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Subject[PATCH v5 08/11] wait_bit: introduce {wait_on,wake_up}_atomic_one
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Add a generic facility for awaiting an atomic_t to reach a value of 1.

Page reference counts typically need to reach 0 to be considered a
free / inactive page. However, ZONE_DEVICE pages allocated via
devm_memremap_pages() are never 'onlined', i.e. the put_page() typically
done at init time to assign pages to the page allocator is skipped.

These pages will have their reference count elevated > 1 by
get_user_pages() when they are under DMA. In order to coordinate DMA to
these pages vs filesytem operations like hole-punch and truncate the
filesystem-dax implementation needs to capture the DMA-idle event i.e.
the 2 to 1 count transition).

For now, this implementation does not have functional behavior change,
follow-on patches will add waiters for these page-idle events.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
drivers/dax/super.c | 2 +-
include/linux/wait_bit.h | 13 ++++++++++
kernel/sched/wait_bit.c | 59 +++++++++++++++++++++++++++++++++++++++-------
3 files changed, 64 insertions(+), 10 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 619b1ed6434c..7e10fa3460e2 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -167,7 +167,7 @@ struct dax_device {
#if IS_ENABLED(CONFIG_FS_DAX)
static void generic_dax_pagefree(struct page *page, void *data)
{
- /* TODO: wakeup page-idle waiters */
+ wake_up_atomic_one(&page->_refcount);
}

struct dax_device *fs_dax_claim_bdev(struct block_device *bdev, void *owner)
diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h
index 61b39eaf7cad..564c9a0141cd 100644
--- a/include/linux/wait_bit.h
+++ b/include/linux/wait_bit.h
@@ -33,10 +33,15 @@ int __wait_on_bit(struct wait_queue_head *wq_head, struct wait_bit_queue_entry *
int __wait_on_bit_lock(struct wait_queue_head *wq_head, struct wait_bit_queue_entry *wbq_entry, wait_bit_action_f *action, unsigned int mode);
void wake_up_bit(void *word, int bit);
void wake_up_atomic_t(atomic_t *p);
+static inline void wake_up_atomic_one(atomic_t *p)
+{
+ wake_up_atomic_t(p);
+}
int out_of_line_wait_on_bit(void *word, int, wait_bit_action_f *action, unsigned int mode);
int out_of_line_wait_on_bit_timeout(void *word, int, wait_bit_action_f *action, unsigned int mode, unsigned long timeout);
int out_of_line_wait_on_bit_lock(void *word, int, wait_bit_action_f *action, unsigned int mode);
int out_of_line_wait_on_atomic_t(atomic_t *p, wait_atomic_t_action_f action, unsigned int mode);
+int out_of_line_wait_on_atomic_one(atomic_t *p, wait_atomic_t_action_f action, unsigned int mode);
struct wait_queue_head *bit_waitqueue(void *word, int bit);
extern void __init wait_bit_init(void);

@@ -262,4 +267,12 @@ int wait_on_atomic_t(atomic_t *val, wait_atomic_t_action_f action, unsigned mode
return out_of_line_wait_on_atomic_t(val, action, mode);
}

+static inline
+int wait_on_atomic_one(atomic_t *val, wait_atomic_t_action_f action, unsigned mode)
+{
+ might_sleep();
+ if (atomic_read(val) == 1)
+ return 0;
+ return out_of_line_wait_on_atomic_one(val, action, mode);
+}
#endif /* _LINUX_WAIT_BIT_H */
diff --git a/kernel/sched/wait_bit.c b/kernel/sched/wait_bit.c
index 84cb3acd9260..8739b1e50df5 100644
--- a/kernel/sched/wait_bit.c
+++ b/kernel/sched/wait_bit.c
@@ -162,28 +162,47 @@ static inline wait_queue_head_t *atomic_t_waitqueue(atomic_t *p)
return bit_waitqueue(p, 0);
}

-static int wake_atomic_t_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync,
- void *arg)
+static struct wait_bit_queue_entry *to_wait_bit_q(
+ struct wait_queue_entry *wq_entry)
+{
+ return container_of(wq_entry, struct wait_bit_queue_entry, wq_entry);
+}
+
+static int __wake_atomic_t_function(struct wait_queue_entry *wq_entry,
+ unsigned mode, int sync, void *arg, int target)
{
struct wait_bit_key *key = arg;
- struct wait_bit_queue_entry *wait_bit = container_of(wq_entry, struct wait_bit_queue_entry, wq_entry);
+ struct wait_bit_queue_entry *wait_bit = to_wait_bit_q(wq_entry);
atomic_t *val = key->flags;

if (wait_bit->key.flags != key->flags ||
wait_bit->key.bit_nr != key->bit_nr ||
- atomic_read(val) != 0)
+ atomic_read(val) != target)
return 0;
return autoremove_wake_function(wq_entry, mode, sync, key);
}

+static int wake_atomic_t_function(struct wait_queue_entry *wq_entry,
+ unsigned mode, int sync, void *arg)
+{
+ return __wake_atomic_t_function(wq_entry, mode, sync, arg, 0);
+}
+
+static int wake_atomic_one_function(struct wait_queue_entry *wq_entry,
+ unsigned mode, int sync, void *arg)
+{
+ return __wake_atomic_t_function(wq_entry, mode, sync, arg, 1);
+}
+
/*
* To allow interruptible waiting and asynchronous (i.e. nonblocking) waiting,
* the actions of __wait_on_atomic_t() are permitted return codes. Nonzero
* return codes halt waiting and return.
*/
static __sched
-int __wait_on_atomic_t(struct wait_queue_head *wq_head, struct wait_bit_queue_entry *wbq_entry,
- wait_atomic_t_action_f action, unsigned int mode)
+int __wait_on_atomic_t(struct wait_queue_head *wq_head,
+ struct wait_bit_queue_entry *wbq_entry,
+ wait_atomic_t_action_f action, unsigned int mode, int target)
{
atomic_t *val;
int ret = 0;
@@ -191,10 +210,10 @@ int __wait_on_atomic_t(struct wait_queue_head *wq_head, struct wait_bit_queue_en
do {
prepare_to_wait(wq_head, &wbq_entry->wq_entry, mode);
val = wbq_entry->key.flags;
- if (atomic_read(val) == 0)
+ if (atomic_read(val) == target)
break;
ret = (*action)(val, mode);
- } while (!ret && atomic_read(val) != 0);
+ } while (!ret && atomic_read(val) != target);
finish_wait(wq_head, &wbq_entry->wq_entry);
return ret;
}
@@ -210,6 +229,17 @@ int __wait_on_atomic_t(struct wait_queue_head *wq_head, struct wait_bit_queue_en
}, \
}

+#define DEFINE_WAIT_ATOMIC_ONE(name, p) \
+ struct wait_bit_queue_entry name = { \
+ .key = __WAIT_ATOMIC_T_KEY_INITIALIZER(p), \
+ .wq_entry = { \
+ .private = current, \
+ .func = wake_atomic_one_function, \
+ .entry = \
+ LIST_HEAD_INIT((name).wq_entry.entry), \
+ }, \
+ }
+
__sched int out_of_line_wait_on_atomic_t(atomic_t *p,
wait_atomic_t_action_f action,
unsigned int mode)
@@ -217,7 +247,7 @@ __sched int out_of_line_wait_on_atomic_t(atomic_t *p,
struct wait_queue_head *wq_head = atomic_t_waitqueue(p);
DEFINE_WAIT_ATOMIC_T(wq_entry, p);

- return __wait_on_atomic_t(wq_head, &wq_entry, action, mode);
+ return __wait_on_atomic_t(wq_head, &wq_entry, action, mode, 0);
}
EXPORT_SYMBOL(out_of_line_wait_on_atomic_t);

@@ -230,6 +260,17 @@ __sched int atomic_t_wait(atomic_t *counter, unsigned int mode)
}
EXPORT_SYMBOL(atomic_t_wait);

+__sched int out_of_line_wait_on_atomic_one(atomic_t *p,
+ wait_atomic_t_action_f action,
+ unsigned int mode)
+{
+ struct wait_queue_head *wq_head = atomic_t_waitqueue(p);
+ DEFINE_WAIT_ATOMIC_ONE(wq_entry, p);
+
+ return __wait_on_atomic_t(wq_head, &wq_entry, action, mode, 1);
+}
+EXPORT_SYMBOL(out_of_line_wait_on_atomic_one);
+
/**
* wake_up_atomic_t - Wake up a waiter on a atomic_t
* @p: The atomic_t being waited on, a kernel virtual address
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