struct backing_dev_info *bdi,
+ struct bdi_writeback *wb)
+{
+ struct bdi_work *work, *ret = NULL;
+
+ rcu_read_lock();
+
+ list_for_each_entry_rcu(work, &bdi->work_list, list) {
+ if (!test_and_clear_bit(wb->nr, &work->seen))
+ continue;
+
+ ret = work;
+ break;
+ }
+
+ rcu_read_unlock();
+ return ret;
+}
+
+static void wb_writeback(struct bdi_writeback *wb)
+{
+ struct backing_dev_info *bdi = wb->bdi;
+ struct bdi_work *work;
+
+ while ((work = get_next_work_item(bdi, wb)) != NULL) {
+ struct super_block *sb = bdi_work_sb(work);
+ long nr_pages = work->nr_pages;
+
+ wb_clear_pending(wb, work);
+ __wb_writeback(wb, nr_pages, sb);
+ }
+}
+
+/*
+ * This will be inlined in bdi_writeback_task() once we get rid of any
+ * dirty inodes on the default_backing_dev_info
+ */
+static void wb_do_writeback(struct bdi_writeback *wb)
+{
+ /*
+ * We get here in two cases:
+ *
+ * schedule_timeout() returned because the dirty writeback
+ * interval has elapsed. If that happens, the work item list
+ * will be empty and we will proceed to do kupdated style writeout.
+ *
+ * Someone called bdi_start_writeback(), which put one/more work
+ * items on the work_list. Process those.
+ */
+ if (list_empty(&wb->bdi->work_list))
+ wb_kupdated(wb);
+ else
+ wb_writeback(wb);
+}
+
+/*
* Handle writeback of dirty data for the device backed by this bdi. Also
* wakes up periodically and does kupdated style flushing.
*/
int bdi_writeback_task(struct bdi_writeback *wb)
{
+ DEFINE_WAIT(wait);
+
while (!kthread_should_stop()) {
unsigned long wait_jiffies;
- DEFINE_WAIT(wait);
+
+ wb_do_writeback(wb);

prepare_to_wait(&wb->wait, &wait, TASK_INTERRUPTIBLE);
wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
schedule_timeout(wait_jiffies);
try_to_freeze();
-
- /*
- * We get here in two cases:
- *
- * schedule_timeout() returned because the dirty writeback
- * interval has elapsed. If that happens, we will be able
- * to acquire the writeback lock and will proceed to do
- * kupdated style writeout.
- *
- * Someone called bdi_start_writeback(), which will acquire
- * the writeback lock. This means our writeback_acquire()
- * below will fail and we call into bdi_pdflush() for
- * pdflush style writeout.
- *
- */
- if (writeback_acquire(wb))
- wb_kupdated(wb);
- else
- wb_writeback(wb);
-
- writeback_release(wb);
- finish_wait(&wb->wait, &wait);
}

+ finish_wait(&wb->wait, &wait);
return 0;
}

void bdi_writeback_all(struct super_block *sb, long nr_pages)
{
- struct backing_dev_info *bdi;
+ struct list_head *entry = &bdi_list;

rcu_read_lock();

-restart:
- list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
+ list_for_each_continue_rcu(entry, &bdi_list) {
+ struct backing_dev_info *bdi;
+ struct list_head *next;
+ struct bdi_work *work;
+
+ bdi = list_entry(entry, struct backing_dev_info, bdi_list);
if (!bdi_has_dirty_io(bdi))
continue;
- if (bdi_start_writeback(bdi, sb, nr_pages))
- goto restart;
+
+ /*
+ * If this allocation fails, we just wakeup the thread and
+ * let it do kupdate writeback
+ */
+ work = kmalloc(sizeof(*work), GFP_ATOMIC);
+ if (work)
+ bdi_work_init(work, sb, nr_pages);
+
+ /*
+ * Prepare to start from previous entry if this one gets moved
+ * to the bdi_pending list.
+ */
+ next = entry->prev;
+ if (bdi_queue_writeback(bdi, work))
+ entry = next;
}

rcu_read_unlock();
}

/*
- * We have only a single wb per bdi, so just return that.
+ * If the filesystem didn't provide a way to map an inode to a dedicated
+ * flusher thread, it doesn't support more than 1 thread. So we know it's
+ * the default thread, return that.
*/
static inline struct bdi_writeback *inode_get_wb(struct inode *inode)
{
- return &inode_to_bdi(inode)->wb;
+ const struct super_operations *sop = inode->i_sb->s_op;
+
+ if (!sop->inode_get_wb)
+ return &inode_to_bdi(inode)->wb;
+
+ return sop->inode_get_wb(inode);
}

/**
@@ -723,8 +877,24 @@ void generic_sync_bdi_inodes(struct super_block *sb,
struct writeback_control *wbc)
{
struct backing_dev_info *bdi = wbc->bdi;
+ struct bdi_writeback *wb;
+
+ /*
+ * Common case is just a single wb thread and that is embedded in
+ * the bdi, so it doesn't need locking
+ */
+ if (!bdi_wblist_needs_lock(bdi))
+ generic_sync_wb_inodes(&bdi->wb, sb, wbc);
+ else {
+ int idx;

- generic_sync_wb_inodes(&bdi->wb, sb, wbc);
+ idx = srcu_read_lock(&bdi->srcu);
+
+ list_for_each_entry_rcu(wb, &bdi->wb_list, list)
+ generic_sync_wb_inodes(wb, sb, wbc);
+
+ srcu_read_unlock(&bdi->srcu, idx);
+ }
}

/*
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index a0c70f1..6ccfa35 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -13,6 +13,8 @@
#include <linux/proportions.h>
#include <linux/kernel.h>
#include <linux/fs.h>
+#include <linux/sched.h>
+#include <linux/srcu.h>
#include <asm/atomic.h>

struct page;
@@ -25,6 +27,7 @@ struct dentry;
enum bdi_state {
BDI_pending, /* On its way to being activated */
BDI_wb_alloc, /* Default embedded wb allocated */
+ BDI_wblist_lock, /* bdi->wb_list now needs locking */
BDI_async_congested, /* The async (write) queue is getting full */
BDI_sync_congested, /* The sync queue is getting full */
BDI_unused, /* Available bits start here */
@@ -41,6 +44,8 @@ enum bdi_stat_item {
#define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))

struct bdi_writeback {
+ struct list_head list; /* hangs off the bdi */
+
struct backing_dev_info *bdi; /* our parent bdi */
unsigned int nr;

@@ -49,13 +54,13 @@ struct bdi_writeback {
struct list_head b_dirty; /* dirty inodes */
struct list_head b_io; /* parked for writeback */
struct list_head b_more_io; /* parked for more writeback */
-
- unsigned long nr_pages;
- struct super_block *sb;
};

+#define BDI_MAX_FLUSHERS 32
+
struct backing_dev_info {
struct rcu_head rcu_head;
+ struct srcu_struct srcu; /* for wb_list read side protection */
struct list_head bdi_list;
unsigned long ra_pages; /* max readahead in PAGE_CACHE_SIZE units */
unsigned long state; /* Always use atomic bitops on this */
@@ -74,8 +79,12 @@ struct backing_dev_info {
unsigned int max_ratio, max_prop_frac;

struct bdi_writeback wb; /* default writeback info for this bdi */
- unsigned long wb_active; /* bitmap of active tasks */
- unsigned long wb_mask; /* number of registered tasks */
+ spinlock_t wb_lock; /* protects update side of wb_list */
+ struct list_head wb_list; /* the flusher threads hanging off this bdi */
+ unsigned long wb_mask; /* bitmask of registered tasks */
+ unsigned int wb_cnt; /* number of registered tasks */
+
+ struct list_head work_list;

struct device *dev;

@@ -97,11 +106,17 @@ int bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
int bdi_writeback_task(struct bdi_writeback *wb);
void bdi_writeback_all(struct super_block *sb, long nr_pages);
void bdi_add_default_flusher_task(struct backing_dev_info *bdi);
+void bdi_add_flusher_task(struct backing_dev_info *bdi);
int bdi_has_dirty_io(struct backing_dev_info *bdi);

extern spinlock_t bdi_lock;
extern struct list_head bdi_list;

+static inline int bdi_wblist_needs_lock(struct backing_dev_info *bdi)
+{
+ return test_bit(BDI_wblist_lock, &bdi->state);
+}
+
static inline int wb_has_dirty_io(struct bdi_writeback *wb)
{
return !list_empty(&wb->b_dirty) ||
@@ -314,4 +329,10 @@ static inline bool mapping_cap_swap_backed(struct address_space *mapping)
return bdi_cap_swap_backed(mapping->backing_dev_info);
}

+static inline int bdi_sched_wait(void *word)
+{
+ schedule();
+ return 0;
+}
+
#endif /* _LINUX_BACKING_DEV_H */
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ecdc544..d3bda5d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1550,11 +1550,14 @@ extern ssize_t vfs_readv(struct file *, const struct iovec __user *,
extern ssize_t vfs_writev(struct file *, const struct iovec __user *,
unsigned long, loff_t *);

+struct bdi_writeback;
+
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);
void (*destroy_inode)(struct inode *);

void (*dirty_inode) (struct inode *);
+ struct bdi_writeback *(*inode_get_wb) (struct inode *);
int (*write_inode) (struct inode *, int);
void (*drop_inode) (struct inode *);
void (*delete_inode) (struct inode *);
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 677a8c6..b4bcb14 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -199,7 +199,42 @@ static int __init default_bdi_init(void)
}
subsys_initcall(default_bdi_init);

-static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
+static int wb_assign_nr(struct backing_dev_info *bdi, struct bdi_writeback *wb)
+{
+ unsigned long mask = BDI_MAX_FLUSHERS - 1;
+ unsigned int nr;
+
+ do {
+ if ((bdi->wb_mask & mask) == mask)
+ return 1;
+
+ nr = find_first_zero_bit(&bdi->wb_mask, BDI_MAX_FLUSHERS);
+ } while (test_and_set_bit(nr, &bdi->wb_mask));
+
+ wb->nr = nr;
+
+ spin_lock(&bdi->wb_lock);
+ bdi->wb_cnt++;
+ spin_unlock(&bdi->wb_lock);
+
+ return 0;
+}
+
+static void bdi_put_wb(struct backing_dev_info *bdi, struct bdi_writeback *wb)
+{
+ clear_bit(wb->nr, &bdi->wb_mask);
+
+ if (wb == &bdi->wb)
+ clear_bit(BDI_wb_alloc, &bdi->state);
+ else
+ kfree(wb);
+
+ spin_lock(&bdi->wb_lock);
+ bdi->wb_cnt--;
+ spin_unlock(&bdi->wb_lock);
+}
+
+static int bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
{
memset(wb, 0, sizeof(*wb));

@@ -208,6 +243,30 @@ static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
INIT_LIST_HEAD(&wb->b_dirty);
INIT_LIST_HEAD(&wb->b_io);
INIT_LIST_HEAD(&wb->b_more_io);
+
+ return wb_assign_nr(bdi, wb);
+}
+
+static struct bdi_writeback *bdi_new_wb(struct backing_dev_info *bdi)
+{
+ struct bdi_writeback *wb;
+
+ /*
+ * Default bdi->wb is already assigned, so just return it
+ */
+ if (!test_and_set_bit(BDI_wb_alloc, &bdi->state))
+ wb = &bdi->wb;
+ else {
+ wb = kmalloc(sizeof(struct bdi_writeback), GFP_KERNEL);
+ if (wb) {
+ if (bdi_wb_init(wb, bdi)) {
+ kfree(wb);
+ wb = NULL;
+ }
+ }
+ }
+
+ return wb;
}

static void bdi_flush_io(struct backing_dev_info *bdi)
@@ -223,35 +282,26 @@ static void bdi_flush_io(struct backing_dev_info *bdi)
generic_sync_bdi_inodes(NULL, &wbc);
}

-static int wb_assign_nr(struct backing_dev_info *bdi, struct bdi_writeback *wb)
+static void bdi_task_init(struct backing_dev_info *bdi,
+ struct bdi_writeback *wb)
{
- set_bit(0, &bdi->wb_mask);
- wb->nr = 0;
- return 0;
-}
+ struct task_struct *tsk = current;
+ int was_empty;

-static void bdi_put_wb(struct backing_dev_info *bdi, struct bdi_writeback *wb)
-{
- clear_bit(wb->nr, &bdi->wb_mask);
- clear_bit(BDI_wb_alloc, &bdi->state);
-}
+ /*
+ * Add us to the active bdi_list. If we are adding threads beyond
+ * the default embedded bdi_writeback, then we need to start using
+ * proper locking. Check the list for empty first, then set the
+ * BDI_wblist_lock flag if there's > 1 entry on the list now
+ */
+ spin_lock(&bdi->wb_lock);

-static struct bdi_writeback *bdi_new_wb(struct backing_dev_info *bdi)
-{
- struct bdi_writeback *wb;
+ was_empty = list_empty(&bdi->wb_list);
+ list_add_tail_rcu(&wb->list, &bdi->wb_list);
+ if (!was_empty)
+ set_bit(BDI_wblist_lock, &bdi->state);

- set_bit(BDI_wb_alloc, &bdi->state);
- wb = &bdi->wb;
- wb_assign_nr(bdi, wb);
- return wb;
-}
-
-static int bdi_start_fn(void *ptr)
-{
- struct bdi_writeback *wb = ptr;
- struct backing_dev_info *bdi = wb->bdi;
- struct task_struct *tsk = current;
- int ret;
+ spin_unlock(&bdi->wb_lock);

tsk->flags |= PF_FLUSHER | PF_SWAPWRITE;
set_freezable();
@@ -260,6 +310,15 @@ static int bdi_start_fn(void *ptr)
* Our parent may run at a different priority, just set us to normal
*/
set_user_nice(tsk, 0);
+}
+
+static int bdi_start_fn(void *ptr)
+{
+ struct bdi_writeback *wb = ptr;
+ struct backing_dev_info *bdi = wb->bdi;
+ int ret;
+
+ bdi_task_init(bdi, wb);

/*
* Clear pending bit and wakeup anybody waiting to tear us down
@@ -267,25 +326,65 @@ static int bdi_start_fn(void *ptr)
clear_bit(BDI_pending, &bdi->state);
wake_up_bit(&bdi->state, BDI_pending);

+ /*
+ * Make us discoverable on the bdi_list again
+ */
+ spin_lock(&bdi_lock);
+ list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
+ spin_unlock(&bdi_lock);
+
ret = bdi_writeback_task(wb);

+ /*
+ * Remove us from the list
+ */
+ spin_lock(&bdi->wb_lock);
+ list_del_rcu(&wb->list);
+ spin_unlock(&bdi->wb_lock);
+
+ /*
+ * wait for rcu grace period to end, so we can free wb
+ */
+ synchronize_srcu(&bdi->srcu);
+
bdi_put_wb(bdi, wb);
return ret;
}

int bdi_has_dirty_io(struct backing_dev_info *bdi)
{
- return wb_has_dirty_io(&bdi->wb);
+ struct bdi_writeback *wb;
+ int ret = 0;
+
+ if (!bdi_wblist_needs_lock(bdi))
+ ret = wb_has_dirty_io(&bdi->wb);
+ else {
+ int idx;
+
+ idx = srcu_read_lock(&bdi->srcu);
+
+ list_for_each_entry_rcu(wb, &bdi->wb_list, list) {
+ ret = wb_has_dirty_io(wb);
+ if (ret)
+ break;
+ }
+
+ srcu_read_unlock(&bdi->srcu, idx);
+ }
+
+ return ret;
}

static int bdi_forker_task(void *ptr)
{
struct bdi_writeback *me = ptr;
+ DEFINE_WAIT(wait);
+
+ bdi_task_init(me->bdi, me);

for (;;) {
struct backing_dev_info *bdi;
struct bdi_writeback *wb;
- DEFINE_WAIT(wait);

/*
* Should never trigger on the default bdi
@@ -301,7 +400,6 @@ static int bdi_forker_task(void *ptr)
if (list_empty(&bdi_pending_list))
schedule();

- finish_wait(&me->wait, &wait);
repeat:
bdi = NULL;
spin_lock_bh(&bdi_lock);
@@ -344,6 +442,7 @@ readd_flush:
}
}

+ finish_wait(&me->wait, &wait);
return 0;
}

@@ -367,34 +466,68 @@ static void bdi_add_to_pending(struct rcu_head *head)
wake_up(&default_backing_dev_info.wb.wait);
}

-/*
- * Add a new flusher task that gets created for any bdi
- * that has dirty data pending writeout
- */
-void bdi_add_default_flusher_task(struct backing_dev_info *bdi)
+static void bdi_add_one_flusher_task(struct backing_dev_info *bdi,
+ int(*func)(struct backing_dev_info *))
{
if (!bdi_cap_writeback_dirty(bdi))
return;

/*
- * Someone already marked this pending for task creation
+ * Check with the helper whether to proceed adding a task. Will only
+ * abort if we two or more simultanous calls to
+ * bdi_add_default_flusher_task() occured, further additions will block
+ * waiting for previous additions to finish.
*/
- if (test_and_set_bit(BDI_pending, &bdi->state))
- return;
+ if (!func(bdi)) {
+ spin_lock_bh(&bdi_lock);
+ list_del_rcu(&bdi->bdi_list);
+ spin_unlock_bh(&bdi_lock);

- spin_lock_bh(&bdi_lock);
- list_del_rcu(&bdi->bdi_list);
- spin_unlock_bh(&bdi_lock);
+ /*
+ * We need to wait for the current grace period to end,
+ * in case others were browsing the bdi_list as well.
+ * So defer the adding and wakeup to after the RCU
+ * grace period has ended.
+ */
+ call_rcu(&bdi->rcu_head, bdi_add_to_pending);
+ }
+}

- /*
- * We need to wait for the current grace period to end,
- * in case others were browsing the bdi_list as well.
- * So defer the adding and wakeup to after the RCU
- * grace period has ended.
- */
- call_rcu(&bdi->rcu_head, bdi_add_to_pending);
+static int flusher_add_helper_block(struct backing_dev_info *bdi)
+{
+ wait_on_bit_lock(&bdi->state, BDI_pending, bdi_sched_wait,
+ TASK_UNINTERRUPTIBLE);
+ return 0;
+}
+
+static int flusher_add_helper_test(struct backing_dev_info *bdi)
+{
+ return test_and_set_bit(BDI_pending, &bdi->state);
+}
+
+/*
+ * Add the default flusher task that gets created for any bdi
+ * that has dirty data pending writeout
+ */
+void bdi_add_default_flusher_task(struct backing_dev_info *bdi)
+{
+ bdi_add_one_flusher_task(bdi, flusher_add_helper_test);
}

+/**
+ * bdi_add_flusher_task - add one more flusher task to this @bdi
+ * @bdi: the bdi
+ *
+ * Add an additional flusher task to this @bdi. Will block waiting on
+ * previous additions, if any.
+ *
+ */
+void bdi_add_flusher_task(struct backing_dev_info *bdi)
+{
+ bdi_add_one_flusher_task(bdi, flusher_add_helper_block);
+}
+EXPORT_SYMBOL(bdi_add_flusher_task);
+
int bdi_register(struct backing_dev_info *bdi, struct device *parent,
const char *fmt, ...)
{
@@ -454,17 +587,13 @@ int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev)
}
EXPORT_SYMBOL(bdi_register_dev);

-static int sched_wait(void *word)
-{
- schedule();
- return 0;
-}
-
/*
* Remove bdi from global list and shutdown any threads we have running
*/
static void bdi_wb_shutdown(struct backing_dev_info *bdi)
{
+ struct bdi_writeback *wb;
+
if (!bdi_cap_writeback_dirty(bdi))
return;

@@ -472,7 +601,8 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
* If setup is pending, wait for that to complete first
* Make sure nobody finds us on the bdi_list anymore
*/
- wait_on_bit(&bdi->state, BDI_pending, sched_wait, TASK_UNINTERRUPTIBLE);
+ wait_on_bit(&bdi->state, BDI_pending, bdi_sched_wait,
+ TASK_UNINTERRUPTIBLE);

/*
* Make sure nobody finds us on the bdi_list anymore
@@ -488,9 +618,11 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
synchronize_rcu();

/*
- * Finally, kill the kernel thread
+ * Finally, kill the kernel threads. We don't need to be RCU
+ * safe anymore, since the bdi is gone from visibility.
*/
- kthread_stop(bdi->wb.task);
+ list_for_each_entry(wb, &bdi->wb_list, list)
+ kthread_stop(wb->task);
}

void bdi_unregister(struct backing_dev_info *bdi)
@@ -515,8 +647,12 @@ int bdi_init(struct backing_dev_info *bdi)
bdi->min_ratio = 0;
bdi->max_ratio = 100;
bdi->max_prop_frac = PROP_FRAC_BASE;
+ spin_lock_init(&bdi->wb_lock);
+ bdi->wb_mask = 0;
+ bdi->wb_cnt = 0;
INIT_LIST_HEAD(&bdi->bdi_list);
- bdi->wb_mask = bdi->wb_active = 0;
+ INIT_LIST_HEAD(&bdi->wb_list);
+ INIT_LIST_HEAD(&bdi->work_list);

bdi_wb_init(&bdi->wb, bdi);

@@ -526,10 +662,15 @@ int bdi_init(struct backing_dev_info *bdi)
goto err;
}

+ err = init_srcu_struct(&bdi->srcu);
+ if (err)
+ goto err;
+
bdi->dirty_exceeded = 0;
err = prop_local_init_percpu(&bdi->completions);

if (err) {
+ cleanup_srcu_struct(&bdi->srcu);
err:
while (i--)
percpu_counter_destroy(&bdi->bdi_stat[i]);
@@ -547,6 +688,8 @@ void bdi_destroy(struct backing_dev_info *bdi)

bdi_unregister(bdi);

+ cleanup_srcu_struct(&bdi->srcu);
+
for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
percpu_counter_destroy(&bdi->bdi_stat[i]);

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 76269f8..de3178a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -667,8 +667,7 @@ void throttle_vm_writeout(gfp_t gfp_mask)

/*
* Start writeback of `nr_pages' pages. If `nr_pages' is zero, write back
- * the whole world. Returns 0 if a pdflush thread was dispatched. Returns
- * -1 if all pdflush threads were busy.
+ * the whole world.
*/
void wakeup_flusher_threads(long nr_pages)
{
@@ -676,7 +675,6 @@ void wakeup_flusher_threads(long nr_pages)
nr_pages = global_page_state(NR_FILE_DIRTY) +
global_page_state(NR_UNSTABLE_NFS);
bdi_writeback_all(NULL, nr_pages);
- return;
}

static void laptop_timer_fn(unsigned long unused);
--
1.6.3.rc0.1.gf800

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